On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.
by
Charles Darwin, M.A.,
Fellow of the Royal, Geological, Linnan, etc. societies;
Author of Journal of researches during H. M. S. Beagle's Voyage round the world.
London: John Murray, Albemarle Street, 1859
WHEN on board H.M.S.
Beagle, as naturalist, I was much struck
with certain facts in the distribution of the inhabitants of South
America, and in the geological relations of the present to the past
inhabitants of that continent. These facts seemed to me to throw some
light on the origin of species — that mystery of mysteries, as
it has been called by one of our greatest philosophers. On my return
home, it occurred to me, in 1837, that something might perhaps be made
out on this question by patiently accumulating and reflecting on all
sorts of facts which could possibly have any bearing on it. After five
years' work I allowed myself to speculate on the subject, and drew up
some short notes; these I enlarged in 1844 into a sketch of the
conclusions, which then seemed to me probable: from that period to the
present day I have steadily pursued the same object. I hope that I may
be excused for entering on these personal details, as I give them to
show that I have not been hasty in coming to a decision.
My work is now nearly finished; but as it will take me two or three
more years to complete it, and as my health is far from strong, I have
been urged to publish this Abstract. I have more especially been
induced to do this, as Mr Wallace, who is now studying the
natural
history of the Malay archipelago, has arrived at almost exactly the
same general conclusions that I have on the origin of species. Last
year he sent to me a memoir on this subject, with a request that I
would forward it to Sir Charles Lyell, who sent it to the Linnean
Society, and it is published in the third volume of the journal of
that Society. Sir C. Lyell and Dr Hooker, who both knew of my work
-- the latter having read my sketch of 1844 — honoured me
by thinking it advisable to publish, with Mr Wallace's excellent
memoir, some brief extracts from my manuscripts.
This Abstract, which I now publish, must necessarily be imperfect.
I cannot here give references and authorities for my
several statements; and I must trust to the reader reposing some
confidence in my accuracy. No doubt errors will have crept in, though
I hope I have always been cautious in trusting to good authorities
alone. I can here give only the general conclusions at which I have
arrived, with a few facts in illustration, but which, I hope, in most
cases will suffice. No one can feel more sensible than I do of the
necessity of hereafter publishing in detail all the facts, with
references, on which my conclusions have been grounded; and I hope in
a future work to do this. For I am well aware that scarcely a single
point is discussed in this volume on which facts cannot be adduced,
often apparently leading to conclusions directly opposite to
those at which I have arrived. A fair result can be obtained only by
fully stating and balancing the facts and arguments on both sides of
each question; and this cannot possibly be here done.
I much regret that want of space prevents my having the
satisfaction of acknowledging the generous assistance which I have
received from very many naturalists, some of them personally unknown
to me. I cannot, however,
let this opportunity pass without expressing
my deep obligations to Dr Hooker, who for the last fifteen years has
aided me in every possible way by his large stores of knowledge and
his excellent judgement.
In considering the Origin of Species, it is quite conceivable that
a naturalist, reflecting on the mutual affinities of organic beings,
on their embryological relations, their geographical distribution,
geological succession, and other such facts, might come to the
conclusion that each species had not been independently created, but
had descended, like varieties, from other species. Nevertheless, such
a conclusion, even if well founded, would be unsatisfactory, until it
could be shown how the innumerable species inhabiting this world have
been modified so as to acquire that perfection of structure and
co-adaptation which most justly excites our admiration. Naturalists
continually refer to external conditions, such as climate, food,
c., as the only possible cause of variation. In one very limited
sense, as we shall hereafter see, this may be true; but it is
preposterous to attribute to mere external conditions, the structure,
for instance, of the woodpecker, with its feet, tail, beak, and
tongue, so admirably adapted to catch insects under the
bark of trees. In the case of the misseltoe, which draws its
nourishment from certain trees, which has seeds that must be
transported by certain birds, and which has flowers with separate
sexes absolutely requiring the agency of certain insects to bring
pollen from one flower to the other, it is equally preposterous to
account for the structure of this parasite, with its relations to
several distinct organic beings, by the effects of external
conditions, or of habit, or of the volition of the plant itself.
The author of the 'Vestiges of Creation' would, I presume, say
that, after a certain unknown number of
generations, some bird had
given birth to a woodpecker, and some plant to the misseltoe, and that
these had been produced perfect as we now see them; but this
assumption seems to me to be no explanation, for it leaves the case of
the coadaptations of organic beings to each other and to their
physical conditions of life, untouched and unexplained.
It is, therefore, of the highest importance to gain a clear insight
into the means of modification and coadaptation. At the commencement
of my observations it seemed to me probable that a careful study of
domesticated animals and of cultivated plants would offer the best
chance of making out this obscure problem. Nor have I been
disappointed; in this and in all other perplexing cases I have
invariably found that our knowledge, imperfect though it be, of
variation under domestication, afforded the best and safest clue. I
may venture to express my conviction of the high value of such
studies, although they have been very commonly neglected by
naturalists.
From these considerations, I shall devote the first chapter of this
Abstract to Variation under Domestication. We shall thus see that a
large amount of hereditary modification is at least possible, and,
what is equally or more important, we shall see how great is the power
of man in accumulating by his Selection successive slight variations.
I will then pass on to the variability of species in a state of
nature; but I shall, unfortunately, be compelled to treat this subject
far too briefly, as it can be treated properly only by giving long
catalogues of facts. We shall, however, be enabled to discuss what
circumstances are most favourable to variation. In the
next chapter the Struggle for Existence amongst all organic beings
throughout the world, which inevitably follows from their high
geometrical powers of
increase, will be treated of. This is the
doctrine of Malthus, applied to the whole animal and vegetable
kingdoms. As many more individuals of each species are born than can
possibly survive; and as, consequently, there is a frequently
recurring struggle for existence, it follows that any being, if it
vary however slightly in any manner profitable to itself, under the
complex and sometimes varying conditions of life, will have a better
chance of surviving, and thus be naturally
selected. From the strong principle of inheritance, any
selected variety will tend to propagate its new and modified form.
This fundamental subject of Natural Selection will be treated at
some length in the fourth chapter; and we shall then see how Natural
Selection almost inevitably causes much Extinction of the less
improved forms of life and induces what I have called Divergence of
Character. In the next chapter I shall discuss the complex and little
known laws of variation and of correlation of growth. In the four
succeeding chapters, the most apparent and gravest difficulties on the
theory will be given: namely, first, the difficulties of transitions,
or understanding how a simple being or a simple organ can be changed
and perfected into a highly developed being or elaborately constructed
organ; secondly the subject of Instinct, or the mental powers of
animals, thirdly, Hybridism, or the infertility of species and the
fertility of varieties when intercrossed; and fourthly, the
imperfection of the Geological Record. In the next chapter I shall
consider the geological succession of organic beings throughout time;
in the eleventh and twelfth, their geographical distribution
throughout space; in the thirteenth, their classification or mutual
affinities, both when mature and in an embryonic condition. In the last
chapter I shall give a
brief recapitulation of the whole work, and a
few concluding remarks.)
No one ought to feel surprise at much remaining as yet unexplained
in regard to the origin of species and varieties, if he makes due
allowance for our profound ignorance in regard to the mutual relations
of all the beings which live around us. Who can explain
why one species ranges widely and is very numerous, and why another
allied species has a narrow range and is rare? Yet these relations are
of the highest importance, for they determine the present welfare,
and, as I believe, the future success and modification of every
inhabitant of this world. Still less do we know of the mutual
relations of the innumerable inhabitants of the world during the many
past geological epochs in its history. Although much remains obscure,
and will long remain obscure, I can entertain no doubt, after the most
deliberate study and dispassionate judgement of which I am capable,
that the view which most naturalists entertain, and which I formerly
entertained — namely, that each species has been independently
created — is erroneous. I am fully convinced that species are
not immutable; but that those belonging to what are called the same
genera are lineal descendants of some other and generally extinct
species, in the same manner as the acknowledged varieties of any one
species are the descendants of that species. Furthermore, I am
convinced that Natural Selection has been the main but not exclusive
means of modification.
VARIATION UNDER
DOMESTICATION
- Causes of Variability
- Effects of Habit
- Correlation of Growth
- Inheritance
- Character of Domestic Varieties
- Difficulty of distinguishing between Varieties and Species
- Origin of Domestic Varieties from one or more Species
- Domestic pigeons, their Differences and Origin
- Principle of Selection anciently followed, its Effects
- Methodical and Unconscious Selection
- Unknown Origin of our Domestic Productions
- Circumstances favourable to Man's power of Selection
WHEN we look to the individuals of the same variety or sub-variety of our
older cultivated plants and animals, one of the first points which
strikes us, is, that they generally differ much more from each other,
than do the individuals of any one species or variety in a state of
nature. When we reflect on the vast diversity of the plants and
animals which have been cultivated, and which have varied during all
ages under the most different climates and treatment, I think we are
driven to conclude that this greater variability is simply due to our
domestic productions having been raised under conditions of life not
so uniform as, and somewhat different from, those to which the
parent-species have been exposed under nature. There is, also, I
think, some probability in the view propounded by Andrew Knight, that
this variability may be partly connected with excess of food. It seems
pretty clear that organic beings must be exposed during several
generations to the new conditions of life to cause any appreciable
amount of variation; and that when the organisation has once begun to
vary, it generally continues to vary for many generations.
No case is
on record of a variable being ceasing to be variable under
cultivation. Our oldest cultivated plants, such as wheat, still often
yield new varieties: our oldest domesticated animals are still capable
of rapid improvement or modification.
It has been disputed at what period of time the causes of variability, whatever they may be, generally act; whether
during the early or late period of development of the embryo, or at
the instant of conception. Geoffroy St Hilaire's experiments show that
unnatural treatment of the embryo causes monstrosities; and
monstrosities cannot be separated by any clear line of distinction
from mere variations. But I am strongly inclined to suspect that the
most frequent cause of variability may be attributed to the male and
female reproductive elements having been affected prior to the act of
conception. Several reasons make me believe in this; but the chief one
is the remarkable effect which confinement or cultivation has on the
functions of the reproductive system; this system appearing to be far
more susceptible than any other part of the organization, to the
action of any change in the conditions of life. Nothing is more easy
than to tame an animal, and few things more difficult than to get it
to breed freely under confinement, even in the many cases when the
male and female unite. How many animals there are which will not
breed, though living long under not very close confinement in their
native country! This is generally attributed to vitiated instincts;
but how many cultivated plants display the utmost vigour, and yet
rarely or never seed! In some few such cases it has been found out
that very trifling changes, such as a little more or less water at
some particular period of growth, will determine whether or not the
plant sets a seed. I cannot here enter on the copious details which I
have collected on
this curious subject; but to show how singular the
laws are which determine the reproduction of animals under
confinement, I may just mention that carnivorous animals, even from
the tropics, breed in this country pretty freely under confinement,
with the exception of the plantigrades or bear family; whereas,
carnivorous birds, with the rarest exceptions, hardly ever lay fertile
eggs. Many exotic plants have pollen utterly worthless, in the same
exact condition as in the most sterile hybrids. When, on the one hand,
we see domesticated animals and plants, though often weak and sickly,
yet breeding quite freely under confinement; and when, on the other
hand, we see individuals, though taken young from a state of nature,
perfectly tamed, long-lived, and healthy (of which I could give
numerous instances), yet having their reproductive system
so seriously affected by unperceived causes as to fail in acting, we
need not be surprised at this system, when it does act under
confinement, acting not quite regularly, and producing offspring not
perfectly like their parents or variable.
Sterility has been said to be the bane of horticulture; but on this
view we owe variability to the same cause which produces sterility;
and variability is the source of all the choicest productions of the
garden. I may add, that as some organisms will breed most freely under
the most unnatural conditions (for instance, the rabbit and ferret
kept in hutches), showing that their reproductive system has not been
thus affected; so will some animals and plants withstand domestication
or cultivation, and vary very slightly — perhaps hardly more
than in a state of nature.
A long list could easily be given of 'sporting plants;' by this
term gardeners mean a single bud or offset, which suddenly assumes a
new and sometimes very different character from that of the rest of
the plant.
Such buds can be propagated by grafting, c., and
sometimes by seed. These 'sports' are extremely rare under nature,
but far from rare under cultivation; and in this case we see that the
treatment of the parent has affected a bud or offset, and not the
ovules or pollen. But it is the opinion of most physiologists that
there is no essential difference between a bud and an ovule in their
earliest stages of formation; so that, in fact,'sports' support my
view, that variability may be largely attributed to the ovules or
pollen, or to both, having been affected by the treatment of the
parent prior to the act of conception. These cases anyhow show that
variation is not necessarily connected, as some authors have supposed,
with the act of generation.
Seedlings from the same fruit, and the young of the same litter,
sometimes differ considerably from each other, though both the young
and the parents, as Muller has remarked, have apparently been exposed
to exactly the same conditions of life; and this shows how unimportant
the direct effects of the conditions of life are in comparison with
the laws of reproduction, and of growth, and of inheritance; for had
the action of the conditions been direct, if any of the young had
varied, all would probably have varied in the same manner. To judge
how much, in the case of any variation, we should
attribute to the direct action of heat, moisture, light, food,
c., is most difficult: my impression is, that with animals such
agencies have produced very little direct effect, though apparently
more in the case of plants. Under this point of view, Mr Buckman's
recent experiments on plants seem extremely valuable. When all or
nearly all the individuals exposed to certain conditions are affected
in the same way, the change at first appears to be directly due to
such conditions; but in some cases it can be shown that quite opposite
conditions produce
similar changes of structure. Nevertheless some
slight amount of change may, I think, be attributed to the direct
action of the conditions of life — as, in some cases, increased
size from amount of food, colour from particular kinds of food and
from light, and perhaps the thickness of fur from climate.
Habit also has a deciding influence, as in the period of flowering
with plants when transported from one climate to another. In animals
it has a more marked effect; for instance, I find in the domestic duck
that the bones of the wing weigh less and the bones of the leg more,
in proportion to the whole skeleton, than do the same bones in the
wild-duck; and I presume that this change may be safely attributed to
the domestic duck flying much less, and walking more, than its wild
parent. The great and inherited development of the udders in cows and
goats in countries where they are habitually milked, in comparison
with the state of these organs in other countries, is another instance
of the effect of use. Not a single domestic animal can be named which
has not in some country drooping ears; and the view suggested by some
authors, that the drooping is due to the disuse of the muscles of the
ear, from the animals not being much alarmed by danger, seems
probable.
There are many laws regulating variation, some few of which can be
dimly seen, and will be hereafter briefly mentioned. I will here only
allude to what may be called correlation of growth. Any change in the
embryo or larva will almost certainly entail changes in the mature
animal. In monstrosities, the correlations between quite distinct
parts are very curious; and many instances are given in Isidore
Geoffroy St Hilaire's great work on this subject. Breeders believe
that long limbs are almost always accompanied by an
elongated head. Some instances of correlation are quite whimsical;
thus
cats with blue eyes are invariably deaf; colour and
constitutional peculiarities go together, of which many remarkable
cases could be given amongst animals and plants. From the facts
collected by Heusinger, it appears that white sheep and pigs are
differently affected from coloured individuals by certain vegetable
poisons. Hairless dogs have imperfect teeth; long-haired and
coarse-haired animals are apt to have, as is asserted, long or many
horns; pigeons with feathered feet have skin between their outer toes;
pigeons with short beaks have small feet, and those with long beaks
large feet. Hence, if man goes on selecting, and thus augmenting, any
peculiarity, he will almost certainly unconsciously modify other parts
of the structure, owing to the mysterious laws of the correlation of
growth.
The result of the various, quite unknown, or dimly seen laws of
variation is infinitely complex and diversified. It is well worth
while carefully to study the several treatises published on some of
our old cultivated plants, as on the hyacinth, potato, even the
dahlia, c.; and it is really surprising to note the endless
points in structure and constitution in which the varieties and sub
varieties differ slightly from each other. The whole organization
seems to have become plastic, and tends to depart in some small degree
from that of the parental type.
Any variation which is not inherited is unimportant for us. But the
number and diversity of inheritable deviations of structure, both
those of slight and those of considerable physiological importance, is
endless. Dr Prosper Lucas's treatise, in two large volumes, is the
fullest and the best on this subject. No breeder doubts how strong is
the tendency to inheritance: like produces like is his fundamental
belief: doubts have been thrown on this principle by theoretical
writers alone. When a
deviation appears not unfrequently, and we see
it in the father and child, we cannot tell whether it may not be due
to the same original cause acting on both; but when amongst
individuals, apparently exposed to the same conditions, any very rare
deviation, due to some extraordinary combination of circumstances,
appears in the parent — say, once amongst several million
individuals — and it reappears in the child, the
mere doctrine of chances almost compels us to attribute its
reappearance to inheritance. Every one must have heard of cases of
albinism, prickly skin, hairy bodies, c. appearing in several
members of the same family. If strange and rare deviations of
structure are truly inherited, less strange and commoner deviations
may be freely admitted to be inheritable. Perhaps the correct way of
viewing the whole subject, would be, to look at the inheritance of
every character whatever as the rule, and non-inheritance as the
anomaly.
The laws governing inheritance are quite unknown; no one can say
why the same peculiarity in different individuals of the same species,
and in individuals of different species, is sometimes inherited and
sometimes not so; why the child often reverts in certain characters to
its grandfather or grandmother or other much more remote ancestor; why
a peculiarity is often transmitted from one sex to both sexes or to
one sex alone, more commonly but not exclusively to the like sex. It
is a fact of some little importance to us, that peculiarities
appearing in the males of our domestic breeds are often transmitted
either exclusively, or in a much greater degree, to males alone. A
much more important rule, which I think may be trusted, is that, at
whatever period of life a peculiarity first appears, it tends to
appear in the offspring at a corresponding age, though sometimes
earlier. In many cases this could
not be otherwise: thus the inherited
peculiarities in the horns of cattle could appear only in the
offspring when nearly mature; peculiarities in the silkworm are known
to appear at the corresponding caterpillar or cocoon stage. But
hereditary diseases and some other facts make me believe that the rule
has a wider extension, and that when there is no apparent reason why a
peculiarity should appear at any particular age, yet that it does tend
to appear in the offspring at the same period at which it first
appeared in the parent. I believe this rule to be of the highest
importance in explaining the laws of embryology. These remarks are of
course confined to the first appearance of
the peculiarity, and not to its primary cause, which may have acted on
the ovules or male element; in nearly the same manner as in the
crossed offspring from a short-horned cow by a long-horned bull, the
greater length of horn, though appearing late in life, is
clearly due to the male element.
Having alluded to the subject of reversion, I may here refer to a
statement often made by naturalists — namely, that our domestic
varieties, when run wild, gradually but certainly revert in character
to their aboriginal stocks. Hence it has been argued that no
deductions can be drawn from domestic races to species in a state of
nature. I have in vain endeavoured to discover on what decisive facts
the above statement has so often and so boldly been made. There would
be great difficulty in proving its truth: we may safely conclude that
very many of the most strongly-marked domestic varieties could not
possibly live in a wild state. In many cases we do not know what the
aboriginal stock was, and so could not tell whether or not nearly
perfect reversion had ensued. It would be quite necessary, in order to
prevent the effects of intercrossing, that only a
single variety
should be turned loose in its new home. Nevertheless, as our
varieties certainly do occasionally revert in some of their characters
to ancestral forms, it seems to me not improbable, that if we could
succeed in naturalising, or were to cultivate, during many
generations, the several races, for instance, of the cabbage, in very
poor soil (in which case, however, some effect would have to be
attributed to the direct action of the poor soil), that they would to
a large extent, or even wholly, revert to the wild aboriginal stock.
Whether or not the experiment would succeed, is not of great
importance for our line of argument; for by the experiment itself the
conditions of life are changed. If it could be shown that our domestic
varieties manifested a strong tendency to reversion, — that is,
to lose their acquired characters, whilst kept under unchanged
conditions, and whilst kept in a considerable body, so that free
intercrossing might check, by blending together, any slight deviations
of structure, in such case, I grant that we could deduce nothing from
domestic varieties in regard to species. But there is not a shadow of
evidence in favour of this view: to assert that we could not breed our
cart and race-horses, long and short-horned cattle and poultry of
various breeds, and esculent vegetables, for an almost infinite number
of generations, would be opposed to all experience. I may add, that
when under nature the conditions of life do change,
variations and reversions of character probably do occur; but natural
selection, as will hereafter be explained, will determine how far the
new characters thus arising shall be preserved.
When we look to the hereditary varieties or races of our domestic
animals and plants, and compare them with species closely allied
together, we generally perceive in each domestic race, as already
remarked, less uniformity of character than in true species. Domestic
races of
the same species, also, often have a somewhat monstrous
character; by which I mean, that, although differing from each other,
and from the other species of the same genus, in several trifling
respects, they often differ in an extreme degree in some one part,
both when compared one with another, and more especially when compared
with all the species in nature to which they are nearest allied. With
these exceptions (and with that of the perfect fertility of varieties
when crossed, — a subject hereafter to be discussed), domestic
races of the same species differ from each other in the same manner
as, only in most cases in a lesser degree than, do closely-allied
species of the same genus in a state of nature. I think this must be
admitted, when we find that there are hardly any domestic races,
either amongst animals or plants, which have not been ranked by some
competent judges as mere varieties, and by other competent judges as
the descendants of aboriginally distinct species. If any marked
distinction existed between domestic races and species, this source of
doubt could not so perpetually recur. It has often been stated that
domestic races do not differ from each other in characters of generic
value. I think it could be shown that this statement is hardly
correct; but naturalists differ most widely in determining what
characters are of generic value; all such valuations being at present
empirical. Moreover, on the view of the origin of genera which I shall
presently give, we have no right to expect often to meet with generic
differences in our domesticated productions.
When we attempt to estimate the amount of structural difference
between the domestic races of the same species, we are soon involved
in doubt, from not knowing whether they have descended
from one or several parent-species. This point, if could be cleared
up, would be interesting; if, for instance, it could be shown that the
greyhound,
bloodhound, terrier, spaniel, and bull-dog, which we all
know propagate their kind so truly, were the offspring of any single
species, then such facts would have great weight in making us doubt
about the immutability of the many very closely allied and natural
species — for instance, of the many foxes — inhabiting
different quarters of the world. I do not believe, as we shall
presently see, that all our dogs have descended from any one wild
species; but, in the case of some other domestic races, there is
presumptive, or even strong, evidence in favour of this view.
It has often been assumed that man has chosen for domestication
animals and plants having an extraordinary inherent tendency to vary,
and likewise to withstand diverse climates. I do not dispute that
these capacities have added largely to the value of most of our
domesticated productions; but how could a savage possibly know, when
he first tamed an animal, whether it would vary in succeeding
generations, and whether it would endure other climates? Has the
little variability of the ass or guinea-fowl, or the small power of
endurance of warmth by the reindeer, or of cold by the common camel,
prevented their domestication? I cannot doubt that if other animals
and plants, equal in number to our domesticated productions, and
belonging to equally diverse classes and countries, were taken from a
state of nature, and could be made to breed for an equal number of
generations under domestication, they would vary on an average as
largely as the parent species of our existing domesticated productions
have varied.
In the case of most of our anciently domesticated animals and
plants, I do not think it is possible to come to any definite
conclusion, whether they have descended from one or several species.
The argument mainly relied on by those who believe in the multiple
origin
of our domestic animals is, that we find in the most ancient
records, more especially on the monuments of Egypt, much diversity in
the breeds; and that some of the breeds closely resemble, perhaps are
identical with, those still existing. Even if this latter fact were
found more strictly and generally true than seems to me to
be the case, what does it show, but that some of our breeds originated
there, four or five thousand years ago? But Mr Horner's
researches have rendered it in some degree probable that man
sufficiently civilized to have manufactured pottery existed in the
valley of the Nile thirteen or fourteen thousand years ago; and who
will pretend to say how long before these ancient periods, savages,
like those of Tierra del Fuego or Australia, who possess a
semi-domestic dog, may not have existed in Egypt?
The whole subject must, I think, remain vague; nevertheless, I may,
without here entering on any details, state that, from geographical
and other considerations, I think it highly probable that our domestic
dogs have descended from several wild species. In regard to sheep and
goats I can form no opinion. I should think, from facts communicated
to me by Mr Blyth, on the habits, voice, and constitution, c., of
the humped Indian cattle, that these had descended from a different
aboriginal stock from our European cattle; and several competent
judges believe that these latter have had more than one wild parent.
With respect to horses, from reasons which I cannot give here, I am
doubtfully inclined to believe, in opposition to several authors, that
all the races have descended from one wild stock. Mr Blyth, whose
opinion, from his large and varied stores of knowledge, I should value
more than that of almost any one, thinks that all the breeds of
poultry have proceeded from the common wild
Indian fowl (Gallus
bankiva). In regard to ducks and rabbits, the breeds of which differ
considerably from each other in structure, I do not doubt that they
all have descended from the common wild duck and rabbit.
The doctrine of the origin of our several domestic races from
several aboriginal stocks, has been carried to an absurd extreme by
some authors. They believe that every race which breeds true, let the
distinctive characters be ever so slight, has had its wild prototype.
At this rate there must have existed at least a score of species of
wild cattle, as many sheep, and several goats in Europe alone, and
several even within Great Britain. One author believes that there
formerly existed in Great Britain eleven wild species of sheep
peculiar to it! When we bear in mind that Britain has
now hardly one peculiar mammal, and France but few distinct from those
of Germany and conversely, and so with Hungary, Spain, c., but
that each of these kingdoms possesses several peculiar breeds of
cattle, sheep, c., we must admit that many domestic breeds have
originated in Europe; for whence could they have been derived, as
these several countries do not possess a number of peculiar species as
distinct parent-stocks? So it is in India. Even in the case of the
domestic dogs of the whole world, which I fully admit have probably
descended from several wild species, I cannot doubt that there has
been an immense amount of inherited variation. Who can believe that
animals closely resembling the Italian greyhound, the bloodhound, the
bull-dog, or Blenheim spaniel, c. — so unlike all wild
Canidae — ever existed freely in a state of nature? It has often
been loosely said that all our races of dogs have been produced by the
crossing of a few aboriginal species; but by crossing we can get only
forms in some degree intermediate between their parents; and if we
account for our several domestic races by this process, we must admit
the former existence of the most extreme forms, as the Italian
greyhound, bloodhound, bull-dog, c., in the wild state. Moreover,
the possibility of making distinct races by crossing has been greatly
exaggerated. There can be no doubt that a race may be modified by
occasional crosses, if aided by the careful selection of those
individual mongrels, which present any desired character; but that a
race could be obtained nearly intermediate between two extremely
different races or species, I can hardly
believe. Sir J. Sebright expressly experimentised for this object, and
failed. The offspring from the first cross between two pure breeds is
tolerably and sometimes (as I have found with pigeons) extremely
uniform, and everything seems simple enough; but when these mongrels
are crossed one with another for several generations, hardly two of
them will be alike, and then the extreme difficulty, or rather utter
hopelessness, of the task becomes apparent. Certainly, a breed
intermediate between two very distinct
breeds could not be got without extreme
care and long-continued selection; nor can I find a single case on
record of a permanent race having been thus formed.
On the Breeds of the Domestic pigeon.
Believing that it is always best to study some special
group, I have, after deliberation, taken up domestic pigeons. I have
kept every breed which I could purchase or obtain, and have been most
kindly favoured with skins from several quarters of the world, more
especially by the Hon. W. Elliot from India, and by the Hon. C.
Murray from Persia. Many treatises in different languages have been
published on pigeons, and some of them are very important, as being of
considerably antiquity. I have associated with several eminent
fanciers, and have been permitted to join two
of the London Pigeon
Clubs. The diversity of the breeds is something astonishing. Compare
the English carrier and the short-faced tumbler, and see the wonderful
difference in their beaks, entailing corresponding differences in
their skulls. The carrier, more especially the male bird, is also
remarkable from the wonderful development of the carunculated skin
about the head, and this is accompanied by greatly elongated eyelids,
very large external orifices to the nostrils, and a wide gape of
mouth. The short-faced tumbler has a beak in outline almost like that
of a finch; and the common tumbler has the singular and strictly
inherited habit of flying at a great height in a compact flock, and
tumbling in the air head over heels. The runt is a bird of great size,
with long, massive beak and large feet; some of the sub-breeds of
runts have very long necks, others very long wings and tails, others
singularly short tails. The barb is allied to the carrier, but,
instead of a very long beak, has a very short and very broad one. The
pouter has a much elongated body, wings, and legs; and its enormously
developed crop, which it glories in inflating, may well excite
astonishment and even laughter. The turbit has a very short and
conical beak, with a line of reversed feathers down the breast; and it
has the habit of continually expanding slightly the upper part of the
oesophagus. The Jacobin has the feathers so much reversed along the
back of the neck that they form a hood, and it has, proportionally to
its size, much elongated wing and tail feathers. The trumpeter and
laugher, as their names express, utter a very different coo from the
other breeds. The fantail has thirty or even forty tail-feathers,
instead of twelve or fourteen, the normal number in all members of the
great pigeon family; and these feathers are kept expanded, and are
carried so erect that in good birds the head and tail
touch; the oil-gland is quite aborted. Several other less distinct
breeds might have been specified.
In the skeletons of the several breeds, the development of the
bones of the face in length and breadth and curvature differs
enormously. The shape, as well as the breadth and length of the ramus
of the lower jaw, varies in a highly remarkable manner. The number of
the caudal and sacral vertebrae vary; as does the number of the ribs,
together with their relative breadth and the presence of processes.
The size and shape of the apertures in the sternum are highly
variable; so is the degree of divergence and relative size of the two
arms of the furcula. The proportional width of the gape of mouth, the
proportional length of the eyelids, of the orifice of the nostrils, of
the tongue (not always in strict correlation with the length of beak),
the size of the crop and of the upper part of the oesophagus; the
development and abortion of the oil-gland; the number of the primary
wing and caudal feathers; the relative length of wing and tail to each
other and to the body; the relative length of leg and of the feet; the
number of scutellae on the toes, the development of skin between the
toes, are all points of structure which are variable. The period at
which the perfect plumage is acquired varies, as does the state of the
down with which the nestling birds are clothed when hatched. The shape
and size of the eggs vary. The manner of flight differs remarkably; as
does in some breeds the voice and disposition. Lastly, in certain
breeds, the males and females have come to differ to a slight degree
from each other.
Altogether at least a score of pigeons might be chosen, which if
shown to an ornithologist, and he were told that they were wild birds,
would certainly, I think, be ranked by him as well-defined species.
Moreover, I do not believe that any ornithologist would place
the
English carrier, the short-faced tumbler, the runt, the barb, pouter,
and fantail in the same genus; more especially as in each of these
breeds several truly-inherited sub-breeds, or species as he might have
called them, could be shown him.
Great as the differences are between the breeds of pigeons, I am
fully convinced that the common opinion of naturalists is correct,
namely, that all have descended from the rock-pigeon
(Columba livia), including under this term several geographical races
or sub-species, which differ from each other in the most trifling
respects. As several of the reasons which have led me to this belief
are in some degree applicable in other cases, I will here briefly give
them. If the several breeds are not varieties, and have not proceeded
from the rock-pigeon, they must have descended from at least seven or
eight aboriginal stocks; for it is impossible to make the present
domestic breeds by the crossing of any lesser number: how, for
instance, could a pouter be produced by crossing two breeds unless one
of the parent-stocks possessed the characteristic enormous crop? The
supposed aboriginal stocks must all have been rock-pigeons, that is,
not breeding or willingly perching on trees. But besides C. livia,
with its geographical sub-species, only two or three other species of
rock-pigeons are known; and these have not any of the characters of
the domestic breeds. Hence the supposed aboriginal stocks must either
still exist in the countries where they were originally domesticated,
and yet be unknown to ornithologists; and this, considering their
size, habits, and remarkable characters, seems very improbable; or
they must have become extinct in the wild state. But birds breeding on
precipices, and good fliers, are unlikely to be exterminated; and the
common rock-pigeon, which has the same habits with the domestic
breeds, has not been exterminated
even on several of the smaller
British islets, or on the shores of the Mediterranean. Hence the
supposed extermination of so many species having similar habits with
the rock-pigeon seems to me a very rash assumption. Moreover, the
several above-named domesticated breeds have been transported to all
parts of the world, and, therefore, some of them must have been
carried back again into their native country; but not one has ever
become wild or feral, though the dovecot-pigeon, which is the
rock-pigeon in a very slightly altered state, has become feral in
several places. Again, all recent experience shows that it is most
difficult to get any wild animal to breed freely under domestication;
yet on the hypothesis of the multiple origin of our pigeons, it must
be assumed that at least seven or eight species were so thoroughly
domesticated in ancient times by half-civilized man, as to be quite
prolific under confinement.
An argument, as it seems to me, of great weight, and applicable in
several other cases, is, that the above-specified breeds, though
agreeing generally in constitution, habits, voice, colouring, and in
most parts of their structure, with the wild rock-pigeon, yet are
certainly highly abnormal in other parts of their structure: we may
look in vain throughout the whole great family of Columbidae for a
beak like that of the English carrier, or that of the short-faced
tumbler, or barb; for reversed feathers like those of the jacobin; for
a crop like that of the pouter; for tail-feathers like those of the
fantail. Hence it must be assumed not only that half-civilized man
succeeded in thoroughly domesticating several species, but that he
intentionally or by chance picked out extraordinarily abnormal
species; and further, that these very species have since all become
extinct or unknown. So many strange contingencies seem to me
improbable in the highest degree.
Some facts in regard to the colouring of pigeons well deserve
consideration. The rock-pigeon is of a slaty-blue, and has a white
rump (the Indian sub-species, C. intermedia of Strickland, having it
bluish); the tail has a terminal dark bar, with the bases of the outer
feathers externally edged with white; the wings have two black bars:
some semi-domestic breeds and some apparently truly wild breeds have,
besides the two black bars, the wings chequered with black. These
several marks do not occur together in any other species of the whole
family. Now, in every one of the domestic breeds, taking thoroughly
well-bred birds, all the above marks, even to the white edging of the
outer tail-feathers, sometimes concur perfectly developed. Moreover,
when two birds belonging to two distinct breeds are crossed, neither
of which is blue or has any of the above-specified marks, the mongrel
offspring are very apt suddenly to acquire these characters; for
instance, I crossed some uniformly white fantails with some uniformly
black barbs, and they produced mottled brown and black birds; these I
again crossed together, and one grandchild of the pure white fantail
and pure black barb was of as beautiful a blue colour, with the white
rump, double black wing-bar, and barred and white-edged tail-feathers,
as any wild rock-pigeon! We can understand these facts, on the
well-known principle of reversion to ancestral characters,
if all the domestic breeds have descended from the rock-pigeon. But if
we deny this, we must make one of the two following highly improbable
suppositions. Either, firstly, that all the several imagined
aboriginal stocks were coloured and marked like the rock-pigeon,
although no other existing species is thus coloured and marked, so
that in each separate breed there might be a tendency to revert to the
very same colours and markings. Or, secondly,
that each breed, even
the purest, has within a dozen or, at most, within a score of
generations, been crossed by the rock-pigeon: I say within a dozen or
twenty generations, for we know of no fact countenancing the belief
that the child ever reverts to some one ancestor, removed by a greater
number of generations. In a breed which has been crossed only once
with some distinct breed, the tendency to reversion to any character
derived from such cross will naturally become less and less, as in
each succeeding generation there will be less of the foreign blood;
but when there has been no cross with a distinct breed, and there is a
tendency in both parents to revert to a character, which has been lost
during some former generation, this tendency, for all that we can see
to the contrary, may be transmitted undiminished for an indefinite
number of generations. These two distinct cases are often confounded
in treatises on inheritance.
Lastly, the hybrids or mongrels from between all the domestic
breeds of pigeons are perfectly fertile. I can state this from my own
observations, purposely made on the most distinct breeds. Now, it is
difficult, perhaps impossible, to bring forward one case of the hybrid
offspring of two animals clearly distinct
being themselves perfectly fertile. Some authors believe that
long-continued domestication eliminates this strong tendency to
sterility: from the history of the dog I think there is some
probability in this hypothesis, if applied to species closely related
together, though it is unsupported by a single experiment. But to
extend the hypothesis so far as to suppose that species, aboriginally
as distinct as carriers, tumblers, pouters, and fantails now are,
should yield offspring perfectly fertile, inter
se, seems to me rash in the extreme.
From these several reasons, namely, the improbability of man having formerly got seven or eight supposed species of pigeons
to breed freely under domestication; these supposed species being
quite unknown in a wild state, and their becoming nowhere feral; these
species having very abnormal characters in certain respects, as
compared with all other Columbidae, though so like in most other
respects to the rock-pigeon; the blue colour and various marks
occasionally appearing in all the breeds, both when kept pure and when
crossed; the mongrel offspring being perfectly fertile; — from
these several reasons, taken together, I can feel no doubt that all
our domestic breeds have descended from the Columba livia with its
geographical sub-species.
In favour of this view, I may add, firstly, that C. livia, or the
rock-pigeon, has been found capable of domestication in Europe and in
India; and that it agrees in habits and in a great number of points of
structure with all the domestic breeds. Secondly, although an English
carrier or short-faced tumbler differs immensely in certain characters
from the rock-pigeon, yet by comparing the several sub-breeds of these
breeds, more especially those brought from distant countries, we can
make an almost perfect series between the extremes of structure.
Thirdly, those characters which are mainly distinctive of each breed,
for instance the wattle and length of beak of the carrier, the
shortness of that of the tumbler, and the number of tail-feathers in
the fantail, are in each breed eminently variable; and the explanation
of this fact will be obvious when we come to treat of selection.
Fourthly, pigeons have been watched, and tended with the utmost care,
and loved by many people. They have been domesticated for thousands of
years in several quarters of the world; the earliest known record of
pigeons is in the fifth gyptian dynasty, about 3000 B.C., as was
pointed out to me by Professor Lepsius; but Mr Birch informs me that
pigeons are given in a bill
of fare in the previous dynasty. In the
time of the Romans, as we hear from Pliny, immense prices were given
for pigeons; 'nay, they are come to this pass, that they can reckon up
their pedigree and race.' Pigeons were much valued by Akber Khan in
India, about the year 1600; never less than 20,000 pigeons were taken
with the court. 'The monarchs of Iran and Turan sent him some very
rare birds;' and, continues the courtly historian, 'His
Majesty by crossing the breeds, which method was never practised
before, has improved them astonishingly.' About this same period the
Dutch were as eager about pigeons as were the old Romans. The
paramount importance of these considerations in explaining the immense
amount of variation which pigeons have undergone, will be obvious when
we treat of Selection. We shall then, also, see how it is that the
breeds so often have a somewhat monstrous character. It is also a most
favourable circumstance for the production of distinct breeds, that
male and female pigeons can be easily mated for life; and thus
different breeds can be kept together in the same aviary.
I have discussed the probable origin of domestic pigeons at some,
yet quite insufficient, length; because when I first kept pigeons and
watched the several kinds, knowing well how true they bred, I felt
fully as much difficulty in believing that they could ever have
descended from a common parent, as any naturalist could in coming to a
similar conclusion in regard to the many species of finches, or other
large groups of birds, in nature. One circumstance has struck me much;
namely, that all the breeders of the various domestic animals and the
cultivators of plants, with whom I have ever conversed, or whose
treatises I have read, are firmly convinced that the several breeds to
which each has attended, are descended from so many aboriginally
distinct species.
Ask, as I have asked, a celebrated raiser of
Hereford cattle, whether his cattle might not have descended from long
horns, and he will laugh you to scorn. I have never met a pigeon, or
poultry, or duck, or rabbit fancier, who was not fully convinced that
each main breed was descended from a distinct species. Van Mons, in
his treatise on pears and apples, shows how utterly he disbelieves
that the several sorts, for instance a Ribston-pippin or Codlin-apple,
could ever have proceeded from the seeds of the same tree. Innumerable
other examples could be given. The explanation, I think, is simple:
from long-continued study they are strongly impressed with the
differences between the several races; and though they well know that
each race varies slightly, for they win their prizes by selecting such
slight differences, yet they ignore all general arguments, and refuse
to sum up in their minds slight differences accumulated during many successive generations. May not those naturalists who,
knowing far less of the laws of inheritance than does the breeder, and
knowing no more than he does of the intermediate links in the long
lines of descent, yet admit that many of our domestic races have
descended from the same parents — may they not learn a lesson of
caution, when they deride the idea of species in a state of nature
being lineal descendants of other species?
Selection.
Let us now briefly consider
the steps by which domestic races have been produced, either from one
or from several allied species. Some little effect may, perhaps, be
attributed to the direct action of the external conditions of life,
and some little to habit; but he would be a bold man who would account
by such agencies for the differences of a dray and race horse, a
greyhound and bloodhound, a carrier and tumbler pigeon. One of the
most remarkable features in our domesticated races
is that we see in
them adaptation, not indeed to the animal's or plant's own good, but
to man's use or fancy. Some variations useful to him have probably
arisen suddenly, or by one step; many botanists, for instance, believe
that the fuller's teazle, with its hooks, which cannot be rivalled by
any mechanical contrivance, is only a variety of the wild Dipsacus;
and this amount of change may have suddenly arisen in a seedling. So
it has probably been with the turnspit dog; and this is known to have
been the case with the ancon sheep. But when we compare the
dray-horse and race-horse, the dromedary and camel, the various breeds
of sheep fitted either for cultivated land or mountain pasture, with
the wool of one breed good for one purpose, and that of another breed
for another purpose; when we compare the many breeds of dogs, each
good for man in very different ways; when we compare the gamecock, so
pertinacious in battle, with other breeds so little quarrelsome, with
'everlasting layers' which never desire to sit, and with the bantam so
small and elegant; when we compare the host of agricultural, culinary,
orchard, and flower-garden races of plants, most useful to man at
different seasons and for different purposes, or so beautiful in his
eyes, we must, I think, look further than to mere variability. We
cannot suppose that all the breeds were suddenly produced as perfect
and as useful as we now see them; indeed, in several
cases, we know that this has not been their history. The key is man's
power of accumulative selection: nature gives successive variations;
man adds them up in certain directions useful to him. In this sense he
may be said to make for himself useful breeds.
The great power of this principle of selection is not hypothetical.
It is certain that several of our eminent breeders have, even within a
single lifetime, modified to
a large extent some breeds of cattle and
sheep. In order fully to realise what they have done, it is almost
necessary to read several of the many treatises devoted to this
subject, and to inspect the animals. Breeders habitually speak of an
animal's organisation as something quite plastic, which they can model
almost as they please. If I had space I could quote numerous passages
to this effect from highly competent authorities. Youatt, who was
probably better acquainted with the works of agriculturalists than
almost any other individual, and who was himself a very good judge of
an animal, speaks of the principle of selection as 'that which enables
the agriculturist, not only to modify the character of his flock, but
to change it altogether. It is the magician's wand, by means of which
he may summon into life whatever form and mould he pleases.' Lord
Somerville, speaking of what breeders have done for sheep, says:
-- 'It would seem as if they had chalked out upon a wall a form
perfect in itself, and then had given it existence.' That most skilful
breeder, Sir John Sebright, used to say, with respect to pigeons, that
'he would produce any given feather in three years, but it would take
him six years to obtain head and beak.' In Saxony the importance of
the principle of selection in regard to merino sheep is so fully
recognised, that men follow it as a trade: the sheep are placed on a
table and are studied, like a picture by a connoisseur; this is done
three times at intervals of months, and the sheep are each time marked
and classed, so that the very best may ultimately be selected for
breeding.
What English breeders have actually effected is proved by the
enormous prices given for animals with a good pedigree; and these have
now been exported to almost every quarter of the world. The
improvement is by no means generally due to crossing different breeds;
all the best breeders are strongly opposed to this practice, except
sometimes amongst closely allied sub-breeds. And when a
cross has been made, the closest selection is far more indispensable
even than in ordinary cases. If selection consisted merely in
separating some very distinct variety, and breeding from it, the
principle would be so obvious as hardly to be worth notice; but its
importance consists in the great effect produced by the accumulation
in one direction, during successive generations, of differences
absolutely inappreciable by an uneducated eye — differences
which I for one have vainly attempted to appreciate. Not one man in a
thousand has accuracy of eye and judgement sufficient to become an
eminent breeder. If gifted with these qualities, and he studies his
subject for years, and devotes his lifetime to it with indomitable
perseverance, he will succeed, and may make great improvements; if he
wants any of these qualities, he will assuredly fail. Few would
readily believe in the natural capacity and years of practice
requisite to become even a skilful pigeon-fancier.
The same principles are followed by horticulturists; but the
variations are here often more abrupt. No one supposes that our
choicest productions have been produced by a single variation from the
aboriginal stock. We have proofs that this is not so in some cases, in
which exact records have been kept; thus, to give a very trifling
instance, the steadily-increasing size of the common gooseberry may be
quoted. We see an astonishing improvement in many florists' flowers,
when the flowers of the present day are compared with drawings made
only twenty or thirty years ago. When a race of plants is once pretty
well established, the seed-raisers do not pick out the best plants,
but merely go over their seed-beds, and pull up the 'rogues,' as they
call the plants that deviate from the proper standard. With animals
this
kind of selection is, in fact, also followed; for hardly any one
is so careless as to allow his worst animals to breed.
In regard to plants, there is another means of observing the
accumulated effects of selection — namely, by comparing the
diversity of flowers in the different varieties of the same species in
the flower-garden; the diversity of leaves, pods, or tubers, or
whatever part is valued, in the kitchen-garden, in comparison with the
flowers of the same varieties; and the diversity of fruit of the same
species in the orchard, in comparison with the leaves and
flowers of the same set of varieties. See how different the leaves of
the cabbage are, and how extremely alike the flowers; how unlike the
flowers of the heartsease are, and how alike the leaves; how much the
fruit of the different kinds of gooseberries differ in size, colour,
shape, and hairiness, and yet the flowers present very slight
differences. It is not that the varieties which differ largely in some
one point do not differ at all in other points; this is hardly ever,
perhaps never, the case. The laws of correlation of growth, the
importance of which should never be overlooked, will ensure some
differences; but, as a general rule, I cannot doubt that the continued
selection of slight variations, either in the leaves, the flowers, or
the fruit, will produce races differing from each other chiefly in
these characters.
It may be objected that the principle of selection has been reduced
to methodical practice for scarcely more than three-quarters of a
century; it has certainly been more attended to of late years, and
many treatises have been published on the subject; and the result, I
may add, has been, in a corresponding degree, rapid and important. But
it is very far from true that the principle is a modern discovery. I
could give several references to the full acknowledgement of the
importance of the principle in works of high antiquity. In rude and
barbarous periods of English history choice animals were often
imported, and laws were passed to prevent their exportation: the
destruction of horses under a certain size was ordered, and this may
be compared to the 'roguing' of plants by nurserymen. The principle
of selection I find distinctly given in an ancient Chinese
encyclopaedia. Explicit rules are laid down by some of the Roman
classical writers. From passages in Genesis, it is clear that the
colour of domestic animals was at that early period attended to.
Savages now sometimes cross their dogs with wild canine animals, to
improve the breed, and they formerly did so, as is attested by
passages in Pliny. The savages in South Africa match their draught
cattle by colour, as do some of the Esquimaux their teams of dogs.
Livingstone shows how much good domestic breeds are valued by the
negroes of the interior of Africa who have not associated with
Europeans. Some of these facts do not show actual selection, but they
show that the breeding of domestic animals was carefully
attended to in ancient times, and is now attended to by the lowest
savages. It would, indeed, have been a strange fact, had attention not
been paid to breeding, for the inheritance of good and bad qualities
is so obvious.
At the present time, eminent breeders try by methodical selection,
with a distinct object in view, to make a new strain or sub-breed,
superior to anything existing in the country. But, for our purpose, a
kind of Selection, which may be called Unconscious, and which results
from every one trying to possess and breed from the best individual
animals, is more important. Thus, a man who intends keeping pointers
naturally tries to get as good dogs as he can, and afterwards breeds
from his own best dogs, but he has no wish or expectation of
permanently altering the breed. Nevertheless I cannot
doubt that this
process, continued during centuries, would improve and modify any
breed, in the same way as Bakewell, Collins, c., by this very
same process, only carried on more methodically, did greatly modify,
even during their own lifetimes, the forms and qualities of their
cattle. Slow and insensible changes of this kind could never be
recognised unless actual measurements or careful drawings of the
breeds in question had been made long ago, which might serve for
comparison. In some cases, however, unchanged or but little changed
individuals of the same breed may be found in less civilised
districts, where the breed has been less improved. There is reason to
believe that King Charles's spaniel has been unconsciously modified to
a large extent since the time of that monarch. Some highly competent
authorities are convinced that the setter is directly derived from the
spaniel, and has probably been slowly altered from it. It is known
that the English pointer has been greatly changed within the last
century, and in this case the change has, it is believed, been chiefly
effected by crosses with the fox-hound; but what concerns us is, that
the change has been effected unconsciously and gradually, and yet so
effectually, that, though the old Spanish pointer certainly came from
Spain, Mr Barrow has not seen, as I am informed by him, any native dog
in Spain like our pointer.
By a similar process of selection, and by careful training, the
whole body of English racehorses have come to surpass in
fleetness and size the parent Arab stock, so that the latter, by the
regulations for the Goodwood Races, are favoured in the weights they
carry. Lord Spencer and others have shown how the cattle of England
have increased in weight and in early maturity, compared with the
stock formerly kept in this country. By comparing the accounts given
in old pigeon treatises of carriers
and tumblers with these breeds as
now existing in Britain, India, and Persia, we can, I think, clearly
trace the stages through which they have insensibly passed, and come
to differ so greatly from the rock-pigeon.
Youatt gives an excellent illustration of the effects of a course
of selection, which may be considered as unconsciously followed, in so
far that the breeders could never have expected or even have wished to
have produced the result which ensued — namely, the production
of two distinct strains. The two flocks of Leicester sheep kept by Mr
Buckley and Mr Burgess, as Mr Youatt remarks, 'have been purely bred
from the original stock of Mr Bakewell for upwards of fifty years.
There is not a suspicion existing in the mind of any one at all
acquainted with the subject that the owner of either of them has
deviated in any one instance from the pure blood of Mr Bakewell's
flock, and yet the difference between the sheep possessed by these two
gentlemen is so great that they have the appearance of being quite
different varieties.'
If there exist savages so barbarous as never to think of the
inherited character of the offspring of their domestic animals, yet
any one animal particularly useful to them, for any special purpose,
would be carefully preserved during famines and other accidents, to
which savages are so liable, and such choice animals would thus
generally leave more offspring than the inferior ones; so that in this
case there would be a kind of unconscious selection going on. We see
the value set on animals even by the barbarians of Tierra del Fuego,
by their killing and devouring their old women, in times of dearth, as
of less value than their dogs.
In plants the same gradual process of improvement, through the
occasional preservation of the best individuals, whether or not
sufficiently distinct to be ranked
at their first appearance as distinct varieties, and whether or not two or more species or
races have become blended together by crossing, may plainly be
recognised in the increased size and beauty which we now see in the
varieties of the heartsease, rose, pelargonium, dahlia, and other
plants, when compared with the older varieties or with their
parent-stocks. No one would ever expect to get a first-rate heartsease
or dahlia from the seed of a wild plant. No one would expect to raise
a first-rate melting pear from the seed of a wild pear, though he
might succeed from a poor seedling growing wild, if it had come from a
garden-stock. The pear, though cultivated in classical times, appears,
from Pliny's description, to have been a fruit of very inferior
quality. I have seen great surprise expressed in horticultural works
at the wonderful skill of gardeners, in having produced such splendid
results from such poor materials; but the art, I cannot doubt, has
been simple, and, as far as the final result is concerned, has been
followed almost unconsciously. It has consisted in always cultivating
the best known variety, sowing its seeds, and, when a slightly better
variety has chanced to appear, selecting
it, and so onwards. But the gardeners of the classical period, who
cultivated the best pear they could procure, never thought what
splendid fruit we should eat; though we owe our excellent fruit, in
some small degree, to their having naturally chosen and preserved the
best varieties they could anywhere find.
A large amount of change in our cultivated plants, thus slowly and
unconsciously accumulated, explains, as I believe, the well-known
fact, that in a vast number of cases we cannot recognise, and
therefore do not know, the wild parent-stocks of the plants which have
been longest cultivated in our flower and kitchen gardens. If it has
taken centuries or thousands of years to improve
or modify most of our
plants up to their present standard of usefulness to man, we can
understand how it is that neither Australia, the Cape of Good Hope,
nor any other region inhabited by quite uncivilised man, has afforded
us a single plant worth culture. It is not that these countries, so
rich in species, do not by a strange chance possess the aboriginal
stocks of any useful plants, but that the native plants have not been
improved by continued selection up to a standard of perfection comparable with that given to the plants in countries
anciently civilised.
In regard to the domestic animals kept by uncivilised man, it
should not be overlooked that they almost always have to struggle for
their own food, at least during certain seasons. And in two countries
very differently circumstanced, individuals of the same species,
having slightly different constitutions or structure, would often
succeed better in the one country than in the other, and thus by a
process of 'natural selection,' as will hereafter be more fully
explained, two sub-breeds might be formed. This, perhaps, partly
explains what has been remarked by some authors, namely, that the
varieties kept by savages have more of the character of species than
the varieties kept in civilised countries.
On the view here given of the all-important part which selection by
man has played, it becomes at once obvious, how it is that our
domestic races show adaptation in their structure or in their habits
to man's wants or fancies. We can, I think, further understand the
frequently abnormal character of our domestic races, and likewise
their differences being so great in external characters and relatively
so slight in internal parts or organs. Man can hardly select, or only
with much difficulty, any deviation of structure excepting such as is
externally visible; and indeed he rarely cares for what is internal.
He can never act by selection, excepting on variations
which are first
given to him in some slight degree by nature. No man would ever try to
make a fantail, till he saw a pigeon with a tail developed in some
slight degree in an unusual manner, or a pouter till he saw a pigeon
with a crop of somewhat unusual size; and the more abnormal or unusual
any character was when it first appeared, the more likely it would be
to catch his attention. But to use such an expression as trying to
make a fantail, is, I have no doubt, in most cases, utterly incorrect.
The man who first selected a pigeon with a slightly larger tail,
never dreamed what the descendants of that pigeon would become through
long-continued, partly unconscious and partly methodical selection.
Perhaps the parent bird of all fantails had only fourteen
tail-feathers somewhat expanded, like the present Java fantail, or
like individuals of other and distinct breeds, in which as
many as seventeen tail-feathers have been counted. Perhaps the first
pouter-pigeon did not inflate its crop much more than the turbit now
does the upper part of its oesophagus, — a habit which is
disregarded by all fanciers, as it is not one of the points of the
breed.
Nor let it be thought that some great deviation of structure would
be necessary to catch the fancier's eye: he perceives extremely small
differences, and it is in human nature to value any novelty, however
slight, in one's own possession. Nor must the value which would
formerly be set on any slight differences in the individuals of the
same species, be judged of by the value which would now be set on
them, after several breeds have once fairly been established. Many
slight differences might, and indeed do now, arise amongst pigeons,
which are rejected as faults or deviations from the standard of
perfection of each breed. The common goose has not given rise to any
marked varieties; hence the Thoulouse and the common breed, which
differ only in colour, that most fleeting of characters, have lately
been exhibited as distinct at our poultry-shows.
I think these views further explain what has sometimes been noticed
-- namely that we know nothing about the origin or history of any
of our domestic breeds. But, in fact, a breed, like a dialect of a
language, can hardly be said to have had a definite origin. A man
preserves and breeds from an individual with some slight deviation of
structure, or takes more care than usual in matching his best animals
and thus improves them, and the improved individuals slowly spread in
the immediate neighbourhood. But as yet they will hardly have a
distinct name, and from being only slightly valued, their history will
be disregarded. When further improved by the same slow and gradual
process, they will spread more widely, and will get recognised as
something distinct and valuable, and will then probably first receive
a provincial name. In semi-civilised countries, with little free
communication, the spreading and knowledge of any new sub-breed will
be a slow process. As soon as the points of value of the new sub-breed
are once fully acknowledged, the principle, as I have called it, of
unconscious selection will always tend, — perhaps more at one
period than at another, as the breed rises or falls in
fashion, — perhaps more in one district than in another,
according to the state of civilisation of the inhabitants --
slowly to add to the characteristic features of the breed, whatever
they may be. But the chance will be infinitely small of any record
having been preserved of such slow, varying, and insensible changes.
I must now say a few words on the circumstances, favourable, or the
reverse, to man's power of selection. A high degree of variability is
obviously favourable, as freely giving the materials for selection to
work on; not that mere individual differences are not amply
sufficient, with extreme care, to allow of the accumulation of a large
amount of modification in almost any desired direction. But as
variations manifestly useful or pleasing to man appear only
occasionally, the chance of their appearance will be much increased by
a large number of individuals being kept; and hence this comes to be
of the highest importance to success. On this principle Marshall has
remarked, with respect to the sheep of parts of Yorkshire, that 'as
they generally belong to poor people, and are mostly in small lots, they never can be improved.' On
the other hand, nurserymen, from raising large stocks of the same
plants, are generally far more successful than amateurs in getting new
and valuable varieties. The keeping of a large number of individuals
of a species in any country requires that the species should be placed
under favourable conditions of life, so as to breed freely in that
country. When the individuals of any species are scanty, all the
individuals, whatever their quality may be, will generally be allowed
to breed, and this will effectually prevent selection. But probably
the most important point of all, is, that the animal or plant should
be so highly useful to man, or so much valued by him, that the closest
attention should be paid to even the slightest deviation in the
qualities or structure of each individual. Unless such attention be
paid nothing can be effected. I have seen it gravely remarked, that it
was most fortunate that the strawberry began to vary just when
gardeners began to attend closely to this plant. No doubt the
strawberry had always varied since it was cultivated, but the slight
varieties had been neglected. As soon, however, as gardeners picked
out individual plants with slightly larger, earlier, or
better fruit, and raised seedlings from them, and again picked out the
best seedlings and bred from them, then, there appeared (aided by some
crossing with distinct species) those many admirable varieties of the
strawberry which have been raised during the last thirty or forty
years.
In the case of animals with separate sexes, facility in preventing
crosses is an important element of success in the formation of new
races, — at least, in a country which is already stocked with
other races. In this respect enclosure of the land plays a part.
Wandering savages or the inhabitants of open plains rarely possess
more than one breed of the same species. Pigeons can be mated for
life, and this is a great convenience to the fancier, for thus many
races may be kept true, though mingled in the same aviary; and this
circumstance must have largely favoured the improvement and formation
of new breeds. Pigeons, I may add, can be propagated in great numbers
and at a very quick rate, and inferior birds may be freely rejected,
as when killed they serve for food. On the other hand, cats, from
their nocturnal rambling habits, cannot be matched, and, although so
much valued by women and children, we hardly ever see a distinct breed
kept up; such breeds as we do sometimes see are almost always imported
from some other country, often from islands. Although I do not doubt
that some domestic animals vary less than others, yet the rarity or
absence of distinct breeds of the cat, the donkey, peacock, goose,
c., may be attributed in main part to selection not having been
brought into play: in cats, from the difficulty in pairing them; in
donkeys, from only a few being kept by poor people, and little
attention paid to their breeding; in peacocks, from not being very
easily reared and a large stock not kept; in geese, from being
valuable only for two purposes, food and feathers, and more especially
from no pleasure having been felt in the display of distinct breeds.
To sum up on the origin of our Domestic Races of animals and
plants. I believe that the conditions of life, from their action on
the reproductive system, are so far of the highest importance as
causing variability. I do not believe that variability is an inherent and necessary contingency, under all circumstances,
with all organic beings, as some authors have thought. The effects of
variability are modified by various degrees of inheritance and of
reversion. Variability is governed by many unknown laws, more
especially by that of correlation of growth. Something may be
attributed to the direct action of the conditions of life. Something
must be attributed to use and disuse. The final result is thus
rendered infinitely complex. In some cases, I do not doubt that the
intercrossing of species, aboriginally distinct, has played an
important part in the origin of our domestic productions. When in any
country several domestic breeds have once been established, their
occasional intercrossing, with the aid of selection, has, no doubt,
largely aided in the formation of new sub-breeds; but the importance
of the crossing of varieties has, I believe, been greatly exaggerated,
both in regard to animals and to those plants which are propagated by
seed. In plants which are temporarily propagated by cuttings, buds,
c., the importance of the crossing both of distinct species and
of varieties is immense; for the cultivator here quite disregards the
extreme variability both of hybrids and mongrels, and the frequent
sterility of hybrids; but the cases of plants not propagated by seed
are of little importance to us, for their endurance is only temporary.
Over all these causes of Change I am convinced that the accumulative
action of Selection, whether applied methodically and more quickly, or
unconsciously and more slowly, but more efficiently, is by far the
predominant power.
VARIATION UNDER NATURE
- Variability
- Individual differences
- Doubtful species
- Wide ranging, much diffused, and common species vary most
- Species of the larger genera in any country vary more than the
species of the smaller genera
- Many of the species of the larger
genera resemble varieties in being very closely, but unequally,
related to each other, and in having restricted ranges
BEFORE applying the principles arrived at in the last chapter to organic
beings in a state of nature, we must briefly discuss whether these
latter are subject to any variation. To treat this subject at all
properly, a long catalogue of dry facts should be given; but these I
shall reserve for my future work. Nor shall I here discuss the various
definitions which have been given of the term species. No one
definition has as yet satisfied all naturalists; yet every naturalist
knows vaguely what he means when he speaks of a species. Generally the
term includes the unknown element of a distinct act of creation. The
term 'variety' is almost equally difficult to define; but here
community of descent is almost universally implied, though it can
rarely be proved. We have also what are called monstrosities; but they
graduate into varieties. By a monstrosity I presume is meant some
considerable deviation of structure in one part, either injurious to
or not useful to the species, and not generally propagated. Some
authors use the term 'variation' in a technical sense, as implying a
modification directly due to the physical conditions of life; and
'variations' in this sense are supposed not to be inherited: but who
can say that the dwarfed condition of shells in the brackish waters of
the Baltic, or dwarfed
plants on Alpine summits, or the thicker fur of
an animal from far northwards, would not in some cases be inherited
for at least some few generations? and in this case I presume that the
form would be called a variety.
Again, we have many slight differences which may be called individual differences, such as are known frequently to
appear in the offspring from the same parents, or which may be
presumed to have thus arisen, from being frequently observed in the
individuals of the same species inhabiting the same confined locality.
No one supposes that all the individuals of the same species are cast
in the very same mould. These individual differences are highly
important for us, as they afford materials for natural selection to
accumulate, in the same manner as man can accumulate in any given
direction individual differences in his domesticated productions.
These individual differences generally affect what naturalists
consider unimportant parts; but I could show by a long catalogue of
facts, that parts which must be called important, whether viewed under
a physiological or classificatory point of view, sometimes vary in the
individuals of the same species. I am convinced that the most
experienced naturalist would be surprised at the number of the cases
of variability, even in important parts of structure, which he could
collect on good authority, as I have collected, during a course of
years. It should be remembered that systematists are far from pleased
at finding variability in important characters, and that there are not
many men who will laboriously examine internal and important organs,
and compare them in many specimens of the same species. I should never
have expected that the branching of the main nerves close to the great
central ganglion of an insect would have been variable in the same
species; I should have expected that changes of this nature could have
been effected only
by slow degrees: yet quite recently Mr Lubbock has
shown a degree of variability in these main nerves in Coccus, which
may almost be compared to the irregular branching of the stem of a
tree. This philosophical naturalist, I may add, has also quite
recently shown that the muscles in the larvae of certain insects are
very far from uniform. Authors sometimes argue in a circle when they
state that important organs never vary; for these same authors
practically rank that character as important (as some few naturalists
have honestly confessed) which does not vary; and, under this point of
view, no instance of any important part varying will ever be found:
but under any other point of view many instances assuredly can be
given.
There is one point connected with individual differences, which
seems to me extremely perplexing: I refer to those genera which have
sometimes been called 'protean' or 'polymorphic,' in which the species
present an inordinate amount of variation; and hardly two naturalists
can agree which forms to rank as species and which as varieties. We
may instance Rubus, Rosa, and Hieracium amongst plants, several genera
of insects, and several genera of Brachiopod shells. In most
polymorphic genera some of the species have fixed and definite
characters. Genera which are polymorphic in one country seem to be,
with some few exceptions, polymorphic in other countries, and
likewise, judging from Brachiopod shells, at former periods of time.
These facts seem to be very perplexing, for they seem to show that
this kind of variability is independent of the conditions of life. I
am inclined to suspect that we see in these polymorphic genera
variations in points of structure which are of no service or
disservice to the species, and which consequently have not been seized
on and rendered definite by natural selection, as hereafter will be
explained.
Those forms which possess in some considerable degree the character
of species, but which are so closely similar to some other forms, or
are so closely linked to them by intermediate gradations, that
naturalists do not like to rank them as distinct species, are in
several respects the most important for us. We have every reason to
believe that many of these doubtful and closely-allied forms have
permanently retained their characters in their own country for a long
time; for as long, as far as we know, as have good and true species.
practically, when a naturalist can unite two forms together by others
having intermediate characters, he treats the one as a variety of the
other, ranking the most common, but sometimes the one first described,
as the species, and the other as the variety. But cases of great
difficulty, which I will not here enumerate, sometimes occur in
deciding whether or not to rank one form as a variety of another, even
when they are closely connected by intermediate links; nor will the
commonly-assumed hybrid nature of the intermediate links always remove
the difficulty. In very many cases, however, one form is ranked as a
variety of another, not because the intermediate links
have actually been found, but because analogy leads the observer to
suppose either that they do now somewhere exist, or may formerly have
existed; and here a wide door for the entry of doubt and conjecture is
opened.
Hence, in determining whether a form should be ranked as a species
or a variety, the opinion of naturalists having sound judgement and
wide experience seems the only guide to follow. We must, however, in
many cases, decide by a majority of naturalists, for few well-marked
and well-known varieties can be named which have not been ranked as
species by at least some competent judges.
That varieties of this doubtful nature are far from uncommon cannot
be disputed. Compare the several floras of Great Britain, of France or
of the United States, drawn up by different botanists, and see what a
surprising number of forms have been ranked by one botanist as good
species, and by another as mere varieties. Mr H. C. Watson, to whom I
lie under deep obligation for assistance of all kinds, has marked for
me 182 British plants, which are generally considered as varieties,
but which have all been ranked by botanists as species; and in making
this list he has omitted many trifling varieties, but which
nevertheless have been ranked by some botanists as species, and he has
entirely omitted several highly polymorphic genera. Under genera,
including the most polymorphic forms, Mr Babington gives 251 species,
whereas Mr Bentham gives only 112, — a difference of 139
doubtful forms! Amongst animals which unite for each birth, and which
are highly locomotive, doubtful forms, ranked by one zoologist as a
species and by another as a variety, can rarely be found within the
same country, but are common in separated areas. How many of those
birds and insects in North America and Europe, which differ very
slightly from each other, have been ranked by one eminent naturalist
as undoubted species, and by another as varieties, or, as they are
often called, as geographical races! Many years ago, when comparing,
and seeing others compare, the birds from the separate islands of the
Galapagos Archipelago, both one with another, and with those from the
American mainland, I was much struck how entirely vague and arbitrary
is the distinction between species and varieties. On the
islets of the little Madeira group there are many insects which are
characterized as varieties in Mr Wollaston's admirable work, but which
it cannot
be doubted would be ranked as distinct species by many
entomologists. Even Ireland has a few animals, now generally regarded
as varieties, but which have been ranked as species by some
zoologists. Several most experienced ornithologists consider our
British red grouse as only a strongly-marked race of a Norwegian
species, whereas the greater number rank it as an undoubted species
peculiar to Great Britain. A wide distance between the homes of two
doubtful forms leads many naturalists to rank both as distinct
species; but what distance, it has been well asked, will suffice? if
that between America and Europe is ample, will that between the
Continent and the Azores, or Madeira, or the Canaries, or Ireland, be
sufficient? It must be admitted that many forms, considered by
highly-competent judges as varieties, have so perfectly the character
of species that they are ranked by other highly-competent judges as
good and true species. But to discuss whether they are rightly called
species or varieties, before any definition of these terms has been
generally accepted, is vainly to beat the air.
Many of the cases of strongly-marked varieties or doubtful species
well deserve consideration; for several interesting lines of argument,
from geographical distribution, analogical variation, hybridism,
c., have been brought to bear on the attempt to determine their
rank. I will here give only a single instance, — the well-known
one of the primrose and cowslip, or Primula veris and elatior. These
plants differ considerably in appearance; they have a different
flavour and emit a different odour; they flower at slightly different
periods; they grow in somewhat different stations; they ascend
mountains to different heights; they have different geographical
ranges; and lastly, according to very numerous experiments made during
several years by
that most careful observer Grtner, they can be
crossed only with much difficulty. We could hardly wish for better
evidence of the two forms being specifically distinct. On the other
hand, they are united by many intermediate links, and it is very
doubtful whether these links are hybrids; and there is, as it seems to
me, an overwhelming amount of experimental evidence,
showing that they descend from common parents, and consequently must
be ranked as varieties.
Close investigation, in most cases, will bring naturalists to an
agreement how to rank doubtful forms. Yet it must be confessed, that
it is in the best-known countries that we find the greatest number of
forms of doubtful value. I have been struck with the fact, that if
any animal or plant in a state of nature be highly useful to man, or
from any cause closely attract his attention, varieties of it will
almost universally be found recorded. These varieties, moreover, will
be often ranked by some authors as species. Look at the common oak,
how closely it has been studied; yet a German author makes more than a
dozen species out of forms, which are very generally considered as
varieties; and in this country the highest botanical authorities and
practical men can be quoted to show that the sessile and pedunculated
oaks are either good and distinct species or mere varieties.
When a young naturalist commences the study of a group of organisms
quite unknown to him, he is at first much perplexed to determine what
differences to consider as specific, and what as varieties; for he
knows nothing of the amount and kind of variation to which the group
is subject; and this shows, at least, how very generally there is some
variation. But if he confine his attention to one class within one
country, he will soon make up his mind how to rank most of the
doubtful forms. His
general tendency will be to make many species, for
he will become impressed, just like the pigeon or poultry-fancier
before alluded to, with the amount of difference in the forms which he
is continually studying; and he has little general knowledge of
analogical variation in other groups and in other countries, by which
to correct his first impressions. As he extends the range of his
observations, he will meet with more cases of difficulty; for he will
encounter a greater number of closely-allied forms. But if his
observations be widely extended, he will in the end generally be
enabled to make up his own mind which to call varieties and which
species; but he will succeed in this at the expense of admitting much
variation, — and the truth of this admission will often be
disputed by other naturalists. When, moreover, he comes to
study allied forms brought from countries not now continuous, in which
case he can hardly hope to find the intermediate links between his
doubtful forms, he will have to trust almost entirely to analogy, and
his difficulties will rise to a climax.
Certainly no clear line of demarcation has as yet been drawn
between species and sub-species — that is, the forms which in
the opinion of some naturalists come very near to, but do not quite
arrive at the rank of species; or, again, between sub-species and
well-marked varieties, or between lesser varieties and individual
differences. These differences blend into each other in an insensible
series; and a series impresses the mind with the idea of an actual
passage.
Hence I look at individual differences, though of small interest to
the systematist, as of high importance for us, as being the first step
towards such slight varieties as are barely thought worth recording in
works on natural history. And I look at varieties which are in any
degree more distinct and permanent, as steps leading to more
strongly
marked and more permanent varieties; and at these latter, as leading
to sub-species, and to species. The passage from one stage of
difference to another and higher stage may be, in some cases, due
merely to the long-continued action of different physical conditions
in two different regions; but I have not much faith in this view; and
I attribute the passage of a variety, from a state in which it differs
very slightly from its parent to one in which it differs more, to the
action of natural selection in accumulating (as will hereafter be more
fully explained) differences of structure in certain definite
directions. Hence I believe a well-marked variety may be justly called
an incipient species; but whether this belief be justifiable must be
judged of by the general weight of the several facts and views given
throughout this work.
It need not be supposed that all varieties or incipient species
necessarily attain the rank of species. They may whilst in this
incipient state become extinct, or they may endure as varieties for
very long periods, as has been shown to be the case by Mr Wollaston
with the varieties of certain fossil land-shells in Madeira. If a
variety were to flourish so as to exceed in numbers the
parent species, it would then rank as the species, and the species as
the variety; or it might come to supplant and exterminate the parent
species; or both might co-exist, and both rank as independent species.
But we shall hereafter have to return to this subject.
From these remarks it will be seen that I look at the term species,
as one arbitrarily given for the sake of convenience to a set of
individuals closely resembling each other, and that it does not
essentially differ from the term variety, which is given to less
distinct and more fluctuating forms. The term variety, again, in
comparison with mere individual differences, is also applied
arbitrarily, and for mere convenience sake.
Guided by theoretical considerations, I thought that some
interesting results might be obtained in regard to the nature and
relations of the species which vary most, by tabulating all the
varieties in several well-worked floras. At first this seemed a
simple task; but Mr H. C. Watson, to whom I am much indebted for
valuable advice and assistance on this subject, soon convinced me that
there were many difficulties, as did subsequently Dr Hooker, even in
stronger terms. I shall reserve for my future work the discussion of
these difficulties, and the tables themselves of the proportional
numbers of the varying species. Dr Hooker permits me to add, that
after having carefully read my manuscript, and examined the tables, he
thinks that the following statements are fairly well established. The
whole subject, however, treated as it necessarily here is with much
brevity, is rather perplexing, and allusions cannot be avoided to the
'struggle for existence,' 'divergence of character,' and other
questions, hereafter to be discussed.
Alph. De Candolle and others have shown that plants which have very
wide ranges generally present varieties; and this might have been
expected, as they become exposed to diverse physical conditions, and
as they come into competition (which, as we shall hereafter see, is a
far more important circumstance) with different sets of organic
beings. But my tables further show that, in any limited country, the
species which are most common, that is abound most in individuals, and
the species which are most widely diffused within their own country
(and this is a different consideration from wide range,
and to a certain extent from commonness), often give rise to varieties
sufficiently well-marked to have been recorded in botanical works.
Hence it is the most flourishing, or, as they may be called, the
dominant species, —
those which range widely over the world, are
the most diffused in their own country, and are the most numerous in
individuals, — which oftenest produce well-marked varieties, or,
as I consider them, incipient species. And this, perhaps, might have
been anticipated; for, as varieties, in order to become in any degree
permanent, necessarily have to struggle with the other inhabitants of
the country, the species which are already dominant will be the most
likely to yield offspring which, though in some slight degree
modified, will still inherit those advantages that enabled their
parents to become dominant over their compatriots.
If the plants inhabiting a country and described in any Flora be
divided into two equal masses, all those in the larger genera being
placed on one side, and all those in the smaller genera on the other
side, a somewhat larger number of the very common and much diffused or
dominant species will be found on the side of the larger genera. This,
again, might have been anticipated; for the mere fact of many species
of the same genus inhabiting any country, shows that there is
something in the organic or inorganic conditions of that country
favourable to the genus; and, consequently, we might have expected to
have found in the larger genera, or those including many species, a
large proportional number of dominant species. But so many causes
tend to obscure this result, that I am surprised that my tables show
even a small majority on the side of the larger genera. I will here
allude to only two causes of obscurity. Fresh-water and salt-loving
plants have generally very wide ranges and are much diffused, but this
seems to be connected with the nature of the stations inhabited by
them, and has little or no relation to the size of the genera to which
the species belong. Again, plants low in the scale of organisation are
generally much more widely diffused than plants higher in the scale;
and here again there is no close relation to the size of the genera.
The cause of lowly-organised plants ranging widely will be discussed
in our chapter on geographical distribution.
From looking at species as only strongly-marked and well-defined
varieties, I was led to anticipate that the species of the larger
genera in each country would oftener present varieties, than the
species of the smaller genera; for wherever many closely related
species (i.e. species of the same genus)
have been formed, many varieties or incipient species ought, as a
general rule, to be now forming. Where many large trees grow, we
expect to find saplings. Where many species of a genus have been
formed through variation, circumstances have been favourable for
variation; and hence we might expect that the circumstances would
generally be still favourable to variation. On the other hand, if we
look at each species as a special act of creation, there is no
apparent reason why more varieties should occur in a group having many
species, than in one having few.
To test the truth of this anticipation I have arranged the plants
of twelve countries, and the coleopterous insects of two districts,
into two nearly equal masses, the species of the larger genera on one
side, and those of the smaller genera on the other side, and it has
invariably proved to be the case that a larger proportion of the
species on the side of the larger genera present varieties, than on
the side of the smaller genera. Moreover, the species of the large
genera which present any varieties, invariably present a larger
average number of varieties than do the species of the small genera.
Both these results follow when another division is made, and when all
the smallest genera, with from only one to four species, are
absolutely excluded from the tables. These
facts are of plain
signification on the view that species are only strongly marked and
permanent varieties; for whenever many species of the same genus have
been formed, or where, if we may use the expression, the manufactory
of species has been active, we ought generally to find the manufactory
still in action, more especially as we have every reason to believe
the process of manufacturing new species to be a slow one. And this
certainly is the case, if varieties be looked at as incipient species;
for my tables clearly show as a general rule that, wherever many
species of a genus have been formed, the species of that genus present
a number of varieties, that is of incipient species, beyond the
average. It is not that all large genera are now varying
much, and are thus increasing in the number of their species, or that
no small genera are now varying and increasing; for if this had been
so, it would have been fatal to my theory; inasmuch as geology plainly
tells us that small genera have in the lapse of time often increased
greatly in size; and that large genera have often come to their
maxima, declined, and disappeared. All that we want to show is, that
where many species of a genus have been formed, on an average many are
still forming; and this holds good.
There are other relations between the species of large genera and
their recorded varieties which deserve notice. We have seen that there
is no infallible criterion by which to distinguish species and
well-marked varieties; and in those cases in which intermediate links
have not been found between doubtful forms, naturalists are compelled
to come to a determination by the amount of difference between them,
judging by analogy whether or not the amount suffices to raise one or
both to the rank of species. Hence the amount of difference is one
very important criterion in settling whether two forms
should be
ranked as species or varieties. Now Fries has remarked in regard to
plants, and Westwood in regard to insects, that in large genera the
amount of difference between the species is often exceedingly small. I
have endeavoured to test this numerically by averages, and, as far as
my imperfect results go, they always confirm the view. I have also
consulted some sagacious and most experienced observers, and, after
deliberation, they concur in this view. In this respect, therefore,
the species of the larger genera resemble varieties, more than do the
species of the smaller genera. Or the case may be put in another way,
and it may be said, that in the larger genera, in which a number of
varieties or incipient species greater than the average are now
manufacturing, many of the species already manufactured still to a
certain extent resemble varieties, for they differ from each other by
a less than usual amount of difference.
Moreover, the species of the large genera are related to each
other, in the same manner as the varieties of any one species are
related to each other. No naturalist pretends that all the species of
a genus are equally distinct from each other; they may
generally be divided into sub-genera, or sections, or lesser groups.
As Fries has well remarked, little groups of species are generally
clustered like satellites around certain other species. And what are
varieties but groups of forms, unequally related to each other, and
clustered round certain forms — that is, round their
parent-species? Undoubtedly there is one most important point of
difference between varieties and species; namely, that the amount of
difference between varieties, when compared with each other or with
their parent-species, is much less than that between the species of
the same genus. But when we come to discuss the principle, as I call
it, of Divergence of Character, we shall see how this may be
explained, and how the lesser differences between varieties will tend
to increase into the greater differences between species.
There is one other point which seems to me worth notice. Varieties
generally have much restricted ranges: this statement is indeed
scarcely more than a truism, for if a variety were found to have a
wider range than that of its supposed parent-species, their
denominations ought to be reversed. But there is also reason to
believe, that those species which are very closely allied to other
species, and in so far resemble varieties, often have much restricted
ranges. For instance, Mr H. C. Watson has marked for me in the
well-sifted London Catalogue of plants (4th edition) 63 plants which
are therein ranked as species, but which he considers as so closely
allied to other species as to be of doubtful value: these 63 reputed
species range on an average over 6.9 of the provinces into which Mr
Watson has divided Great Britain. Now, in this same catalogue, 53
acknowledged varieties are recorded, and these range over 7.7
provinces; whereas, the species to which these varieties belong range
over 14.3 provinces. So that the acknowledged varieties have very
nearly the same restricted average range, as have those very closely
allied forms, marked for me by Mr Watson as doubtful species, but
which are almost universally ranked by British botanists as good and
true species.
Finally, then, varieties have the same general characters as
species, for they cannot be distinguished from species, --
except, firstly, by the discovery of intermediate linking forms, and
the occurrence of such links cannot affect the actual
characters of the forms which they connect; and except, secondly, by a
certain amount of difference, for two forms, if differing very little,
are generally ranked as varieties, notwithstanding that intermediate
linking forms have not been discovered; but the amount of difference
considered necessary to give to two forms the rank of species is quite
indefinite. In genera having more than the average number of species
in any country, the species of these genera have more than the average
number of varieties. In large genera the species are apt to be
closely, but unequally, allied together, forming little clusters round
certain species. Species very closely allied to other species
apparently have restricted ranges. In all these several respects the
species of large genera present a strong analogy with varieties. And
we can clearly understand these analogies, if species have once
existed as varieties, and have thus originated: whereas, these
analogies are utterly inexplicable if each species has been
independently created.
We have, also, seen that it is the most flourishing and dominant
species of the larger genera which on an average vary most; and
varieties, as we shall hereafter see, tend to become converted into
new and distinct species. The larger genera thus tend to become
larger; and throughout nature the forms of life which are now dominant
tend to become still more dominant by leaving many modified and
dominant descendants. But by steps hereafter to be explained, the
larger genera also tend to break up into smaller genera. And thus, the
forms of life throughout the universe become divided into groups
subordinate to groups.
STRUGGLE FOR EXISTENCE
- Bears on natural selection
- The term used in a wide sense
- Geometrical powers of increase
- Rapid increase of naturalised animals and plants
- Nature of the checks to increase
- Competition universal
- Effects of climate
- Protection from the number of individuals
- Complex relations of all animals and plants throughout nature
- Struggle for life most severe between
individuals and varieties of the same species; often severe between
species of the same genus
- The relation of organism to organism the most important of all
relations
BEF0RE entering on the
subject of this chapter, I must make a few preliminary remarks, to
show how the struggle for existence bears on Natural Selection. It has
been seen in the last chapter that amongst organic beings in a state
of nature there is some individual variability; indeed I am not aware
that this has ever been disputed. It is immaterial for us whether a
multitude of doubtful forms be called species or sub-species or
varieties; what rank, for instance, the two or three hundred doubtful
forms of British plants are entitled to hold, if the existence of any
well-marked varieties be admitted. But the mere existence of
individual variability and of some few well-marked varieties, though
necessary as the foundation for the work, helps us but little in
understanding how species arise in nature. How have all those
exquisite adaptations of one part of the organisation to another part,
and to the conditions of life, and of one distinct organic being to
another being, been perfected? We see these beautiful co-adaptations
most plainly in the woodpecker and missletoe; and only a little less
plainly in the humblest parasite which clings
to the hairs of a
quadruped or feathers of a bird; in the structure of the beetle which
dives through the water; in the plumed seed which is wafted by the
gentlest breeze; in short, we see beautiful adaptations
everywhere and in every part of the organic world.
Again, it may be asked, how is it that varieties, which I have
called incipient species, become ultimately converted into good and
distinct species, which in most cases obviously differ from each other
far more than do the varieties of the same species? How do those
groups of species, which constitute what are called distinct genera,
and which differ from each other more than do the species of the same
genus, arise? All these results, as we shall more fully see in the
next chapter, follow inevitably from the struggle for life. Owing to
this struggle for life, any variation, however slight and from
whatever cause proceeding, if it be in any degree profitable to an
individual of any species, in its infinitely complex relations to
other organic beings and to external nature, will tend to the
preservation of that individual, and will generally be inherited by
its offspring. The offspring, also, will thus have a better chance of
surviving, for, of the many individuals of any species which are
periodically born, but a small number can survive. I have called this
principle, by which each slight variation, if useful, is preserved, by
the term of Natural Selection, in order to mark its relation to man's
power of selection. We have seen that man by selection can certainly
produce great results, and can adapt organic beings to his own uses,
through the accumulation of slight but useful variations, given to him
by the hand of Nature. But Natural Selection, as we shall hereafter
see, is a power incessantly ready for action, and is as immeasurably
superior to man's feeble efforts, as the works of Nature are to those
of Art.
We will now discuss in a little more detail the struggle for
existence. In my future work this subject shall be treated, as it well
deserves, at much greater length. The elder De Candolle and Lyell have
largely and philosophically shown that all organic beings are exposed
to severe competition. In regard to plants, no one has treated this
subject with more spirit and ability than W. Herbert, Dean of
Manchester, evidently the result of his great horticultural knowledge.
Nothing is easier than to admit in words the truth of the universal
struggle for life, or more difficult — at least I have found it
so — than constantly to bear this conclusion in
mind. Yet unless it be thoroughly engrained in the mind, I am
convinced that the whole economy of nature, with every fact on
distribution, rarity, abundance, extinction, and variation, will be
dimly seen or quite misunderstood. We behold the face of nature bright
with gladness, we often see superabundance of food; we do not see, or
we forget, that the birds which are idly singing round us mostly live
on insects or seeds, and are thus constantly destroying life; or we
forget how largely these songsters, or their eggs, or their nestlings
are destroyed by birds and beasts of prey; we do not always bear in
mind, that though food may be now superabundant, it is not so at all
seasons of each recurring year.
I should premise that I use the term Struggle for Existence in a
large and metaphorical sense, including dependence of one being on
another, and including (which is more important) not only the life of
the individual, but success in leaving progeny. Two canine animals in
a time of dearth, may be truly said to struggle with each other which
shall get food and live. But a plant on the edge of a desert is said
to struggle for life against the drought, though more properly it
should be said to be dependent on the moisture. A plant which annually
produces a thousand seeds, of which on an average only one comes to
maturity, may be more truly said to struggle with the plants of the
same and other kinds which already clothe the ground. The missletoe is
dependent on the apple and a few other trees, but can only in a
far-fetched sense be said to struggle with these trees, for if too
many of these parasites grow on the same tree, it will languish and
die. But several seedling missletoes, growing close together on the
same branch, may more truly be said to struggle with each other. As
the missletoe is disseminated by birds, its existence depends on
birds; and it may metaphorically be said to struggle with other
fruit-bearing plants, in order to tempt birds to devour and thus
disseminate its seeds rather than those of other plants. In these
several senses, which pass into each other, I use for convenience sake
the general term of struggle for existence.
A struggle for existence inevitably follows from the high rate at
which all organic beings tend to increase. Every being, which during its natural lifetime produces several eggs or seeds,
must suffer destruction during some period of its life, and during
some season or occasional year, otherwise, on the principle of
geometrical increase, its numbers would quickly become so inordinately
great that no country could support the product. Hence, as more
individuals are produced than can possibly survive, there must in
every case be a struggle for existence, either one individual with
another of the same species, or with the individuals of distinct
species, or with the physical conditions of life. It is the doctrine
of Malthus applied with manifold force to the whole animal and
vegetable kingdoms; for in this case there can be no artificial
increase of food, and no prudential restraint from marriage. Although
some species may
be now increasing, more or less rapidly, in numbers,
all cannot do so, for the world would not hold them.
There is no exception to the rule that every organic being
naturally increases at so high a rate, that if not destroyed, the
earth would soon be covered by the progeny of a single pair. Even
slow-breeding man has doubled in twenty-five years, and at this rate,
in a few thousand years, there would literally not be standing room
for his progeny. Linnaeus has calculated that if an annual plant
produced only two seeds — and there is no plant so unproductive
as this — and their seedlings next year produced two, and so on,
then in twenty years there would be a million plants. The elephant is
reckoned to be the slowest breeder of all known animals, and I have
taken some pains to estimate its probable minimum rate of natural
increase: it will be under the mark to assume that it breeds when
thirty years old, and goes on breeding till ninety years old, bringing
forth three pairs of young in this interval; if this be so, at the end
of the fifth century there would be alive fifteen million elephants,
descended from the first pair.
But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three
following seasons. Still more striking is the evidence from our
domestic animals of many kinds which have run wild in
several parts of the world: if the statements of the rate of increase
of slow-breeding cattle and horses in South America, and latterly in
Australia, had not been well authenticated, they would have been quite
incredible. So it is with plants: cases could be given of introduced
plants which have become common throughout whole islands in a period
of less than ten years, Several of the plants now most numerous over
the wide plains of La Plata, clothing square leagues of surface almost
to the exclusion of all other plants, have been introduced from
Europe; and there are plants which now range in India, as I hear from
Dr Falconer, from Cape Comorin to the Himalaya, which have been
imported from America since its discovery. In such cases, and endless
instances could be given, no one supposes that the fertility of these
animals or plants has been suddenly and temporarily increased in any
sensible degree. The obvious explanation is that the conditions of
life have been very favourable, and that there has consequently been
less destruction of the old and young, and that nearly all the young
have been enabled to breed. In such cases the geometrical ratio of
increase, the result of which never fails to be surprising, simply
explains the extraordinarily rapid increase and wide diffusion of
naturalised productions in their new homes.
In a state of nature almost every plant produces seed, and amongst
animals there are very few which do not annually pair. Hence we may
confidently assert, that all plants and animals are tending to
increase at a geometrical ratio, that all would most rapidly stock
every station in which they could any how exist, and that the
geometrical tendency to increase must be checked by destruction at
some period of life. Our familiarity with the larger domestic animals
tends, I think, to mislead us: we see no great destruction falling on
them, and we forget that thousands are annually slaughtered for food,
and that in a state of nature an equal number would have somehow to be
disposed of.
The only difference between organisms which annually produce eggs
or seeds by the thousand, and those which produce extremely few, is,
that the slow-breeders would require a few more years to people, under
favourable conditions, a whole district, let it be ever
so large. The condor lays a couple of eggs and the ostrich a score,
and yet in the same country the condor may be the more numerous of the
two: the Fulmar petrel lays but one egg, yet it is believed to be the
most numerous bird in the world. One fly deposits hundreds of eggs,
and another, like the hippobosca, a single one; but this difference
does not determine how many individuals of the two species can be
supported in a district. A large number of eggs is of some importance
to those species, which depend on a rapidly fluctuating amount of
food, for it allows them rapidly to increase in number. But the real
importance of a large number of eggs or seeds is to make up for much
destruction at some period of life; and this period in the great
majority of cases is an early one. If an animal can in any way protect
its own eggs or young, a small number may be produced, and yet the
average stock be fully kept up; but if many eggs or young are
destroyed, many must be produced, or the species will become extinct.
It would suffice to keep up the full number of a tree, which lived on
an average for a thousand years, if a single seed were produced once
in a thousand years, supposing that this seed were never destroyed,
and could be ensured to germinate in a fitting place. So that in all
cases, the average number of any animal or plant depends only
indirectly on the number of its eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind — never to forget that every
single organic being around us may be said to be striving to the
utmost to increase in numbers; that each lives by a struggle at some
period of its life; that heavy destruction inevitably falls either on
the young or old, during each generation or at recurrent intervals.
Lighten any check, mitigate the
destruction ever so little, and the
number of the species will almost instantaneously increase to any
amount. The face of Nature may be compared to a yielding surface, with
ten thousand sharp wedges packed close together and driven inwards by
incessant blows, sometimes one wedge being struck, and then another
with greater force.
What checks the natural tendency of each species to increase in
number is most obscure. Look at the most vigorous species; by as much as it swarms in numbers, by so much will its
tendency to increase be still further increased. We know not exactly
what the checks are in even one single instance. Nor will this
surprise any one who reflects how ignorant we are on this head, even
in regard to mankind, so incomparably better known than any other
animal. This subject has been ably treated by several authors, and I
shall, in my future work, discuss some of the checks at considerable
length, more especially in regard to the feral animals of South
America. Here I will make only a few remarks, just to recall to the
reader's mind some of the chief points. Eggs or very young animals
seem generally to suffer most, but this is not invariably the case.
With plants there is a vast destruction of seeds, but, from some
observations which I have made, I believe that it is the seedlings
which suffer most from germinating in ground already thickly stocked
with other plants. Seedlings, also, are destroyed in vast numbers by
various enemies; for instance, on a piece of ground three feet long
and two wide, dug and cleared, and where there could be no choking
from other plants, I marked all the seedlings of our native weeds as
they came up, and out of the 357 no less than 295 were destroyed,
chiefly by slugs and insects. If turf which has long been mown, and
the case would be the same with turf closely browsed by quadrupeds, be
let to grow,
the more vigorous plants gradually kill the less
vigorous, though fully grown, plants: thus out of twenty species
growing on a little plot of turf (three feet by four) nine species
perished from the other species being allowed to grow up freely.
The amount of food for each species of course gives the extreme
limit to which each can increase; but very frequently it is not the
obtaining food, but the serving as prey to other animals, which
determines the average numbers of a species. Thus, there seems to be
little doubt that the stock of partridges, grouse, and hares on any
large estate depends chiefly on the destruction of vermin. If not one
head of game were shot during the next twenty years in England, and,
at the same time, if no vermin were destroyed, there would, in all
probability, be less game than at present, although hundreds of
thousands of game animals are now annually killed. On the other hand,
in some cases, as with the elephant and rhinoceros, none
are destroyed by beasts of prey: even the tiger in India most rarely
dares to attack a young elephant protected by its dam.
Climate plays an important part in determining the average numbers
of a species, and periodical seasons of extreme cold or drought, I
believe to be the most effective of all checks. I estimated that the
winter of 1854-55 destroyed four-fifths of the birds in my own
grounds; and this is a tremendous destruction, when we remember that
ten per cent. is an extraordinarily severe mortality from epidemics
with man. The action of climate seems at first sight to be quite
independent of the struggle for existence; but in so far as climate
chiefly acts in reducing food, it brings on the most severe struggle
between the individuals, whether of the same or of distinct species,
which subsist on the same kind of food. Even when climate, for
instance extreme
cold, acts directly, it will be the least vigorous,
or those which have got least food through the advancing winter, which
will suffer most. When we travel from south to north, or from a damp
region to a dry, we invariably see some species gradually getting
rarer and rarer, and finally disappearing; and the change of climate
being conspicuous, we are tempted to attribute the whole effect to its
direct action. But this is a very false view: we forget that each
species, even where it most abounds, is constantly suffering enormous
destruction at some period of its life, from enemies or from
competitors for the same place and food; and if these enemies or
competitors be in the least degree favoured by any slight change of
climate, they will increase in numbers, and, as each area is already
fully stocked with inhabitants, the other species will decrease. When
we travel southward and see a species decreasing in numbers, we may
feel sure that the cause lies quite as much in other species being
favoured, as in this one being hurt. So it is when we travel
northward, but in a somewhat lesser degree, for the number of species
of all kinds, and therefore of competitors, decreases northwards;
hence in going northward, or in ascending a mountain, we far oftener
meet with stunted forms, due to the directly
injurious action of climate, than we do in proceeding southwards or in
descending a mountain. When we reach the Arctic regions, or
snow-capped summits, or absolute deserts, the struggle
for life is almost exclusively with the elements.
That climate acts in main part indirectly by favouring other
species, we may clearly see in the prodigious number of plants in our
gardens which can perfectly well endure our climate, but which never
become naturalised, for they cannot compete with our native plants,
nor resist destruction by our native animals.
When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics — at least,
this seems generally to occur with our game animals — often
ensue: and here we have a limiting check independent of the struggle
for life. But even some of these so-called epidemics appear to be due
to parasitic worms, which have from some cause, possibly in part
through facility of diffusion amongst the crowded animals, been
disproportionably favoured: and here comes in a sort of struggle
between the parasite and its prey.
On the other hand, in many cases, a large stock of individuals of
the same species, relatively to the numbers of its enemies, is
absolutely necessary for its preservation. Thus we can easily raise
plenty of corn and rape-seed, c., in our fields, because the
seeds are in great excess compared with the number of birds which feed
on them; nor can the birds, though having a superabundance of food at
this one season, increase in number proportionally to the supply of
seed, as their numbers are checked during winter: but any one who has
tried, knows how troublesome it is to get seed from a few wheat or
other such plants in a garden; I have in this case lost every single
seed. This view of the necessity of a large stock of the same species
for its preservation, explains, I believe, some singular facts in
nature, such as that of very rare plants being sometimes extremely
abundant in the few spots where they do occur; and that of some social
plants being social, that is, abounding in individuals, even on the
extreme confines of their range. For in such cases, we may believe,
that a plant could exist only where the conditions of its life were so
favourable that many could exist together, and thus save each other
from utter destruction. I should add that the good effects of frequent
intercrossing, and the ill effects
of close interbreeding, probably
come into play in some of these cases; but on this
intricate subject I will not here enlarge.
Many cases are on record showing how complex and unexpected are the
checks and relations between organic beings, which have to struggle
together in the same country. I will give only a single instance,
which, though a simple one, has interested me. In Staffordshire, on
the estate of a relation where I had ample means of investigation,
there was a large and extremely barren heath, which had never been
touched by the hand of man; but several hundred acres of exactly the
same nature had been enclosed twenty-five years previously and planted
with Scotch fir. The change in the native vegetation of the planted
part of the heath was most remarkable, more than is generally seen in
passing from one quite different soil to another: not only the
proportional numbers of the heath-plants were wholly changed, but
twelve species of plants (not counting grasses and carices) flourished
in the plantations, which could not be found on the heath. The effect
on the insects must have been still greater, for six insectivorous
birds were very common in the plantations, which were not to be seen
on the heath; and the heath was frequented by two or three distinct
insectivorous birds. Here we see how potent has been the effect of the
introduction of a single tree, nothing whatever else having been done,
with the exception that the land had been enclosed, so that cattle
could not enter. But how important an element enclosure is, I plainly
saw near Farnham, in Surrey. Here there are extensive heaths, with a
few clumps of old Scotch firs on the distant hill-tops: within the
last ten years large spaces have been enclosed, and self-sown firs are
now springing up in multitudes, so close together that all cannot
live.
When I ascertained that these young trees had not been sown or
planted, I was so much surprised at their numbers that I went to
several points of view, whence I could examine hundreds of acres of
the unenclosed heath, and literally I could not see a single Scotch
fir, except the old planted clumps. But on looking closely between the
stems of the heath, I found a multitude of seedlings and little trees,
which had been perpetually browsed down by the cattle. In one square
yard, at a point some hundreds yards distant from one of the old
clumps, I counted thirty-two little trees; and one of
them, judging from the rings of growth, had during twenty-six years
tried to raise its head above the stems of the heath, and had failed.
No wonder that, as soon as the land was enclosed, it became thickly
clothed with vigorously growing young firs. Yet the heath was so
extremely barren and so extensive that no one would ever have imagined
that cattle would have so closely and effectually searched it for
food.
Here we see that cattle absolutely determine the existence of the
Scotch fir; but in several parts of the world insects determine the
existence of cattle. Perhaps Paraguay offers the most curious instance
of this; for here neither cattle nor horses nor dogs have ever run
wild, though they swarm southward and northward in a feral state; and
Azara and Rengger have shown that this is caused by the greater number
in Paraguay of a certain fly, which lays its eggs in the navels of
these animals when first born. The increase of these flies, numerous
as they are, must be habitually checked by some means, probably by
birds. Hence, if certain insectivorous birds (whose numbers are
probably regulated by hawks or beasts of prey) were to increase in
Paraguay, the flies would decrease — then cattle and horses
would become feral, and this would certainly greatly alter (as
indeed
I have observed in parts of South America) the vegetation: this again
would largely affect the insects; and this, as we just have seen in
Staffordshire, the insectivorous birds, and so onwards in
ever-increasing circles of complexity. We began this series by
insectivorous birds, and we have ended with them. Not that in nature
the relations can ever be as simple as this. Battle within battle must
ever be recurring with varying success; and yet in the long-run the
forces are so nicely balanced, that the face of nature remains uniform
for long periods of time, though assuredly the merest trifle would
often give the victory to one organic being over another. Nevertheless
so profound is our ignorance, and so high our presumption, that we
marvel when we hear of the extinction of an organic being; and as we
do not see the cause, we invoke cataclysms to desolate the world, or
invent laws on the duration of the forms of life!
I am tempted to give one more instance showing how plants and
animals, most remote in the scale of nature, are bound
together by a web of complex relations. I shall hereafter have
occasion to show that the exotic Lobelia fulgens, in this part of
England, is never visited by insects, and consequently, from its
peculiar structure, never can set a seed. Many of our orchidaceous
plants absolutely require the visits of moths to remove their
pollen-masses and thus to fertilise them. I have, also, reason to
believe that humble-bees are indispensable to the fertilisation of the
heartsease (Viola tricolor), for other bees do not visit this flower.
From experiments which I have tried, I have found that the visits of
bees, if not indispensable, are at least highly beneficial to the
fertilisation of our clovers; but humble-bees alone visit the common
red clover (Trifolium pratense), as other bees cannot reach the
nectar. Hence I have very little doubt, that if the whole genus of
humble-bees became
extinct or very rare in England, the heartsease and
red clover would become very rare, or wholly disappear. The number of
humble-bees in any district depends in a great degree on the number of
field-mice, which destroy their combs and nests; and Mr H. Newman, who
has long attended to the habits of humble-bees, believes that 'more
than two thirds of them are thus destroyed all over England.' Now the
number of mice is largely dependent, as every one knows, on the number
of cats; and Mr Newman says, 'Near villages and small towns I have
found the nests of humble-bees more numerous than elsewhere, which I
attribute to the number of cats that destroy the mice.' Hence it is
quite credible that the presence of a feline animal in large numbers
in a district might determine, through the intervention first of mice
and then of bees, the frequency of certain flowers in that district!
In the case of every species, many different checks, acting at
different periods of life, and during different seasons or years,
probably come into play; some one check or some few being generally
the most potent, but all concurring in determining the average number
or even the existence of the species. In some cases it can be shown
that widely-different checks act on the same species in different
districts. When we look at the plants and bushes clothing an entangled
bank, we are tempted to attribute their proportional numbers and kinds
to what we call chance. But how false a view is this! Every one has
heard that when an American forest is cut down, a very
different vegetation springs up; but it has been observed that the
trees now growing on the ancient Indian mounds, in the Southern United
States, display the same beautiful diversity and proportion of kinds
as in the surrounding virgin forests. What a struggle between the
several kinds of trees
must here have gone on during long centuries,
each annually scattering its seeds by the thousand; what war between
insect and insect — between insects, snails, and other animals
with birds and beasts of prey — all striving to increase, and
all feeding on each other or on the trees or their seeds and
seedlings, or on the other plants which first clothed the ground and
thus checked the growth of the trees! Throw up a handful of feathers,
and all must fall to the ground according to definite laws; but how
simple is this problem compared to the action and reaction of the
innumerable plants and animals which have determined, in the course of
centuries, the proportional numbers and kinds of trees now growing on
the old Indian ruins!
The dependency of one organic being on another, as of a parasite on
its prey, lies generally between beings remote in the scale of nature.
This is often the case with those which may strictly be said to
struggle with each other for existence, as in the case of locusts and
grass-feeding quadrupeds. But the struggle almost invariably will be
most severe between the individuals of the same species, for they
frequent the same districts, require the same food, and are exposed to
the same dangers. In the case of varieties of the same species, the
struggle will generally be almost equally severe, and we sometimes see
the contest soon decided: for instance, if several varieties of wheat
be sown together, and the mixed seed be resown, some of the varieties
which best suit the soil or climate, or are naturally the most
fertile, will beat the others and so yield more seed, and will
consequently in a few years quite supplant the other varieties. To
keep up a mixed stock of even such extremely close varieties as the
variously coloured sweet-peas, they must be each year harvested
separately, and the seed then mixed in due proportion, otherwise the
weaker kinds will steadily decrease in numbers and disappear. So again
with the varieties of sheep: it has been asserted that
certain mountain-varieties will starve out other mountain-varieties,
so that they cannot be kept together. The same result has followed
from keeping together different varieties of the medicinal leech. It
may even be doubted whether the varieties of any one of our domestic
plants or animals have so exactly the same strength, habits, and
constitution, that the original proportions of a mixed stock could be
kept up for half a dozen generations, if they were allowed to struggle
together, like beings in a state of nature, and if the seed or young
were not annually sorted.
As species of the same genus have usually, though by no means
invariably, some similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between species
of the same genus, when they come into competition with each other,
than between species of distinct genera. We see this in the recent
extension over parts of the United States of one species of swallow
having caused the decrease of another species. The recent increase of
the missel-thrush in parts of Scotland has caused the decrease of the
song-thrush. How frequently we hear of one species of rat taking the
place of another species under the most different climates! In Russia
the small Asiatic cockroach has everywhere driven before it its great
congener. One species of charlock will supplant another, and so in
other cases. We can dimly see why the competition should be most
severe between allied forms, which fill nearly the same place in the
economy of nature; but probably in no one case could we precisely say
why one species has been victorious over another in the great battle
of life.
A corollary of the highest importance may be deduced from the
foregoing remarks, namely, that the structure of every organic being
is related, in the most essential yet often hidden manner, to that of
all other organic beings, with which it comes into competition for
food or residence, or from which it has to escape, or on which it
preys. This is obvious in the structure of the teeth and talons of the
tiger; and in that of the legs and claws of the parasite which clings
to the hair on the tiger's body. But in the beautifully plumed seed of
the dandelion, and in the flattened and fringed legs of the
water-beetle, the relation seems at first confined to the
elements of air and water. Yet the advantage of plumed seeds no doubt
stands in the closest relation to the land being already thickly
clothed by other plants; so that the seeds may be widely distributed
and fall on unoccupied ground. In the water-beetle, the structure of
its legs, so well adapted for diving, allows it to compete with other
aquatic insects, to hunt for its own prey, and to escape serving as
prey to other animals.
The store of nutriment laid up within the seeds of many plants
seems at first sight to have no sort of relation to other plants. But
from the strong growth of young plants produced from such seeds (as
peas and beans), when sown in the midst of long grass, I suspect that
the chief use of the nutriment in the seed is to favour the growth of
the young seedling, whilst struggling with other plants growing
vigorously all around.
Look at a plant in the midst of its range, why does it not double
or quadruple its numbers? We know that it can perfectly well withstand
a little more heat or cold, dampness or dryness, for elsewhere it
ranges
into slightly hotter or colder, damper or drier districts. In
this case we can clearly see that if we wished in imagination to give
the plant the power of increasing in number, we should have to give it
some advantage over its competitors, or over the animals which preyed
on it. On the confines of its geographical range, a change of
constitution with respect to climate would clearly be an advantage to
our plant; but we have reason to believe that only a few plants or
animals range so far, that they are destroyed by the rigour of the
climate alone. Not until we reach the extreme confines of life, in the
arctic regions or on the borders of an utter desert, will competition
cease. The land may be extremely cold or dry, yet there will be
competition between some few species, or between the individuals of
the same species, for the warmest or dampest spots.
Hence, also, we can see that when a plant or animal is placed in a
new country amongst new competitors, though the climate may be exactly
the same as in its former home, yet the conditions of its life will
generally be changed in an essential manner. If we wished to increase
its average numbers in its new home, we should have to modify it in a
different way to what we should have done in its native
country; for we should have to give it some advantage over a different
set of competitors or enemies.
It is good thus to try in our imagination to give any form some
advantage over another. Probably in no single instance should we know
what to do, so as to succeed. It will convince us of our ignorance on
the mutual relations of all organic beings; a conviction as necessary,
as it seems to be difficult to acquire. All that we can do, is to keep
steadily in mind that each organic being is striving to increase at a
geometrical
ratio; that each at some period of its life, during some
season of the year, during each generation or at intervals, has to
struggle for life, and to suffer great destruction. When we reflect on
this struggle, we may console ourselves with the full belief, that the
war of nature is not incessant, that no fear is felt, that death is
generally prompt, and that the vigorous, the healthy, and the happy
survive and multiply.
NATURAL SELECTION
- Natural Selection
- its power compared with man's selection
- its power on characters of trifling importance
- its power at all ages and on both sexes
- Sexual Selection
- On the generality
of intercrosses between individuals of the same species
- Circumstances favourable and unfavourable to Natural Selection,
namely, intercrossing, isolation, number of individuals
- Slow action
- Extinction caused by Natural Selection
- Divergence of Character, related to the diversity of inhabitants of
any small area, and to naturalisation
- Action of Natural Selection, through Divergence of
Character and Extinction, on the descendants from a common parent
- Explains the Grouping of all organic beings
How will the struggle for existence,
discussed too briefly in the last chapter, act in regard to variation?
Can the principle of selection, which we have seen is so potent in the
hands of man, apply in nature? I think we shall see that it can act
most effectually. Let it be borne in mind in what an endless number of
strange peculiarities our domestic productions, and, in a lesser
degree, those under nature, vary; and how strong the hereditary
tendency is. Under domestication, it may be truly said that the, whole
organisation becomes in some degree plastic. Let it be borne in mind
how infinitely complex and close-fitting are the mutual relations of
all organic beings to each other and to their physical conditions of
life. Can it, then, be thought improbable, seeing that variations
useful to man have undoubtedly occurred, that other variations useful
in some way to each being in the great and complex battle of life,
should sometimes occur in the course of thousands of generations? If
such do occur, can we doubt (remembering
that many more individuals
are born than can possibly survive) that individuals having any
advantage, however slight, over others, would have the best chance of
surviving and of procreating their kind? On the other
hand, we may feel sure that any variation in the least degree
injurious would be rigidly destroyed. This preservation of favourable
variations and the rejection of injurious variations, I call Natural
Selection. Variations neither useful nor injurious would not be
affected by natural selection, and would be left a fluctuating
element, as perhaps we see in the species called polymorphic.
We shall best understand the probable course of natural selection
by taking the case of a country undergoing some physical change, for
instance, of climate. The proportional numbers of its inhabitants
would almost immediately undergo a change, and some species might
become extinct. We may conclude, from what we have seen of the
intimate and complex manner in which the inhabitants of each country
are bound together, that any change in the numerical proportions of
some of the inhabitants, independently of the change of climate
itself, would most seriously affect many of the others. If the country
were open on its borders, new forms would certainly immigrate, and
this also would seriously disturb the relations of some of the former
inhabitants. Let it be remembered how powerful the influence of a
single introduced tree or mammal has been shown to be. But in the case
of an island, or of a country partly surrounded by barriers, into
which new and better adapted forms could not freely enter, we should
then have
places in the economy of nature which would assuredly be
better filled up, if some of the original inhabitants were in some
manner modified; for, had the area been open to immigration, these
same places would have been seized on by intruders. In such case,
every slight modification, which in the course of ages chanced to
arise, and which in any way favoured the individuals of any of the
species, by better adapting them to their altered conditions, would
tend to be preserved; and natural selection would thus have free scope
for the work of improvement.
We have reason to believe, as stated in the first chapter, that a
change in the conditions of life, by specially acting on the
reproductive system, causes or increases variability; and in the
foregoing case the conditions of life are supposed to have undergone a
change, and this would manifestly be favourable to
natural selection, by giving a better chance of profitable variations
occurring; and unless profitable variations do occur, natural
selection can do nothing. Not that, as I believe, any extreme amount
of variability is necessary; as man can certainly produce great
results by adding up in any given direction mere individual
differences, so could Nature, but far more easily, from having
incomparably longer time at her disposal. Nor do I believe that any
great physical change, as of climate, or any unusual degree of
isolation to check immigration, is actually necessary to produce new
and unoccupied places for natural selection to fill up by modifying
and improving some of the varying inhabitants. For as all the
inhabitants of each country are struggling together with nicely
balanced forces, extremely slight modifications in the structure or
habits of one inhabitant would often give it an advantage over others;
and still further modifications of the same kind would often still
further increase the advantage. No country can be named in which all
the native inhabitants are now so perfectly adapted to each other and
to the physical conditions under which they live, that none of
them
could anyhow be improved; for in all countries, the natives have been
so far conquered by naturalised productions, that they have allowed
foreigners to take firm possession of the land. And as foreigners
have thus everywhere beaten some of the natives, we may safely
conclude that the natives might have been modified with advantage, so
as to have better resisted such intruders.
As man can produce and certainly has produced a great result by his
methodical and unconscious means of selection, what may not nature
effect? Man can act only on external and visible characters: nature
cares nothing for appearances, except in so far as they may be useful
to any being. She can act on every internal organ, on every shade of
constitutional difference, on the whole machinery of life. Man
selects only for his own good; Nature only for that of the being
which she tends. Every selected character is fully exercised by her;
and the being is placed under well-suited conditions of life. Man
keeps the natives of many climates in the same country; he seldom
exercises each selected character in some peculiar and fitting manner;
he feeds a long and a short beaked pigeon on the same food; he does
not exercise a long-backed or long-legged quadruped in
any peculiar manner; he exposes sheep with long and short wool to the
same climate. He does not allow the most vigorous males to struggle
for the females. He does not rigidly destroy all inferior animals, but
protects during each varying season, as far as lies in his power, all
his productions. He often begins his selection by some half-monstrous
form; or at least by some modification prominent enough to catch his
eye, or to be plainly useful to him. Under nature, the slightest
difference of structure or constitution may well turn the
nicely-balanced scale in the
struggle for life, and so be preserved.
How fleeting are the wishes and efforts of man! how short his time!
and consequently how poor will his products be, compared with those
accumulated by nature during whole geological periods. Can we wonder,
then, that nature's productions should be far 'truer' in character
than man's productions; that they should be infinitely better adapted
to the most complex conditions of life, and should plainly bear the
stamp of far higher workmanship?
It may be said that natural selection is daily and hourly
scrutinising, throughout the world, every variation, even the
slightest; rejecting that which is bad, preserving and adding up all
that is good; silently and insensibly working, whenever and wherever
opportunity offers, at the improvement of each organic being in
relation to its organic and inorganic conditions of life. We see
nothing of these slow changes in progress, until the hand of time has
marked the long lapses of ages, and then so imperfect is our view into
long past geological ages, that we only see that the forms of life are
now different from what they formerly were.
Although natural selection can act only through and for the good of
each being, yet characters and structures, which we are apt to
consider as of very trifling importance, may thus be acted on. When
we see leaf-eating insects green, and bark-feeders mottled-grey; the
alpine ptarmigan white in winter, the red-grouse the colour of
heather, and the black-grouse that of peaty earth, we must believe
that these tints are of service to these birds and insects in
preserving them from danger. Grouse, if not destroyed at some period
of their lives, would increase in countless numbers; they are known to
suffer largely from birds of prey; and hawks are guided
by eyesight to their prey, — so much so, that on
parts of the
Continent persons are warned not to keep white pigeons, as being the
most liable to destruction. Hence I can see no reason to doubt that
natural selection might be most effective in giving the proper colour
to each kind of grouse, and in keeping that colour, when once
acquired, true and constant. Nor ought we to think that the occasional
destruction of an animal of any particular colour would produce little
effect: we should remember how essential it is in a flock of white
sheep to destroy every lamb with the faintest trace of black. In
plants the down on the fruit and the colour of the flesh are
considered by botanists as characters of the most trifling importance:
yet we hear from an excellent horticulturist, Downing, that in the
United States smooth-skinned fruits suffer far more from a beetle, a
curculio, than those with down; that purple plums suffer far more from
a certain disease than yellow plums; whereas another disease attacks
yellow-fleshed peaches far more than those with other coloured flesh.
If, with all the aids of art, these slight differences make a great
difference in cultivating the several varieties, assuredly, in a state
of nature, where the trees would have to struggle with other trees and
with a host of enemies, such differences would effectually settle
which variety, whether a smooth or downy, a yellow or purple fleshed
fruit, should succeed.
In looking at many small points of difference between species,
which, as far as our ignorance permits us to judge, seem to be quite
unimportant, we must not forget that climate, food, c., probably
produce some slight and direct effect. It is, however, far more
necessary to bear in mind that there are many unknown laws of
correlation of growth, which, when one part of the organisation is
modified through variation, and the modifications are accumulated by
natural selection for
the good of the being, will cause other
modifications, often of the most unexpected nature.
As we see that those variations which under domestication appear at
any particular period of life, tend to reappear in the offspring at
the same period; — for instance, in the seeds of the many
varieties of our culinary and agricultural plants; in the
caterpillar and cocoon stages of the varieties of the silkworm; in the
eggs of poultry, and in the colour of the down of their chickens; in
the horns of our sheep and cattle when nearly adult; — so in a
state of nature, natural selection will be enabled to act on and
modify organic beings at any age, by the accumulation of profitable
variations at that age, and by their inheritance at a corresponding
age. If it profit a plant to have its seeds more and more widely
disseminated by the wind, I can see no greater difficulty in this
being effected through natural selection, than in the cotton-planter
increasing and improving by selection the down in the pods on his
cotton-trees. Natural selection may modify and adapt the larva of an
insect to a score of contingencies, wholly different from those which
concern the mature insect. These modifications will no doubt affect,
through the laws of correlation, the structure of the adult; and
probably in the case of those insects which live only for a few hours,
and which never feed, a large part of their structure is merely the
correlated result of successive changes in the structure of their
larvae. So, conversely, modifications in the adult will probably often
affect the structure of the larva; but in all cases natural selection
will ensure that modifications consequent on other modifications at a
different period of life, shall not be in the least degree injurious:
for if they became so, they would cause the extinction of the species.
Natural selection will modify the structure of the
young in
relation to the parent, and of the parent in relation to the young. In
social animals it will adapt the structure of each individual for the
benefit of the community; if each in consequence profits by the
selected change. What natural selection cannot do, is to modify the
structure of one species, without giving it any advantage, for the
good of another species; and though statements to this effect may be
found in works of natural history, I cannot find one case which will
bear investigation. A structure used only once in an animal's whole
life, if of high importance to it, might be modified to any extent by
natural selection; for instance, the great jaws possessed by certain
insects, and used exclusively for opening the cocoon — or the
hard tip to the beak of nestling birds, used for breaking the egg. It
has been asserted, that of the best short-beaked
tumbler-pigeons more perish in the egg than are able to get out of it;
so that fanciers assist in the act of hatching. Now, if nature had to
make the beak of a full-grown pigeon very short for the bird's own
advantage, the process of modification would be very slow, and there
would be simultaneously the most rigorous selection of the young birds
within the egg, which had the most powerful and hardest beaks, for all
with weak beaks would inevitably perish: or, more delicate and more
easily broken shells might be selected, the thickness of the shell
being known to vary like every other structure.
Sexual Selection.
Inasmuch as
peculiarities often appear under domestication in one sex and become
hereditarily attached to that sex, the same fact probably occurs under
nature, and if so, natural selection will be able to modify one sex in
its functional relations to the other sex, or in relation to wholly
different habits of life in the two sexes, as is sometimes the case
with insects. And this leads me to say a few words on what I call
Sexual Selection. This depends, not on a struggle for existence, but
on a struggle between the males for possession of the females; the
result is not death to the unsuccessful competitor, but few or no
offspring. Sexual selection is, therefore, less rigorous than natural
selection. Generally, the most vigorous males, those which are best
fitted for their places in nature, will leave most progeny. But in
many cases, victory will depend not on general vigour, but on having
special weapons, confined to the male sex. A hornless stag or spurless
cock would have a poor chance of leaving offspring. Sexual selection
by always allowing the victor to breed might surely give indomitable
courage, length to the spur, and strength to the wing to strike in the
spurred leg, as well as the brutal cock-fighter, who knows well that
he can improve his breed by careful selection of the best cocks. How
low in the scale of nature this law of battle descends, I know not;
male alligators have been described as fighting, bellowing, and
whirling round, like Indians in a war-dance, for the possession of the
females; male salmons have been seen fighting all day long; male
stag-beetles often bear wounds from the huge mandibles of other males.
The war is, perhaps, severest between the males of
polygamous animals, and these seem oftenest provided with special
weapons. The males of carnivorous animals are already well armed;
though to them and to others, special means of defence may be given
through means of sexual selection, as the mane to the lion, the
shoulder-pad to the boar, and the hooked jaw to the male salmon; for
the shield may be as important for victory, as the sword or spear.
Amongst birds, the contest is often of a more peaceful character.
All those who have attended to the subject,
believe that there is the
severest rivalry between the males of many species to attract by
singing the females. The rock-thrush of Guiana, birds of paradise, and
some others, congregate; and successive males display their gorgeous
plumage and perform strange antics before the females, which standing
by as spectators, at last choose the most attractive partner. Those
who have closely attended to birds in confinement well know that they
often take individual preferences and dislikes: thus Sir R. Heron has
described how one pied peacock was eminently attractive to all his hen
birds. It may appear childish to attribute any effect to such
apparently weak means: I cannot here enter on the details necessary to
support this view; but if man can in a short time give elegant
carriage and beauty to his bantams, according to his standard of
beauty, I can see no good reason to doubt that female birds, by
selecting, during thousands of generations, the most melodious or
beautiful males, according to their standard of beauty, might produce
a marked effect. I strongly suspect that some well-known laws with
respect to the plumage of male and female birds, in comparison with
the plumage of the young, can be explained on the view of plumage
having been chiefly modified by sexual selection, acting when the
birds have come to the breeding age or during the breeding season; the
modifications thus produced being inherited at corresponding ages or
seasons, either by the males alone, or by the males and females; but I
have not space here to enter on this subject.
Thus it is, as I believe, that when the males and females of any
animal have the same general habits of life, but differ in structure,
colour, or ornament, such differences have been mainly caused by
sexual selection; that is, individual males have had, in
successive generations, some slight advantage over other
males, in
their weapons, means of defence, or charms; and have transmitted these
advantages to their male offspring. Yet, I would not wish to
attribute all such sexual differences to this agency: for we see
peculiarities arising and becoming attached to the male sex in our
domestic animals (as the wattle in male carriers, horn-like
protuberances in the cocks of certain fowls, c.), which we cannot
believe to be either useful to the males in battle, or attractive to
the females. We see analogous cases under nature, for instance, the
tuft of hair on the breast of the turkey-cock, which can hardly be
either useful or ornamental to this bird; — indeed, had the tuft
appeared under domestication, it would have been called a monstrosity.
Illustrations of the action of Natural
Selection.
In order to make it clear how, as I believe, natural
selection acts, I must beg permission to give one or two imaginary
illustrations. Let us take the case of a wolf, which preys on various
animals, securing some by craft, some by strength, and some by
fleetness; and let us suppose that the fleetest prey, a deer for
instance, had from any change in the country increased in numbers, or
that other prey had decreased in numbers, during that season of the
year when the wolf is hardest pressed for food. I can under such
circumstances see no reason to doubt that the swiftest and slimmest
wolves would have the best chance of surviving, and so be preserved or
selected, — provided always that they retained strength to
master their prey at this or at some other period of the year, when
they might be compelled to prey on other animals. I can see no more
reason to doubt this, than that man can improve the fleetness of his
greyhounds by careful and methodical selection, or by that unconscious
selection which results from each man trying
to keep the best dogs
without any thought of modifying the breed.
Even without any change in the proportional numbers of the animals
on which our wolf preyed, a cub might be born with an innate tendency
to pursue certain kinds of prey. Nor can this be thought very
improbable; for we often observe great differences in the natural
tendencies of our domestic animals; one cat, for
instance, taking to catch rats, another mice; one cat, according to Mr.
St. John, bringing home winged game, another hares or rabbits, and
another hunting on marshy ground and almost nightly catching woodcocks
or snipes. The tendency to catch rats rather than mice is known to be
inherited. Now, if any slight innate change of habit or of structure
benefited an individual wolf, it would have the best chance of
surviving and of leaving offspring. Some of its young would probably
inherit the same habits or structure, and by the repetition of this
process, a new variety might be formed which would either supplant or
coexist with the parent-form of wolf. Or, again, the wolves inhabiting
a mountainous district, and those frequenting the lowlands, would
naturally be forced to hunt different prey; and from the continued
preservation of the individuals best fitted for the two sites, two
varieties might slowly be formed. These varieties would cross and
blend where they met; but to this subject of intercrossing we shall
soon have to return. I may add, that, according to Mr. Pierce, there
are two varieties of the wolf inhabiting the Catskill Mountains in the
United States, one with a light greyhound-like form, which pursues
deer, and the other more bulky, with shorter legs, which more
frequently attacks the shepherd's flocks.
Let us now take a more complex case. Certain plants excrete a sweet
juice, apparently for the sake of eliminating something injurious from
their sap: this is
effected by glands at the base of the stipules in
some Leguminosae, and at the back of the leaf of the common laurel.
This juice, though small in quantity, is greedily sought by insects.
Let us now suppose a little sweet juice or nectar to be excreted by
the inner bases of the petals of a flower. In this case insects in
seeking the nectar would get dusted with pollen, and would certainly
often transport the pollen from one flower to the stigma of another
flower. The flowers of two distinct individuals of the same species
would thus get crossed; and the act of crossing, we have good reason
to believe (as will hereafter be more fully alluded to), would produce
very vigorous seedlings, which consequently would have the best chance
of flourishing and surviving. Some of these seedlings would probably
inherit the nectar-excreting power. Those in individual
flowers which had the largest glands or nectaries, and which excreted
most nectar, would be oftenest visited by insects, and would be
oftenest crossed; and so in the long-run would gain the upper hand.
Those flowers, also, which had their stamens and pistils placed, in
relation to the size and habits of the particular insects which
visited them, so as to favour in any degree the transportal of their
pollen from flower to flower, would likewise be favoured or selected.
We might have taken the case of insects visiting flowers for the sake
of collecting pollen instead of nectar; and as pollen is formed for
the sole object of fertilisation, its destruction appears a simple
loss to the plant; yet if a little pollen were carried, at first
occasionally and then habitually, by the pollen-devouring insects from
flower to flower, and a cross thus effected, although nine-tenths of
the pollen were destroyed, it might still be a great gain to the
plant; and those individuals which produced more and more pollen, and
had larger and larger anthers, would be selected.
When our plant, by this process of the continued preservation or
natural selection of more and more attractive flowers, had been
rendered highly attractive to insects, they would, unintentionally on
their part, regularly carry pollen from flower to flower; and that
they can most effectually do this, I could easily show by many
striking instances. I will give only one — not as a very
striking case, but as likewise illustrating one step in the separation
of the sexes of plants, presently to be alluded to. Some holly-trees
bear only male flowers, which have four stamens producing rather a
small quantity of pollen, and a rudimentary pistil; other holly-trees
bear only female flowers; these have a full-sized pistil, and four
stamens with shrivelled anthers, in which not a grain of pollen can be
detected. Having found a female tree exactly sixty yards from a male
tree, I put the stigmas of twenty flowers, taken from different
branches, under the microscope, and on all, without exception, there
were pollen-grains, and on some a profusion of pollen. As the wind had
set for several days from the female to the male tree, the pollen
could not thus have been carried. The weather had been cold and
boisterous, and therefore not favourable to bees, nevertheless every
female flower which I examined had been effectually
fertilised by the bees, accidentally dusted with pollen, having flown
from tree to tree in search of nectar. But to return to our imaginary
case: as soon as the plant had been rendered so highly attractive to
insects that pollen was regularly carried from flower to flower,
another process might commence. No naturalist doubts the advantage of
what has been called the 'physiological division of labour;' hence we
may believe that it would be advantageous to a plant to produce
stamens alone in one flower or on one whole plant, and pistils alone
in
another flower or on another plant. In plants under culture and
placed under new conditions of life, sometimes the male organs and
sometimes the female organs become more or less impotent; now if we
suppose this to occur in ever so slight a degree under nature, then as
pollen is already carried regularly from flower to flower, and as a
more complete separation of the sexes of our plant would be
advantageous on the principle of the division of labour, individuals
with this tendency more and more increased, would be continually
favoured or selected, until at last a complete separation of the sexes
would be effected.
Let us now turn to the nectar-feeding insects in our imaginary
case: we may suppose the plant of which we have been slowly increasing
the nectar by continued selection, to be a common plant; and that
certain insects depended in main part on its nectar for food. I could
give many facts, showing how anxious bees are to save time; for
instance, their habit of cutting holes and sucking the nectar at the
bases of certain flowers, which they can, with a very little more
trouble, enter by the mouth. Bearing such facts in mind, I can see no
reason to doubt that an accidental deviation in the size and form of
the body, or in the curvature and length of the proboscis, c.,
far too slight to be appreciated by us, might profit a bee or other
insect, so that an individual so characterised would be able to obtain
its food more quickly, and so have a better chance of living and
leaving descendants. Its descendants would probably inherit a tendency
to a similar slight deviation of structure. The tubes of the corollas
of the common red and incarnate clovers (Trifolium pratense and
incarnatum) do not on a hasty glance appear to differ in length; yet
the hive-bee can easily suck the nectar out of the
incarnate clover, but not out of the common red
clover, which is
visited by humble-bees alone; so that whole fields of the red clover
offer in vain an abundant supply of precious nectar to the hive-bee.
Thus it might be a great advantage to the hive-bee to have a slightly
longer or differently constructed proboscis. On the other hand, I have
found by experiment that the fertility of clover greatly depends on
bees visiting and moving parts of the corolla, so as to push the
pollen on to the stigmatic surface. Hence, again, if humble-bees were
to become rare in any country, it might be a great advantage to the
red clover to have a shorter or more deeply divided tube to its
corolla, so that the hive-bee could visit its flowers. Thus I can
understand how a flower and a bee might slowly become, either
simultaneously or one after the other, modified and adapted in the
most perfect manner to each other, by the continued preservation of
individuals presenting mutual and slightly favourable deviations of
structure.
I am well aware that this doctrine of natural selection,
exemplified in the above imaginary instances, is open to the same
objections which were at first urged against Sir Charles Lyell's noble
views on 'the modern changes of the earth, as illustrative of
geology;' but we now very seldom hear the action, for instance, of the
coast-waves, called a trifling and insignificant cause, when applied
to the excavation of gigantic valleys or to the formation of the
longest lines of inland cliffs. Natural selection can act only by the
preservation and accumulation of infinitesimally small inherited
modifications, each profitable to the preserved being; and as modern
geology has almost banished such views as the excavation of a great
valley by a single diluvial wave, so will natural selection, if it be
a true principle, banish the belief of the continued creation of new
organic
beings, or of any great and sudden modification in their
structure.
On the Intercrossing of Individuals.
I must here introduce a short digression. In the case of animals and
plants with separated sexes, it is of course obvious that two
individuals must always unite for each birth; but in the case of
hermaphrodites this is far from obvious. Nevertheless I am strongly
inclined to believe that with all hermaphrodites two
individuals, either occasionally or habitually, concur for the
reproduction of their kind. This view, I may add, was first suggested
by Andrew Knight. We shall presently see its importance; but I must
here treat the subject with extreme brevity, though I have the
materials prepared for an ample discussion. All vertebrate animals,
all insects, and some other large groups of animals, pair for each
birth. Modern research has much diminished the number of supposed
hermaphrodites, and of real hermaphrodites a large number pair; that
is, two individuals regularly unite for reproduction, which is all
that concerns us. But still there are many hermaphrodite animals which
certainly do not habitually pair, and a vast majority of plants are
hermaphrodites. What reason, it may be asked, is there for supposing
in these cases that two individuals ever concur in reproduction? As it
is impossible here to enter on details, I must trust to some general
considerations alone.
In the first place, I have collected so large a body of facts,
showing, in accordance with the almost universal belief of breeders,
that with animals and plants a cross between different varieties, or
between individuals of the same variety but of another strain, gives
vigour and fertility to the offspring; and on the other hand, that
close interbreeding diminishes vigour and
fertility; that
these facts alone incline me to believe that it is a
general law of nature (utterly ignorant though we be of the meaning of
the law) that no organic being self-fertilises itself for an eternity
of generations; but that a cross with another individual is
occasionally — perhaps at very long intervals --
indispensable.
On the belief that this is a law of nature, we can, I think,
understand several large classes of facts, such as the following,
which on any other view are inexplicable. Every hybridizer knows how
unfavourable exposure to wet is to the fertilisation of a flower, yet
what a multitude of flowers have their anthers and stigmas fully
exposed to the weather! but if an occasional cross be indispensable,
the fullest freedom for the entrance of pollen from another individual
will explain this state of exposure, more especially as the plant's
own anthers and pistil generally stand so close together that
self-fertilisation seems almost inevitable. Many flowers,
on the other hand, have their organs of fructification closely
enclosed, as in the great papilionaceous or pea-family; but in
several, perhaps in all, such flowers, there is a very curious
adaptation between the structure of the flower and the manner in which
bees suck the nectar; for, in doing this, they either push the
flower's own pollen on the stigma, or bring pollen from another
flower. So necessary are the visits of bees to papilionaceous flowers,
that I have found, by experiments published elsewhere, that their
fertility is greatly diminished if these visits be prevented. Now, it
is scarcely possible that bees should fly from flower to flower, and
not carry pollen from one to the other, to the great good, as I
believe, of the plant. Bees will act like a camel-hair pencil, and it
is quite sufficient just to touch the anthers of one flower and then
the stigma of another with the same brush to ensure fertilisation; but
it must not be
supposed that bees would thus produce a multitude of
hybrids between distinct species; for if you bring on the same brush a
plant's own pollen and pollen from another species, the former will
have such a prepotent effect, that it will invariably and completely
destroy, as has been shown by Grtner, any influence from the foreign
pollen.
When the stamens of a flower suddenly spring towards the pistil, or
slowly move one after the other towards it, the contrivance seems
adapted solely to ensure self-fertilisation; and no doubt it is useful
for this end: but, the agency of insects is often required to cause
the stamens to spring forward, as Klreuter has shown to be the case
with the barberry; and curiously in this very genus, which seems to
have a special contrivance for self-fertilisation, it is well known
that if very closely-allied forms or varieties are planted near each
other, it is hardly possible to raise pure seedlings, so largely do
they naturally cross. In many other cases, far from there being any
aids for self-fertilisation, there are special contrivances, as I
could show from the writings of C. C. Sprengel and from my own
observations, which effectually prevent the stigma receiving pollen
from its own flower: for instance, in Lobelia fulgens, there is a
really beautiful and elaborate contrivance by which every one of the
infinitely numerous pollen-granules are swept out of the conjoined
anthers of each flower, before the stigma of that
individual flower is ready to receive them; and as this flower is
never visited, at least in my garden, by insects, it never sets a
seed, though by placing pollen from one flower on the stigma of
another, I raised plenty of seedlings; and whilst another species of
Lobelia growing close by, which is visited by bees, seeds freely. In
very many other cases, though there be no special mechanical
contrivance to prevent the stigma of a flower receiving its own
pollen, yet, as
C. C. Sprengel has shown, and as I can confirm, either
the anthers burst before the stigma is ready for fertilisation, or the
stigma is ready before the pollen of that flower is ready, so that
these plants have in fact separated sexes, and must habitually be
crossed. How strange are these facts! How strange that the pollen and
stigmatic surface of the same flower, though placed so close together,
as if for the very purpose of self-fertilisation, should in so many
cases be mutually useless to each other! How simply are these facts
explained on the view of an occasional cross with a distinct
individual being advantageous or indispensable!
If several varieties of the cabbage, radish, onion, and of some
other plants, be allowed to seed near each other, a large majority, as
I have found, of the seedlings thus raised will turn out mongrels: for
instance, I raised 233 seedling cabbages from some plants of different
varieties growing near each other, and of these only 78 were true to
their kind, and some even of these were not perfectly true. Yet the
pistil of each cabbage-flower is surrounded not only by its own six
stamens, but by those of the many other flowers on the same plant.
How, then, comes it that such a vast number of the seedlings are
mongrelised? I suspect that it must arise from the pollen of a
distinct variety having a prepotent effect
over a flower's own pollen; and that this is part of the general law
of good being derived from the intercrossing of distinct individuals
of the same species. When distinct species
are crossed the case is
directly the reverse, for a plant's own pollen is always prepotent
over foreign pollen; but to this subject we shall return in a future
chapter.
In the case of a gigantic tree covered with innumerable flowers, it
may be objected that pollen could seldom be carried from tree to tree, and at most only from flower
to flower on the same
tree, and that flowers on the same tree can be considered as distinct
individuals only in a limited sense. I believe this objection to be
valid, but that nature has largely provided against it by giving to
trees a strong tendency to bear flowers with separated sexes. When the
sexes are separated, although the male and female flowers may be
produced on the same tree, we can see that pollen must be regularly
carried from flower to flower; and this will give a better chance of
pollen being occasionally carried from tree to tree. That trees
belonging to all Orders have their sexes more often separated than
other plants, I find to be the case in this country; and at my request
Dr Hooker tabulated the trees of New Zealand, and Dr Asa Gray those of
the United States, and the result was as I anticipated. On the other
hand, Dr Hooker has recently informed me that he finds that the rule
does not hold in Australia; and I have made these few remarks on the
sexes of trees simply to call attention to the subject.
Turning for a very brief space to animals: on the land there are
some hermaphrodites, as land-mollusca and earth-worms; but these all
pair. As yet I have not found a single case of a terrestrial animal
which fertilises itself. We can understand this remarkable fact, which
offers so strong a contrast with terrestrial plants, on the view of an
occasional cross being indispensable, by considering the medium in
which terrestrial animals live, and the nature of the fertilising
element; for we know of no means, analogous to the action of insects
and of the wind in the case of plants, by which an occasional cross
could be effected with terrestrial animals without the concurrence of
two individuals. Of aquatic animals, there are many self-fertilising
hermaphrodites; but here currents in the water offer an obvious means
for an occasional cross. And, as in the case of flowers, I have as yet
failed, after consultation with one of the highest authorities,
namely, Professor Huxley, to discover a single case of an
hermaphrodite animal with the organs of reproduction so perfectly
enclosed within the body, that access from without and the occasional
influence of a distinct individual can be shown to be physically
impossible. Cirripedes long appeared to me to present a case of very
great difficulty under this point of view; but I have been enabled, by
a fortunate chance, elsewhere to prove that two
individuals, though both are self-fertilising hermaphrodites, do
sometimes cross.
It must have struck most naturalists as a strange anomaly that, in
the case of both animals and plants, species of the same family and
even of the same genus, though agreeing closely with each other in
almost their whole organisation, yet are not rarely, some of them
hermaphrodites, and some of them unisexual. But if, in fact, all
hermaphrodites do occasionally intercross with other individuals, the
difference between hermaphrodites and unisexual species, as far as
function is concerned, becomes very small.
From these several considerations and from the many special facts
which I have collected, but which I am not here able to give, I am
strongly inclined to suspect that, both in the vegetable and animal
kingdoms, an occasional intercross with a distinct individual is a law
of nature. I am well aware that there are, on this view, many cases of
difficulty, some of which I am trying to investigate. Finally then, we
may conclude that in many organic beings, a cross between two
individuals is an obvious necessity for each birth; in many others it
occurs perhaps only at long intervals; but in none, as I suspect, can
self-fertilisation go on for perpetuity.
Circumstances favourable to Natural
Selection.
This
is an extremely intricate subject. A large
amount of inheritable and diversified variability is favourable, but
I believe mere individual differences suffice for the work. A large
number of individuals, by giving a better chance for the appearance
within any given period of profitable variations, will compensate for
a lesser amount of variability in each individual, and is, I believe, an
extremely important element of success. Though nature grants vast
periods of time for the work of natural selection, she does not grant
an indefinite period; for as all organic beings are striving, it may
be said, to seize on each place in the economy of nature, if any one
species does not become modified and improved in a corresponding
degree with its competitors, it will soon be exterminated.
In man's methodical selection, a breeder selects for some definite
object, and free intercrossing will wholly stop his work. But when
many men, without intending to alter the breed, have a nearly common
standard of perfection, and all try to get and breed from the best
animals, much improvement and modification surely but slowly follow
from this unconscious process of selection, notwithstanding a large
amount of crossing with inferior animals. Thus it will be in nature;
for within a confined area, with some place in its polity not so
perfectly occupied as might be, natural selection will always tend to
preserve all the individuals varying in the right direction, though in
different degrees, so as better to fill up the unoccupied place. But
if the area be large, its several districts will almost certainly
present different conditions of life; and then if natural selection be
modifying and improving a species in the several districts, there will
be intercrossing with the other individuals of the same species on the
confines of each. And in this case the effects of intercrossing can
hardly be counterbalanced
by natural selection always tending to
modify all the individuals in each district in exactly the same manner
to the conditions of each; for in a continuous area, the conditions
will generally graduate away insensibly from one district to another.
The intercrossing will most affect those animals which unite for each
birth, which wander much, and which do not breed at a very quick rate.
Hence in animals of this nature, for instance in birds, varieties will
generally be confined to separated countries; and this I believe to be
the case. In hermaphrodite organisms which cross only occasionally,
and likewise in animals which unite for each birth, but which wander
little and which can increase at a very rapid rate, a new and improved
variety might be quickly formed on any one spot, and might there
maintain itself in a body, so that whatever intercrossing took place
would be chiefly between the individuals of the same new variety. A
local variety when once thus formed might subsequently slowly spread
to other districts. On the above principle, nurserymen always prefer
getting seed from a large body of plants of the same variety, as the
chance of intercrossing with other varieties is thus lessened.
Even in the case of slow-breeding animals, which unite for each birth, we must not overrate the effects of intercrosses
in retarding natural selection; for I can bring a considerable
catalogue of facts, showing that within the same area, varieties of
the same animal can long remain distinct, from haunting different
stations, from breeding at slightly different seasons, or from
varieties of the same kind preferring to pair together.
Intercrossing plays a very important part in nature in keeping the
individuals of the same species, or of the same variety, true and
uniform in character. It will obviously thus act far more efficiently
with those animals
which unite for each birth; but I have already
attempted to show that we have reason to believe that occasional
intercrosses take place with all animals and with all plants. Even if
these take place only at long intervals, I am convinced that the young
thus produced will gain so much in vigour and fertility over the
offspring from long-continued self-fertilisation, that they will have
a better chance of surviving and propagating their kind; and thus, in
the long run, the influence of intercrosses, even at rare intervals,
will be great. If there exist organic beings which never intercross,
uniformity of character can be retained amongst them, as long as their
conditions of life remain the same, only through the principle of
inheritance, and through natural selection destroying any which depart
from the proper type; but if their conditions of life change and they
undergo modification, uniformity of character can be given to their
modified offspring, solely by natural selection preserving the same
favourable variations.
Isolation, also, is an important element in the process of natural
selection. In a confined or isolated area, if not very large, the
organic and inorganic conditions of life will generally be in a great
degree uniform; so that natural selection will tend to modify all the
individuals of a varying species throughout the area in the same
manner in relation to the same conditions. Intercrosses, also, with
the individuals of the same species, which otherwise would have
inhabited the surrounding and differently circumstanced districts,
will be prevented. But isolation probably acts more efficiently in
checking the immigration of better adapted organisms, after any
physical change, such as of climate or elevation of the land, c.;
and thus new places in the natural economy of the country
are left open for the old inhabitants to struggle for, and become
adapted to, through modifications in their structure and constitution.
Lastly, isolation, by checking immigration and consequently
competition, will give time for any new variety to be slowly improved;
and this may sometimes be of importance in the production of new
species. If, however, an isolated area be very small, either from
being surrounded by barriers, or from having very peculiar physical
conditions, the total number of the individuals supported on it will
necessarily be very small; and fewness of individuals will greatly
retard the production of new species through natural selection, by
decreasing the chance of the appearance of favourable variations.
If we turn to nature to test the truth of these remarks, and look
at any small isolated area, such as an oceanic island, although the
total number of the species inhabiting it, will be found to be small,
as we shall see in our chapter on geographical distribution; yet of
these species a very large proportion are endemic, — that is,
have been produced there, and nowhere else. Hence an oceanic island at
first sight seems to have been highly favourable for the production of
new species. But we may thus greatly deceive ourselves, for to
ascertain whether a small isolated area, or a large open area like a
continent, has been most favourable for the production of new organic
forms, we ought to make the comparison within equal times; and this we
are incapable of doing.
Although I do not doubt that isolation is of considerable importance
in the production of new species, on the whole I am inclined to
believe that largeness of area is of more importance, more especially
in the production of species, which will prove capable of enduring for
a long period, and of spreading widely. Throughout a great and open
area, not only will there be a better chance of favourable variations
arising from the large number of individuals of the same species
there
supported, but the conditions of life are infinitely complex from the
large number of already existing species; and if some of these many
species become modified and improved, others will have to be improved
in a corresponding degree or they will be exterminated. Each new form,
also, as soon as it has been much improved, will be able
to spread over the open and continuous area, and will thus come into
competition with many others. Hence more new places will be formed,
and the competition to fill them will be more severe, on a large than
on a small and isolated area. Moreover, great areas, though now
continuous, owing to oscillations of level, will often have recently
existed in a broken condition, so that the good effects of isolation
will generally, to a certain extent, have concurred. Finally, I
conclude that, although small isolated areas probably have been in
some respects highly favourable for the production of new species, yet
that the course of modification will generally have been more rapid on
large areas; and what is more important, that the new forms produced
on large areas, which already have been victorious over many
competitors, will be those that will spread most widely, will give
rise to most new varieties and species, and will thus play an
important part in the changing history of the organic world.
We can, perhaps, on these views, understand some facts which will
be again alluded to in our chapter on geographical distribution; for
instance, that the productions of the smaller continent of Australia
have formerly yielded, and apparently are now yielding, before those
of the larger Europaeo-Asiatic area. Thus, also, it is that
continental productions have everywhere become so largely naturalised
on islands. On a small island, the race for life will have been less
severe, and there will have been less modification and less
extermination.
Hence, perhaps, it comes that the flora of Madeira,
according to Oswald Heer, resembles the extinct tertiary flora of
Europe. All fresh-water basins, taken together, make a small area
compared with that of the sea or of the land; and, consequently, the
competition between fresh-water productions will have been less severe
than elsewhere; new forms will have been more slowly formed, and old
forms more slowly exterminated. And it is in fresh water that we find
seven genera of Ganoid fishes, remnants of a once preponderant order:
and in fresh water we find some of the most anomalous forms now known
in the world, as the Ornithorhynchus and Lepidosiren, which, like
fossils, connect to a certain extent orders now widely separated in
the natural scale. These anomalous forms may almost be called living
fossils; they have endured to the present day, from
having inhabited a confined area, and from having thus been exposed to
less severe competition.
To sum up the circumstances favourable and unfavourable to natural
selection, as far as the extreme intricacy of the subject permits. I
conclude, looking to the future, that for terrestrial productions a
large continental area, which will probably undergo many oscillations
of level, and which consequently will exist for long periods in a
broken condition, will be the most favourable for the production of
many new forms of life, likely to endure long and to spread widely.
For the area will first have existed as a continent, and the
inhabitants, at this period numerous in individuals and kinds, will
have been subjected to very severe competition. When converted by
subsidence into large separate islands, there will still exist many
individuals of the same species on each island: intercrossing on the
confines of the range of each species will thus be checked: after
physical changes of any kind, immigration will be prevented,
so that
new places in the polity of each island will have to be filled up by
modifications of the old inhabitants; and time will be allowed for the
varieties in each to become well modified and perfected. When, by
renewed elevation, the islands shall be re-converted into a
continental area, there will again be severe competition: the most
favoured or improved varieties will be enabled to spread: there will
be much extinction of the less improved forms, and the relative
proportional numbers of the various inhabitants of the renewed
continent will again be changed; and again there will be a fair field
for natural selection to improve still further the inhabitants, and
thus produce new species.
That natural selection will always act with extreme slowness, I
fully admit. Its action depends on there being places in the polity of
nature, which can be better occupied by some of the inhabitants of the
country undergoing modification of some kind. The existence of such
places will often depend on physical changes, which are generally very
slow, and on the immigration of better adapted forms having been
checked. But the action of natural selection will probably still
oftener depend on some of the inhabitants becoming slowly modified;
the mutual relations of many of the other inhabitants
being thus disturbed. Nothing can be effected, unless favourable
variations occur, and variation itself is apparently always a very
slow process. The process will often be greatly retarded by free
intercrossing. Many will exclaim that these several causes are amply
sufficient wholly to stop the action of natural selection. I do not
believe so. On the other hand, I do believe that natural selection
will always act very slowly, often only at long intervals of time, and
generally on only a very few of the inhabitants of the same region at
the same time. I further believe, that this very slow, intermittent
action of natural selection accords perfectly well with what geology
tells us of the rate and manner at which the inhabitants of this world
have changed.
Slow though the process of selection may be, if feeble man can do
much by his powers of artificial selection, I can see no limit to the
amount of change, to the beauty and infinite complexity of the
coadaptations between all organic beings, one with another and with
their physical conditions of life, which may be effected in the long
course of time by nature's power of selection.
Extinction.
This subject will be more
fully discussed in our chapter on Geology; but it must be here alluded
to from being intimately connected with natural selection. Natural
selection acts solely through the preservation of variations in some
way advantageous, which consequently endure. But as from the high
geometrical powers of increase of all organic beings, each area is
already fully stocked with inhabitants, it follows that as each
selected and favoured form increases in number, so will the less
favoured forms decrease and become rare. Rarity, as geology tells us,
is the precursor to extinction. We can, also, see that any form
represented by few individuals will, during fluctuations in the
seasons or in the number of its enemies, run a good chance of utter
extinction. But we may go further than this; for as new forms are
continually and slowly being produced, unless we believe that the
number of specific forms goes on perpetually and almost indefinitely
increasing, numbers inevitably must become extinct. That the number of
specific forms has not indefinitely increased, geology
shows us plainly; and indeed we can see reason why they should not
have thus increased, for the number of places in the polity of nature
is not indefinitely great, — not that we
have any means of
knowing that any one region has as yet got its maximum of species.
probably no region is as yet fully stocked, for at the Cape of Good
Hope, where more species of plants are crowded together than in any
other quarter of the world, some foreign plants have become
naturalised, without causing, as far as we know, the extinction of any
natives.
Furthermore, the species which are most numerous in individuals
will have the best chance of producing within any given period
favourable variations. We have evidence of this, in the facts given in
the second chapter, showing that it is the common species which afford
the greatest number of recorded varieties, or incipient species.
Hence, rare species will be less quickly modified or improved within
any given period, and they will consequently be beaten in the race for
life by the modified descendants of the commoner species.
From these several considerations I think it inevitably follows,
that as new species in the course of time are formed through natural
selection, others will become rarer and rarer, and finally extinct.
The forms which stand in closest competition with those undergoing
modification and improvement, will naturally suffer most. And we have
seen in the chapter on the Struggle for Existence that it is the most
closely-allied forms, — varieties of the same species, and
species of the same genus or of related genera, — which, from
having nearly the same structure, constitution, and habits, generally
come into the severest competition with each other. Consequently, each
new variety or species, during the progress of its formation, will
generally press hardest on its nearest kindred, and tend to
exterminate them. We see the same process of extermination amongst our
domesticated productions, through the selection of improved forms by
man. Many curious
instances could be given showing how quickly new
breeds of cattle, sheep, and other animals, and varieties of flowers,
take the place of older and inferior kinds. In Yorkshire, it is
historically known that the ancient black cattle were displaced by the
long-horns, and that these 'were swept away by the
short-horns' (I quote the words of an agricultural writer) 'as if by
some murderous pestilence.'
Divergence of Character.
The principle,
which I have designated by this term, is of high importance on my
theory, and explains, as I believe, several important facts. In the
first place, varieties, even strongly-marked ones, though having
somewhat of the character of species — as is shown by the
hopeless doubts in many cases how to rank them — yet certainly
differ from each other far less than do good and distinct species.
Nevertheless, according to my view, varieties are species in the
process of formation, or are, as I have called them, incipient species.
How, then, does the lesser difference between varieties become
augmented into the greater difference between species? That this does
habitually happen, we must infer from most of the innumerable species
throughout nature presenting well-marked differences; whereas
varieties, the supposed prototypes and parents of future well-marked
species, present slight and ill-defined differences. Mere chance, as
we may call it, might cause one variety to differ in some character
from its parents, and the offspring of this variety again to differ
from its parent in the very same character and in a greater degree;
but this alone would never account for so habitual and large an amount
of difference as that between varieties of the same species and
species of the same genus.
As has always been my practice, let us seek light on
this head from
our domestic productions. We shall here find something analogous. A
fancier is struck by a pigeon having a slightly shorter beak; another
fancier is struck by a pigeon having a rather longer beak; and on the
acknowledged principle that 'fanciers do not and will not admire a
medium standard, but like extremes,' they both go on (as has actually
occurred with tumbler-pigeons) choosing and breeding from birds with
longer and longer beaks, or with shorter and shorter beaks. Again, we
may suppose that at an early period one man preferred swifter horses;
another stronger and more bulky horses. The early differences would be
very slight; in the course of time, from the continued selection of
swifter horses by some breeders, and of stronger ones by others, the
differences would become greater, and would be noted as forming two
sub-breeds; finally, after the lapse of centuries, the sub-breeds
would become converted into two well-established and distinct breeds.
As the differences slowly become greater, the inferior animals with
intermediate characters, being neither very swift nor very strong,
will have been neglected, and will have tended to disappear. Here,
then, we see in man's productions the action of what may be called the
principle of divergence, causing differences, at first barely
appreciable, steadily to increase, and the breeds to diverge in
character both from each other and from their common parent.
But how, it may be asked, can any analogous principle apply in
nature? I believe it can and does apply most efficiently, from the
simple circumstance that the more diversified the descendants from any
one species become in structure, constitution, and habits, by so much
will they be better enabled to seize on many and widely diversified
places in the polity of nature, and so be enabled to increase in
numbers.
We can clearly see this in the case of animals with simple
habits. Take the case of a carnivorous quadruped, of which the number
that can be supported in any country has long ago arrived at its full
average. If its natural powers of increase be allowed to act, it can
succeed in increasing (the country not undergoing any change in its
conditions) only by its varying descendants seizing on places at
present occupied by other animals: some of them, for instance, being
enabled to feed on new kinds of prey, either dead or alive; some
inhabiting new stations, climbing trees, frequenting water, and some
perhaps becoming less carnivorous. The more diversified in habits and
structure the descendants of our carnivorous animal became, the more
places they would be enabled to occupy. What applies to one animal
will apply throughout all time to all animals — that is, if they
vary — for otherwise natural selection can do nothing. So it
will be with plants. It has been experimentally proved, that if a plot
of ground be sown with several distinct genera of grasses, a greater
number of plants and a greater weight of dry herbage can thus be
raised. The same has been found to hold good when first one
variety and then
several mixed varieties of wheat have been sown on equal spaces of
ground. Hence, if any one species of grass were to go on varying, and
those varieties were continually selected which differed from each
other in at all the same manner as distinct species and genera of
grasses differ from each other, a greater number of individual plants
of this species of grass, including its modified descendants, would
succeed in living on the same piece of ground. And we well know that
each species and each variety of grass is annually sowing almost
countless seeds; and thus, as it may be said, is striving its utmost
to increase its numbers. Consequently, I cannot doubt that in the
course of many thousands of generations, the most distinct varieties
of any one species of grass would always have the best chance of
succeeding and of increasing in numbers, and thus of supplanting the
less distinct varieties; and varieties, when rendered very distinct
from each other, take the rank of species.
The truth of the principle, that the
greatest amount of life can be supported by great diversification of
structure, is seen under many natural circumstances. In an extremely
small area, especially if freely open to immigration, and where the
contest between individual and individual must be severe, we always
find great diversity in its inhabitants. For instance, I found that a
piece of turf, three feet by four in size, which had been exposed for
many years to exactly the same conditions, supported twenty species of
plants, and these belonged to eighteen genera and to eight orders,
which shows how much these plants differed from each other. So it is
with the plants and insects on small and uniform islets; and so in
small ponds of fresh water. Farmers find that they can raise most food
by a rotation of plants belonging to the most different orders: nature
follows what may be called a simultaneous rotation. Most of the
animals and plants which live close round any small piece of ground,
could live on it (supposing it not to be in any way peculiar in its
nature), and may be said to be striving to the utmost to live there;
but, it is seen, that where they come into the closest competition
with each other, the advantages of diversification of structure, with
the accompanying differences of habit and constitution, determine that
the inhabitants, which thus jostle each other most
closely, shall, as a general rule, belong to what we call different
genera and orders.
The same principle is seen in the naturalisation of
plants through
man's agency in foreign lands. It might have been expected that the
plants which have succeeded in becoming naturalised in any land would
generally have been closely allied to the indigenes; for these are
commonly looked at as specially created and adapted for their own
country. It might, also, perhaps have been expected that naturalised
plants would have belonged to a few groups more especially adapted to
certain stations in their new homes. But the case is very different;
and Alph. De Candolle has well remarked in his great and admirable
work, that floras gain by naturalisation, proportionally with the
number of the native genera and species, far more in new genera than
in new species. To give a single instance:
in the last edition of Dr Asa Gray's 'Manual of the Flora of the
Northern United States,' 260 naturalised plants are enumerated, and
these belong to 162 genera. We thus see that these naturalised plants
are of a highly diversified nature. They differ, moreover, to a large
extent from the indigenes, for out of the 162 genera, no less than 100
genera are not there indigenous, and thus a large proportional
addition is made to the genera of these States.
By considering the nature of the plants or animals which have
struggled successfully with the indigenes of any country, and have
there become naturalised, we can gain some crude idea in what manner
some of the natives would have had to be modified, in order to have
gained an advantage over the other natives; and we may, I think, at
least safely infer that diversification of structure, amounting to new
generic differences, would have been profitable to them.
The advantage of diversification in the inhabitants of the same
region is, in fact, the same as that of the physiological division of
labour in the organs of the same individual body — a subject so
well elucidated by
Milne Edwards. No physiologist doubts that a
stomach by being adapted to digest vegetable matter alone, or flesh
alone, draws most nutriment from these substances. So in the general
economy of any land, the more widely and perfectly the animals and
plants are diversified for different habits of life, so
will a greater number of individuals be capable of there supporting
themselves. A set of animals, with their organisation but little
diversified, could hardly compete with a set more perfectly
diversified in structure. It may be doubted, for instance, whether the
Australian marsupials, which are divided into groups differing but
little from each other, and feebly representing, as Mr Waterhouse and
others have remarked, our carnivorous, ruminant, and rodent mammals,
could successfully compete with these well-pronounced orders. In the
Australian mammals, we see the process of diversification in an early
and incomplete stage of development.
After the foregoing discussion, which ought to have been much
amplified, we may, I think, assume that the modified descendants of
any one species will succeed by so much the better as they become more
diversified in structure, and are thus enabled to encroach on places
occupied by other beings. Now let us see how this principle of great
benefit being derived from divergence of character, combined with the
principles of natural selection and of extinction, will tend to act.
The accompanying diagram will aid us in
understanding this rather perplexing subject. Let A to L represent the
species of a genus large in its own country; these species are
supposed to resemble each other in unequal degrees, as is so generally
the case in nature, and as is represented in the diagram by the
letters standing at unequal distances. I have said a large genus,
because we have seen in the second chapter,
that on an average more of
the species of large genera vary than of small genera; and the varying
species of the large genera present a greater number of varieties. We
have, also, seen that the species, which are the commonest and the
most widely-diffused, vary more than rare species with restricted
ranges. Let (A) be a common, widely-diffused, and varying species,
belonging to a genus large in its own country. The little fan of
diverging dotted lines of unequal lengths proceeding from (A), may
represent its varying offspring. The variations are supposed to be
extremely slight, but of the most diversified nature; they are not
supposed all to appear simultaneously, but often after long intervals
of time; nor are they all supposed to endure for equal periods. Only
those variations which are in some way profitable will be
preserved or naturally selected. And here the importance of the
principle of benefit being derived from divergence of character comes
in; for this will generally lead to the most different or divergent
variations (represented by the outer dotted lines) being preserved and
accumulated by natural selection. When a dotted line reaches one of
the horizontal lines, and is there marked by a small numbered letter,
a sufficient amount of variation is supposed to have been accumulated
to have formed a fairly well-marked variety, such as would be thought
worthy of record in a systematic work.
The intervals between the horizontal lines in the diagram, may
represent each a thousand generations; but it would have been better
if each had represented ten thousand generations. After a thousand
generations, species (A) is supposed to have produced two fairly
well-marked varieties, namely a1
and m1. These two varieties will
generally continue to be exposed to the same conditions which made
their parents variable,
and the tendency to variability is in itself
hereditary, consequently they will tend to vary, and generally to vary
in nearly the same manner as their parents varied. Moreover, these two
varieties, being only slightly modified forms, will tend to inherit
those advantages which made their common parent (A) more numerous than
most of the other inhabitants of the same country; they will likewise
partake of those more general advantages which made the genus to which
the parent-species belonged, a large genus in its own country. And
these circumstances we know to be favourable to the production of new
varieties.
If, then, these two varieties be variable, the most divergent of
their variations will generally be preserved during the next thousand
generations. And after this interval, variety
a1 is
supposed in the diagram to have produced variety a2, which will, owing to the
principle of divergence, differ more from (A) than did variety
a1.
Variety m1 is supposed to have
produced two varieties, namely m
2 and s2,
differing from each other, and more
considerably from their common parent (A). We may continue the process
by similar steps for any length of time; some of the varieties, after
each thousand generations, producing only a single
variety, but in a more and more modified condition, some producing two
or three varieties, and some failing to produce any. Thus the
varieties or modified descendants, proceeding from the common parent
(A), will generally go on increasing in number and diverging in
character. In the diagram the process is represented up to the
ten-thousandth generation, and under a condensed and simplified form
up to the fourteen-thousandth generation.
But I must here remark that I do not suppose that the process ever goes
on so regularly as is represented in the diagram, though in itself
made somewhat irregular.
I am far from thinking that the most
divergent varieties will invariably prevail and multiply: a medium
form may often long endure, and may or may not produce more than one
modified descendant; for natural selection will always act according
to the nature of the places which are either unoccupied or not
perfectly occupied by other beings; and this will depend on infinitely
complex relations. But as a general rule, the more diversified in
structure the descendants from any one species can be rendered, the
more places they will be enabled to seize on, and the more their
modified progeny will be increased. In our diagram the line of
succession is broken at regular intervals by small numbered letters
marking the successive forms which have become sufficiently distinct
to be recorded as varieties. But these breaks are imaginary, and
might have been inserted anywhere, after intervals long enough to have
allowed the accumulation of a considerable amount of divergent
variation.
As all the modified descendants from a common and widely-diffused
species, belonging to a large genus, will tend to partake of the same
advantages which made their parent successful in life, they will
generally go on multiplying in number as well as diverging in
character: this is represented in the diagram by the several divergent
branches proceeding from (A). The modified offspring from the later
and more highly improved branches in the lines of descent, will, it is
probable, often take the place of, and so destroy, the earlier and
less improved branches: this is represented in the diagram by some of
the lower branches not reaching to the upper horizontal lines. In some
cases I do not doubt that the process of modification
will be confined to a single line of descent, and the number of the
descendants will not be increased; although the amount
of divergent
modification may have been increased in the successive generations.
This case would be represented in the diagram, if all the lines
proceeding from (A) were removed, excepting that from a1 to
a
10
In the same way, for instance, the English race-horse and English
pointer have apparently both gone on slowly diverging in character
from their original stocks, without either having given off any fresh
branches or races.
After ten thousand generations, species (A) is supposed to have
produced three forms, a10,
f10,
and m
10,
which, from having diverged in character
during the successive generations, will have come to differ largely,
but perhaps unequally, from each other and from their common parent.
If we suppose the amount of change between each horizontal line in our
diagram to be excessively small, these three forms may still be only
well-marked varieties; or they may have arrived at the doubtful
category of sub-species; but we have only to suppose the steps in the
process of modification to be more numerous or greater in amount, to
convert these three forms into well-defined species: thus the diagram
illustrates the steps by which the small differences distinguishing
varieties are increased into the larger differences distinguishing
species. By continuing the same process for a greater number of
generations (as shown in the diagram in a condensed and simplified
manner), we get eight species, marked by the letters between
a14 and
m14, all descended from (A).
Thus, as I believe, species are multiplied and genera are formed.
In a large genus it is probable that more than one species would
vary. In the diagram I have assumed that a second species (I) has
produced, by analogous steps, after ten thousand generations, either
two well-marked varieties (w10 and z10) or two species, according to the amount of change
supposed to be represented between the horizontal lines. After
fourteen thousand generations, six new species, marked by the letters
n14 to z14, are supposed to have been
produced. In each genus, the species, which are already extremely
different in character, will generally tend to produce the greatest
number of modified descendants; for these will have the
best chance of filling new and widely different places in the polity
of nature: hence in the diagram I have chosen the extreme species (A),
and the nearly extreme species (I), as those which have largely
varied, and have given rise to new varieties and species. The other
nine species (marked by capital letters) of our original genus, may
for a long period continue transmitting unaltered descendants; and
this is shown in the diagram by the dotted lines not prolonged far
upwards from want of space.
But during the process of modification, represented in the diagram,
another of our principles, namely that of extinction, will have played
an important part. As in each fully stocked country natural selection
necessarily acts by the selected form having some advantage in the
struggle for life over other forms, there will be a constant tendency
in the improved descendants of any one species to supplant and
exterminate in each stage of descent their predecessors and their
original parent. For it should be remembered that the competition will
generally be most severe between those forms which are most nearly
related to each other in habits, constitution, and structure. Hence
all the intermediate forms between the earlier and later states, that
is between the less and more improved state of a species, as well as
the original parent-species itself, will generally tend to become
extinct. So it probably will be with many whole collateral lines of
descent, which will be conquered by later and improved lines of
descent. If, however, the
modified offspring of a species get into
some distinct country, or become quickly adapted to some quite new
station, in which child and parent do not come into competition, both
may continue to exist.
If then our diagram be assumed to represent a considerable amount
of modification, species (A) and all the earlier varieties will have
become extinct, having been replaced by eight new species (a14 to m14); and (I) will have been
replaced by six (n14 to
z14) new species.
But we may go further than this. The original species of our genus
were supposed to resemble each other in unequal degrees, as is so
generally the case in nature; species (A) being more nearly related to
B, C, and D, than to the other species; and species (I)
more to G, H, K, L, than to the others. These two species (A) and (I),
were also supposed to be very common and widely diffused species, so
that they must originally have had some advantage over most of the
other species of the genus. Their modified descendants, fourteen in
number at the fourteen-thousandth generation, will probably have
inherited some of the same advantages: they have also been modified
and improved in a diversified manner at each stage of descent, so as
to have become adapted to many related places in the natural economy
of their country. It seems, therefore, to me extremely probable that
they will have taken the places of, and thus exterminated, not only
their parents (A) and (I), but likewise some of the original species
which were most nearly related to their parents. Hence very few of
the original species will have transmitted offspring to the
fourteen-thousandth generation. We may suppose that only one (F), of
the two species which were least closely related to the other nine
original species, has transmitted descendants to this late stage of
descent.
The new species in our diagram descended from the original eleven
species, will now be fifteen in number. Owing to the divergent
tendency of natural selection, the extreme amount of difference in
character between species a14 and z14 will be much greater than that between the most
different of the original eleven species. The new species, moreover,
will be allied to each other in a widely different manner. Of the
eight descendants from (A) the three marked a14, q14, p14, will be nearly related
from having recently branched off from a14; b14
and f14, from having
diverged at an earlier period from a5, will be in some degree distinct from the three
first-named species; and lastly, o14, e14,
and m14, will be nearly
related one to the other, but from having diverged at the first
commencement of the process of modification, will be widely different
from the other five species, and may constitute a sub-genus or even a
distinct genus. The six descendants from (I) will form two sub-genera or even genera. But
as the original species (I) differed largely from (A), standing nearly
at the extreme points of the original genus, the six descendants from
(I) will, owing to inheritance, differ considerably from the eight
descendants from (A); the two groups, moreover, are
supposed to have gone on diverging in different directions. The
intermediate species, also (and this is a very important
consideration), which connected the original species (A) and (I), have
all become, excepting (F), extinct, and have left no descendants.
Hence the six new species descended from (I), and the eight descended
from (A), will have to be ranked as very distinct genera, or even as
distinct sub-families.
Thus it is, as I believe, that two or more genera are produced by
descent, with modification, from two or more species of the same
genus. And the two or
more parent-species are supposed to have
descended from some one species of an earlier genus. In our diagram,
this is indicated by the broken lines, beneath the capital letters,
converging in sub-branches downwards towards a single point; this
point representing a single species, the supposed single parent of our
several new sub-genera and genera.
It is worth while to reflect for a moment on the character of the
new species F14, which is supposed not to have
diverged much in character, but to have retained the form of (F),
either unaltered or altered only in a slight degree. In this case, its
affinities to the other fourteen new species will be of a curious and
circuitous nature. Having descended from a form which stood between
the two parent-species (A) and (I), now supposed to be extinct and
unknown, it will be in some degree intermediate in character between
the two groups descended from these species. But as these two groups
have gone on diverging in character from the type of their parents,
the new species (F14) will not be directly
intermediate between them, but rather between types of the two groups;
and every naturalist will be able to bring some such case before his
mind.
In the diagram, each horizontal line has hitherto been supposed to
represent a thousand generations, but each may represent a million or
hundred million generations, and likewise a section of the successive
strata of the earth's crust including extinct remains. We shall, when
we come to our chapter on Geology, have to refer again to this
subject, and I think we shall then see that the diagram throws light on
the affinities of extinct beings, which, though generally belonging to
the same orders, or families, or genera, with those now living, yet
are often, in some degree, intermediate in character
between existing groups; and we can understand this fact, for
the
extinct species lived at very ancient epochs when the branching lines
of descent had diverged less.
I see no reason to limit the process of modification, as now
explained, to the formation of genera alone. If, in our diagram, we
suppose the amount of change represented by each successive group of
diverging dotted lines to be very great, the forms marked a214 to p14, those marked b14 and f14, and those marked o14 to m14, will form three very distinct genera. We shall also
have two very distinct genera descended from (I) and as these latter
two genera, both from continued divergence of character and from
inheritance from a different parent, will differ widely from the three
genera descended from (A), the two little groups of genera will form
two distinct families, or even orders, according to the amount of
divergent modification supposed to be represented in the diagram. And
the two new families, or orders, will have descended from two species
of the original genus; and these two species are supposed to have
descended from one species of a still more ancient and unknown genus.
We have seen that in each country it is the species of the larger
genera which oftenest present varieties or incipient species. This,
indeed, might have been expected; for as natural selection acts
through one form having some advantage over other forms in the
struggle for existence, it will chiefly act on those which already
have some advantage; and the largeness of any group shows that its
species have inherited from a common ancestor some advantage in
common. Hence, the struggle for the production of new and modified
descendants, will mainly lie between the larger groups, which are all
trying to increase in number. One large group will slowly conquer
another large group, reduce its numbers, and thus lessen its chance of
further variation and improvement. Within the same large
group, the
later and more highly perfected sub-groups, from branching out and
seizing on many new places in the polity of Nature, will constantly
tend to supplant and destroy the earlier and less improved sub-groups.
Small and broken groups and sub-groups will finally tend to disappear.
Looking to the future, we can predict that the groups of
organic beings which are now large and triumphant, and which are least
broken up, that is, which as yet have suffered least extinction, will
for a long period continue to increase. But which groups will
ultimately prevail, no man can predict; for we well know that many
groups, formerly most extensively developed, have now become extinct.
Looking still more remotely to the future, we may predict that, owing
to the continued and steady increase of the larger groups, a multitude
of smaller groups will become utterly extinct, and leave no modified
descendants; and consequently that of the species living at any one
period, extremely few will transmit descendants to a remote futurity.
I shall have to return to this subject in the chapter on
Classification, but I may add that on this view of extremely few of
the more ancient species having transmitted descendants, and on the
view of all the descendants of the same species making a class, we can
understand how it is that there exist but very few classes in each
main division of the animal and vegetable kingdoms. Although extremely
few of the most ancient species may now have living and modified
descendants, yet at the most remote geological period, the earth may
have been as well peopled with many species of many genera, families,
orders, and classes, as at the present day.
Summary of Chapter.
If during the long
course of ages and under varying conditions of life, organic beings
vary at all in the several parts of their organisation, and I think this
cannot be disputed; if there be, owing to the high geometrical powers
of increase of each species, at some age, season, or year, a severe
struggle for life, and this certainly cannot be disputed; then,
considering the infinite complexity of the relations of all organic
beings to each other and to their conditions of existence, causing an
infinite diversity in structure, constitution, and habits, to be
advantageous to them, I think it would be a most extraordinary fact if
no variation ever had occurred useful to each being's own welfare, in
the same way as so many variations have occurred useful to man. But if
variations useful to any organic being do occur, assuredly individuals
thus characterised will have the best chance of being preserved in the
struggle for life; and from the strong principle of
inheritance they will tend to produce offspring similarly
characterised. This principle of preservation, I have called, for the
sake of brevity, Natural Selection. Natural selection, on the
principle of qualities being inherited at corresponding ages, can
modify the egg, seed, or young, as easily as the adult. Amongst many
animals, sexual selection will give its aid to ordinary selection, by
assuring to the most vigorous and best adapted males the greatest
number of offspring. Sexual selection will also give characters useful
to the males alone, in their struggles with other males.
Whether natural selection has really thus acted in nature, in
modifying and adapting the various forms of life to their several
conditions and stations, must be judged of by the general tenour and
balance of evidence given in the following chapters. But we already
see how it entails extinction; and how largely extinction has acted in
the world's history, geology plainly declares. Natural selection,
also, leads to divergence of
character; for more living beings can be
supported on the same area the more they diverge in structure, habits,
and constitution, of which we see proof by looking at the inhabitants
of any small spot or at naturalised productions. Therefore during the
modification of the descendants of any one species, and during the
incessant struggle of all species to increase in numbers, the more
diversified these descendants become, the better will be their
chance of succeeding in the battle of life. Thus the small differences
distinguishing varieties of the same species, will steadily tend to
increase till they come to equal the greater differences between
species of the same genus, or even of distinct genera.
We have seen that it is the common, the widely-diffused, and
widely-ranging species, belonging to the larger genera, which vary
most; and these will tend to transmit to their modified offspring
that superiority which now makes them dominant in their own countries.
Natural selection, as has just been remarked, leads to divergence of
character and to much extinction of the less improved and intermediate
forms of life. On these principles, I believe, the nature of the
affinities of all organic beings may be explained. It is a truly
wonderful fact — the wonder of which we are apt to overlook from
familiarity — that all animals and all plants
throughout all time and space should be related to each other in group
subordinate to group, in the manner which we everywhere behold --
namely, varieties of the same species most closely related together,
species of the same genus less closely and unequally related together,
forming sections and sub-genera, species of distinct genera much less
closely related, and genera related in different degrees, forming
sub-families, families, orders, sub-classes, and classes. The several
subordinate groups in any class cannot be
ranked in a single file, but
seem rather to be clustered round points, and these round other
points, and so on in almost endless cycles. On the view that each
species has been independently created, I can see no explanation of
this great fact in the classification of all organic beings; but, to
the best of my judgment, it is explained through inheritance and the
complex action of natural selection, entailing extinction and
divergence of character, as we have seen illustrated in the diagram.
The affinities of all the beings of the same class have sometimes
been represented by a great tree. I believe this simile largely speaks
the truth. The green and budding twigs may represent existing species;
and those produced during each former year may represent the long
succession of extinct species. At each period of growth all the
growing twigs have tried to branch out on all sides, and to overtop
and kill the surrounding twigs and branches, in the same manner as
species and groups of species have tried to overmaster other species
in the great battle for life. The limbs divided into great branches,
and these into lesser and lesser branches, were themselves once, when
the tree was small, budding twigs; and this connexion of the former
and present buds by ramifying branches may well represent the
classification of all extinct and living species in groups subordinate
to groups. Of the many twigs which flourished when the tree was a mere
bush, only two or three, now grown into great branches, yet survive
and bear all the other branches; so with the species which lived
during long-past geological periods, very few now have living and
modified descendants. From the first growth of the tree, many a limb
and branch has decayed and dropped off; and these lost branches of
various sizes may represent those whole orders, families,
and genera which have now no living representatives, and
which are
known to us only from having been found in a fossil state. As we here
and there see a thin straggling branch springing from a fork low down
in a tree, and which by some chance has been favoured and is still
alive on its summit, so we occasionally see an animal like the
Ornithorhynchus or Lepidosiren, which in some small degree connects by
its affinities two large branches of life, and which has apparently
been saved from fatal competition by having inhabited a protected
station. As buds give rise by growth to fresh buds, and these, if
vigorous, branch out and overtop on all sides many a feebler branch,
so by generation I believe it has been with the great Tree of Life,
which fills with its dead and broken branches the crust of the earth,
and covers the surface with its ever branching and beautiful
ramifications.
LAWS OF VARIATION
- Effects of external conditions
- Use and disuse, combined
with natural selection; organs of flight and of vision
- Acclimatisation
- Correlation of growth
- Compensation and economy of growth
- False correlations
- Multiple, rudimentary, and lowly organised structures variable
- Parts developed in an unusual manner are highly variable:
specific character more variable than generic: secondary sexual characters
variable
- Species of the same genus vary in an analogous manner
- Reversions to long-lost characters
- Summary
I HAVE hitherto sometimes spoken as if the variations
-- so common and multiform in organic beings under domestication,
and in a lesser degree in those in a state of nature — had been
due to chance. This, of course, is a wholly incorrect expression, but
it serves to acknowledge plainly our ignorance of the cause of each
particular variation. Some authors believe it to be as much the
function of the reproductive system to produce individual differences,
or very slight deviations of structure, as to make the child like its
parents. But the much greater variability, as well as the greater
frequency of monstrosities, under domestication or cultivation, than
under nature, leads me to believe that deviations of structure are in
some way due to the nature of the conditions of life, to which the
parents and their more remote ancestors have been exposed during
several generations. I have remarked in the first chapter — but
a long catalogue of facts which cannot be here given would be
necessary to show the truth of the remark — that the
reproductive system is eminently susceptible to changes in the
conditions of life; and to
this system being functionally disturbed in
the parents, I chiefly attribute the varying or plastic condition of
the offspring. The male and female sexual elements seem to be affected
before that union takes place which is to form a new
being. In the case of 'sporting' plants, the bud, which in its
earliest condition does not apparently differ essentially from an
ovule, is alone affected. But why, because the reproductive system is
disturbed, this or that part should vary more or less, we are
profoundly ignorant. Nevertheless, we can here and there dimly catch a
faint ray of light, and we may feel sure that there must be some cause
for each deviation of structure, however slight.
How much direct effect difference of climate, food, c.,
produces on any being is extremely doubtful. My impression is, that
the effect is extremely small in the case of animals, but perhaps
rather more in that of plants. We may, at least, safely conclude that
such influences cannot have produced the many striking and complex
co-adaptations of structure between one organic being and another,
which we see everywhere throughout nature. Some little influence may
be attributed to climate, food, c.: thus, E. Forbes speaks
confidently that shells at their southern limit, and when living in
shallow water, are more brightly coloured than those of the same
species further north or from greater depths. Gould believes that
birds of the same species are more brightly coloured under a clear
atmosphere, than when living on islands or near the coast. So with
insects, Wollaston is convinced that residence near the sea affects
their colours. Moquin-Tandon gives a list of plants which when growing
near the sea-shore have their leaves in some degree fleshy, though not
elsewhere fleshy. Several other such cases could be given.
The fact of varieties of one species, when they range
into the zone
of habitation of other species, often acquiring in a very slight
degree some of the characters of such species, accords with our view
that species of all kinds are only well-marked and permanent
varieties. Thus the species of shells which are confined to tropical
and shallow seas are generally brighter-coloured than those confined
to cold and deeper seas. The birds which
are confined to continents are, according to Mr Gould,
brighter-coloured than those of islands. The insect-species confined
to sea-coasts, as every collector knows,
are often brassy or lurid. Plants which live exclusively on the
sea-side are very apt to have fleshy leaves. He who believes in the
creation of each species, will have to say
that this shell, for instance, was created with bright
colours for a warm sea; but that this other shell became
bright-coloured by variation when it ranged into warmer or shallower
waters.
When a variation is of the slightest use to a being, we cannot tell
how much of it to attribute to the accumulative action of natural
selection, and how much to the conditions of life. Thus, it is well
known to furriers that animals of the same species have thicker and
better fur the more severe the climate is under which they have lived;
but who can tell how much of this difference may be due to the
warmest-clad individuals having been favoured and preserved during
many generations, and how much to the direct action of the severe
climate? for it would appear that climate has some direct action on
the hair of our domestic quadrupeds.
Instances could be given of the same variety being produced under
conditions of life as different as can well be conceived; and, on the
other hand, of different varieties being produced from the same
species under the same conditions. Such facts show how indirectly
the
conditions of life must act. Again, innumerable instances are known to
every naturalist of species keeping true, or not varying at all,
although living under the most opposite climates. Such considerations
as these incline me to lay very little weight on the direct action of
the conditions of life. Indirectly, as already remarked, they seem to
play an important part in affecting the reproductive system, and in
thus inducing variability; and natural selection will then accumulate
all profitable variations, however slight, until they become plainly
developed and appreciable by us.
Effects of Use and Disuse.
From the
facts alluded to in the first chapter, I think there can be little
doubt that use in our domestic animals strengthens and enlarges
certain parts, and disuse diminishes them; and that such modifications
are inherited. Under free nature, we can have no standard of
comparison, by which to judge of the effects of long-continued use or
disuse, for we know not the parent-forms; but many animals have
structures which can be explained by the effects of disuse. As
Professor Owen has remarked, there is no greater anomaly
in nature than a bird that cannot fly; yet there are several in this
state. The logger-headed duck of South America can only flap along the
surface of the water, and has its wings in nearly the same condition
as the domestic Aylesbury duck. As the larger ground-feeding birds
seldom take flight except to escape danger, I believe that the nearly
wingless condition of several birds, which now inhabit or have lately
inhabited several oceanic islands, tenanted by no beast of prey, has
been caused by disuse. The ostrich indeed inhabits continents and is
exposed to danger from which it cannot escape by flight, but by
kicking it can defend itself from enemies, as well as any of the
smaller
quadrupeds. We may imagine that the early progenitor of the
ostrich had habits like those of a bustard, and that as natural
selection increased in successive generations the size and weight of
its body, its legs were used more, and its wings less, until they
became incapable of flight.
Kirby has remarked (and I have observed the same fact) that the
anterior tarsi, or feet, of many male dung-feeding beetles are very
often broken off; he examined seventeen specimens in his own
collection, and not one had even a relic left. In the Onites apelles
the tarsi are so habitually lost, that the insect has been described
as not having them. In some other genera they are present, but in a
rudimentary condition. In the Ateuchus or sacred beetle of the
Egyptians, they are totally deficient. There is not sufficient
evidence to induce us to believe that mutilations are ever inherited;
and I should prefer explaining the entire absence of the anterior
tarsi in Ateuchus, and their rudimentary condition in some other
genera, by the long-continued effects of disuse in their progenitors;
for as the tarsi are almost always lost in many dung-feeding beetles,
they must be lost early in life, and therefore cannot be much used by
these insects.
In some cases we might easily put down to disuse modifications of
structure which are wholly, or mainly, due to natural selection. Mr.
Wollaston has discovered the remarkable fact that 200 beetles, out of
the 550 species inhabiting Madeira, are so far deficient in wings that
they cannot fly; and that of the twenty-nine endemic genera, no less
than twenty-three genera have all their species in this condition!
Several facts, namely, that beetles in many parts of the
world are very frequently blown to sea and perish; that the beetles in
Madeira, as observed by Mr Wollaston, lie much concealed, until the
wind lulls and the sun shines; that the proportion of wingless beetles
is larger on the exposed Dezertas than in Madeira itself; and
especially the extraordinary fact, so strongly insisted on by Mr.
Wollaston, of the almost entire absence of certain large groups of
beetles, elsewhere excessively numerous, and which groups have habits
of life almost necessitating frequent flight; — these several
considerations have made me believe that the wingless condition of so
many Madeira beetles is mainly due to the action of natural selection,
but combined probably with disuse. For during thousands of successive
generations each individual beetle which flew least, either from its
wings having been ever so little less perfectly developed or from
indolent habit, will have had the best chance of surviving from not
being blown out to sea; and, on the other hand, those beetles which
most readily took to flight will oftenest have been blown to sea and
thus have been destroyed.
The insects in Madeira which are not ground-feeders, and which, as
the flower-feeding coleoptera and lepidoptera, must habitually use
their wings to gain their subsistence, have, as Mr. Wollaston suspects,
their wings not at all reduced, but even enlarged. This is quite
compatible with the action of natural selection. For when a new insect
first arrived on the island, the tendency of natural selection to
enlarge or to reduce the wings, would depend on whether a greater
number of individuals were saved by successfully battling with the
winds, or by giving up the attempt and rarely or never flying. As with
mariners ship-wrecked near a coast, it would have been better for the
good swimmers if they had been able to swim still further, whereas it
would have been better for the bad swimmers if they had not been able
to swim at all and had stuck to the wreck.
The eyes of moles and of some burrowing rodents are rudimentary in
size, and in some cases are quite covered up by skin and fur. This
state of the eyes is probably due to gradual reduction from disuse,
but aided perhaps by natural selection. In South America, a burrowing
rodent, the tuco-tuco, or Ctenomys, is even more subterranean in its
habits than the mole; and I was assured by a Spaniard,
who had often caught them, that they were frequently blind; one which
I kept alive was certainly in this condition, the cause, as appeared
on dissection, having been inflammation of the nictitating membrane.
As frequent inflammation of the eyes must be injurious to any animal,
and as eyes are certainly not indispensable to animals with
subterranean habits, a reduction in their size with the adhesion of
the eyelids and growth of fur over them, might in such case be an
advantage; and if so, natural selection would constantly aid the
effects of disuse.
It is well known that several animals, belonging to the most
different classes, which inhabit the caves of Styria and of Kentucky,
are blind. In some of the crabs the foot-stalk for the eye remains,
though the eye is gone; the stand for the telescope is there, though
the telescope with its glasses has been lost. As it is difficult to
imagine that eyes, though useless, could be in any way injurious to
animals living in darkness, I attribute their loss wholly to disuse.
In one of the blind animals, namely, the cave-rat, the eyes are of
immense size; and Professor Silliman thought that it regained, after
living some days in the light, some slight power of vision. In the
same manner as in Madeira the wings of some of the insects have been
enlarged, and the wings of others have been reduced by natural
selection aided by use and disuse, so in the case of the cave-rat
natural selection seems to have struggled with the loss of light and
to have increased the size of the eyes; whereas with all the other
inhabitants of the caves, disuse by itself seems to have done its
work.
It is difficult to imagine conditions of life more similar than
deep limestone caverns under a nearly similar climate; so that on the
common view of the blind animals having been separately created for
the American and European caverns, close similarity in their
organisation and affinities might have been expected; but, as Schidte
and others have remarked, this is not the case, and the cave-insects
of the two continents are not more closely allied than might have
been anticipated from the general resemblance of the other inhabitants
of North America and Europe. On my view we must suppose that American
animals, having ordinary powers of vision, slowly migrated by
successive generations from the outer world into the
deeper and deeper recesses of the Kentucky caves, as did European
animals into the caves of Europe. We have some evidence of this
gradation of habit; for, as Schidte remarks, 'animals not far remote
from ordinary forms, prepare the transition from light to darkness.
Next follow those that are constructed for twilight; and, last of all,
those destined for total darkness.' By the time that an animal had
reached, after numberless generations, the deepest recesses, disuse
will on this view have more or less perfectly obliterated its eyes,
and natural selection will often have effected other changes, such as
an increase in the length of the antennae or palpi, as a compensation
for blindness. Notwithstanding such modifications, we might expect
still to see in the cave-animals of America, affinities to the other
inhabitants of that continent, and in those of Europe, to the
inhabitants of the European continent. And this is the case with some
of the American cave-animals, as I hear from
Professor Dana; and some
of the European cave-insects are very closely allied to those of the
surrounding country. It would be most difficult to give any rational
explanation of the affinities of the blind cave-animals to the other
inhabitants of the two continents on the ordinary view of their
independent creation. That several of the inhabitants of the caves of
the Old and New Worlds should be closely related, we might expect from
the well-known relationship of most of their other productions. Far
from feeling any surprise that some of the cave-animals should be very
anomalous, as Agassiz has remarked in regard to the blind fish, the
Amblyopsis, and as is the case with the blind Proteus with reference
to the reptiles of Europe, I am only surprised that more wrecks of
ancient life have not been preserved, owing to the less severe
competition to which the inhabitants of these dark abodes will
probably have been exposed.
Acclimatisation.
Habit is hereditary
with plants, as in the period of flowering, in the amount of rain
requisite for seeds to germinate, in the time of sleep, c., and
this leads me to say a few words on acclimatisation. As it is
extremely common for species of the same genus to inhabit very hot and
very cold countries, and as I believe that all the
species of the same genus have descended from a single parent, if this
view be correct, acclimatisation must be readily effected during
long-continued descent. It is notorious that each species is adapted
to the climate of its own home: species from an arctic or even from a
temperate region cannot endure a tropical climate, or conversely. So
again, many succulent plants cannot endure a damp climate. But the
degree of adaptation of species to the climates under which they live
is often overrated.
We may infer this from our frequent inability to
predict whether or not an imported plant will endure our climate, and
from the number of plants and animals brought from warmer countries
which here enjoy good health. We have reason to believe that species
in a state of nature are limited in their ranges by the competition of
other organic beings quite as much as, or more than, by adaptation to
particular climates. But whether or not the adaptation be generally
very close, we have evidence, in the case of some few plants, of their
becoming, to a certain extent, naturally habituated to different
temperatures, or becoming acclimatised: thus the pines and
rhododendrons, raised from seed collected by Dr Hooker from trees
growing at different heights on the Himalaya were found in this
country to possess different constitutional powers of resisting cold.
Mr Thwaites informs me that he has observed similar facts in Ceylon,
and analogous observations have been made by Mr H. C. Watson on
European species of plants brought from the Azores to England. In
regard to animals, several authentic cases could be given of species
within historical times having largely extended their range from
warmer to cooler latitudes, and conversely; but we do not positively
know that these animals were strictly adapted to their native climate,
but in all ordinary cases we assume such to be the case; nor do we
know that they have subsequently become acclimatised to their new
homes.
As I believe that our domestic animals were originally chosen by
uncivilised man because they were useful and bred readily under
confinement, and not because they were subsequently found capable of
far-extended transportation, I think the common and extraordinary
capacity in our domestic animals of not only withstanding the most
different climates but of being perfectly
fertile (a far
severer test) under them, may be used as an argument that a large
proportion of other animals, now in a state of nature, could easily be
brought to bear widely different climates. We must not, however, push
the foregoing argument too far, on account of
the probable origin of some of our domestic
animals from several wild stocks: the blood, for instance, of a
tropical and arctic wolf or wild dog may perhaps be mingled in our
domestic breeds. The rat and mouse cannot be considered as domestic
animals, but they have been transported by man to many parts of the
world, and now have a far wider range than any other rodent, living
free under the cold climate of Faroe in the north and of the Falklands
in the south, and on many islands in the torrid zones. Hence I am
inclined to look at adaptation to any special climate as a quality
readily grafted on an innate wide flexibility of constitution, which
is common to most animals. On this view,
the capacity of enduring the most different
climates by man himself and by his domestic animals, and such facts as
that former species of the elephant and rhinoceros were capable of
enduring a glacial climate, whereas the living species are now
all tropical or sub-tropical in their habits,
ought not to be looked at as anomalies, but
merely as examples of a very common flexibility of constitution,
brought, under peculiar circumstances, into play.
How much of the acclimatisation of species to any peculiar climate
is due to mere habit, and how much to the natural selection of
varieties having different innate constitutions, and how much to
means combined, is a very obscure question.
That habit or custom has some influence I must believe, both from
analogy, and from the incessant advice given in agricultural works,
even in the ancient Encyclopaedias of China, to be very cautious
in
transposing animals from one district to another; for it is not likely
that man should have succeeded in selecting so many breeds and
sub-breeds with constitutions specially fitted for their own
districts: the result must, I think, be due to habit. On the other
hand, I can see no reason to doubt that
natural selection will continually tend to preserve those individuals
which are born with constitutions best adapted to their native
countries. In treatises on many kinds of cultivated plants, certain varieties are said to withstand certain climates
better than others: this is very strikingly shown in works on fruit
trees published in the United States, in which certain varieties are
habitually recommended for the northern, and others for the southern
States; and as most of these varieties are of recent origin, they
cannot owe their constitutional differences to habit. The case of the
Jerusalem artichoke, which is never propagated by seed, and of which
consequently new varieties have not been produced, has even been
advanced — for it is now as tender as ever it was — as
proving that acclimatisation cannot be effected! The case, also, of
the kidney-bean has been often cited for a similar purpose, and with
much greater weight; but until some one will sow, during a score of
generations, his kidney-beans so early that a very large proportion
are destroyed by frost, and then collect seed from the few survivors,
with care to prevent accidental crosses, and then again get seed from
these seedlings, with the same precautions, the experiment cannot be
said to have been even tried. Nor let it be supposed that no
differences in the constitution of seedling kidney-beans ever appear,
for an account has been published how much more hardy some seedlings
appeared to be than others.
On the whole, I think we may conclude that habit,
use, and disuse,
have, in some cases, played a considerable part in the modification of
the constitution, and of the structure of various organs; but that the
effects of use and disuse have often been largely combined with, and
sometimes overmastered by, the natural selection of innate
differences.
Correlation of Growth.
I mean by this
expression that the whole organisation is so tied together during its
growth and development, that when slight variations in any one part
occur, and are accumulated through natural selection, other parts
become modified. This is a very important subject, most imperfectly
understood. The most obvious case is, that modifications accumulated
solely for the good of the young or larva, will, it may safely be
concluded, affect the structure of the adult; in the same manner as
any malconformation affecting the early embryo, seriously affects the
whole organisation of the adult. The several parts of the
body which are homologous, and which, at an early embryonic period,
are alike, seem liable to vary in an allied manner: we see this in the
right and left sides of the body varying in the same manner; in the
front and hind legs, and even in the jaws
and limbs, varying together, for the lower jaw is believed to be
homologous with the limbs. These tendencies, I do not doubt, may be
mastered more or less completely by natural selection: thus a family
of stags once existed with an antler only on one side; and if this had
been of any great use to the breed it might
probably have been rendered permanent by natural selection.
Homologous parts, as has been remarked by some authors, tend to
cohere; this is often seen in monstrous plants; and nothing is more
common than the union of homologous parts in normal structures, as the
union of
the petals of the corolla into a
tube. Hard parts seem to affect the form of adjoining soft parts; it
is believed by some authors that the diversity in the shape of the
pelvis in birds causes the remarkable diversity in the shape of their
kidneys. Others believe that the shape of the pelvis in the human
mother influences by pressure the shape of the head of
the child. In snakes, according to Schlegel, the
shape of the body and the manner of swallowing determine the position
of several of the most important viscera.
The nature of the bond of correlation is very frequently quite
obscure. M. Is. Geoffroy St Hilaire has forcibly remarked, that
certain malconformations very frequently, and that others rarely
coexist, without our being able to assign any reason. What can be more
singular than the relation between blue eyes and deafness in cats, and
the tortoise-shell colour with the female sex; the feathered feet and
skin between the outer toes in pigeons, and the presence of more or
less down on the young birds when first hatched, with the future
colour of their plumage; or, again, the relation between the hair and
teeth in the naked Turkish dog, though here probably homology comes
into play? With respect to this latter case of correlation, I think it
can hardly be accidental, that if we pick out the two orders of
mammalia which are most abnormal in their dermal coverings, viz.
Cetacea (whales) and Edentata (armadilloes, scaly ant-eaters,
c.), that these are likewise the most abnormal in
their teeth.
I know of no case better adapted to show the importance of the laws
of correlation in modifying important structures, independently of
utility and, therefore, of natural selection, than that of the
difference between the outer and inner flowers in some Compositous and
Umbelliferous plants. Every one knows the
difference in the ray and
central florets of, for instance, the daisy, and this difference is
often accompanied with the abortion of parts of the flower. But, in
some Compositous plants, the seeds also differ in shape and sculpture;
and even the ovary itself, with its accessory parts, differs, as has
been described by Cassini. These differences have been attributed by
some authors to pressure, and the shape of the seeds in the
ray-florets in some Compositae countenances this idea; but, in the
case of the corolla of the Umbelliferae,
it is by no means, as Dr Hooker informs me, in species with the
densest heads that the inner and outer
flowers most frequently differ. It might have been thought that the
development of the ray-petals by drawing nourishment from certain
other parts of the flower had caused their abortion; but in some
Compositae there is a difference in the seeds of the outer and inner
florets without any difference in the corolla. Possibly, these several
differences may be connected with some difference in the flow of
nutriment towards the central and external flowers: we know, at least,
that in irregular flowers, those nearest to the axis are oftenest
subject to peloria, and become regular. I may add, as an instance of
this, and of a striking case of correlation, that I have recently
observed in some garden pelargoniums, that the central flower of the
truss often loses the patches of darker colour in the two upper
petals; and that when this occurs, the adherent nectary is quite
aborted; when the colour is absent from only one of the two upper
petals, the nectary is only much shortened.
With respect to the difference in the corolla of the central and
exterior flowers of a head or umbel, I do not feel at all sure that C.
C. Sprengel's idea that the ray-florets serve to attract insects,
whose agency is highly advantageous in the fertilisation of plants of
these two orders, is so far-fetched, as it may at first appear: and if
it be advantageous, natural selection may have come into
play. But in regard to the differences both in the internal and
external structure of the seeds, which are not always correlated with
any differences in the flowers, it seems impossible that they can be
in any way advantageous to the plant: yet in the Umbelliferae these
differences are of such apparent importance — the seeds being in
some cases, according to Tausch, orthospermous in the exterior flowers
and coelospermous in the central flowers, — that the elder De
Candolle founded his main divisions of the
order on analogous differences. Hence we see that modifications of
structure, viewed by systematists as of high value, may be wholly due
to unknown laws of correlated growth, and without being, as far as we
can see, of the slightest service to the species.
We may often falsely attribute to correlation of growth, structures
which are common to whole groups of species, and which in
truth are simply due to inheritance; for an
ancient progenitor may have acquired through natural selection some
one modification in structure, and, after thousands of generations,
some other and independent modification; and these two modifications,
having been transmitted to a whole group of descendants with diverse
habits, would naturally be thought to be correlated in some necessary
manner. So, again, I do not doubt that some
apparent correlations, occurring throughout whole orders, are entirely
due to the manner alone in which natural
selection can act. For instance, Alph. De Candolle has remarked that
winged seeds are never found in fruits which do not open: I should
explain the rule by the fact
that seeds could not gradually become winged
through natural selection, except in fruits which opened; so that the
individual plants producing
seeds which were a little better fitted to
be wafted further, might get an advantage over those producing seed
less fitted for dispersal; and this process could not possibly go on
in fruit which did not open.
The elder Geoffroy and Goethe propounded, at about
the same period, their law of compensation or
balancement of growth; or, as Goethe expressed it, 'in order to spend
on one side, nature is forced to economise on the other side.' I think
this holds true to a certain extent with our domestic productions: if
nourishment flows to one part or organ in excess, it rarely flows, at
least in excess, to another part; thus it is difficult to
get a cow to give much milk and to fatten readily. The same varieties
of the cabbage do not yield abundant and nutritious foliage and a
copious supply of oil-bearing seeds. When the seeds in our fruits
become atrophied, the fruit itself gains largely in size and quality.
In our poultry, a large tuft of feathers on the head is generally
accompanied by a diminished comb, and a large beard by diminished
wattles. With species in a state of nature it can hardly be
maintained that the law is of universal application; but many good
observers, more especially botanists, believe in its truth. I will
not, however, here give any instances, for I see hardly any way of
distinguishing between the effects, on the one hand, of a part being
largely developed through natural selection and another and adjoining
part being reduced by this same process or by disuse, and, on the
other hand, the actual withdrawal of nutriment from one part owing to
the excess of growth in another and adjoining part.
I suspect, also, that some of the cases of compensation which have
been advanced, and likewise some other facts, may be merged under a
more general principle, namely, that natural selection is continually
trying to economise in every part of the organisation. If under
changed conditions of life a structure before useful becomes less
useful, any diminution, however slight, in its development, will be
seized on by natural selection, for it will profit the individual not
to have its nutriment wasted in building up an useless structure. I
can thus only understand a fact with which I was much struck when
examining cirripedes, and of which many other instances could be
given: namely, that when a cirripede is parasitic within another and
is thus protected, it loses more or less
completely its own shell or carapace. This is the case with the male
Ibla, and in a truly extraordinary manner with the Proteolepas: for
the carapace in all other cirripedes consists of the three
highly-important anterior segments of the head enormously developed,
and furnished with great nerves and muscles;
but in the parasitic and protected Proteolepas,
the whole anterior part of the head is reduced to the merest rudiment
attached to the bases of the prehensile antennae. Now the saving of a
large and complex structure, when rendered superfluous by
the parasitic habits of the Proteolepas, though effected by slow
steps, would be a decided advantage to each successive individual of
the species; for in the struggle for life
to which every animal is exposed, each individual
Proteolepas would have a better chance of supporting itself, by less
nutriment being wasted in developing a structure now become useless.
Thus, as I believe, natural selection will always succeed in the
long run in reducing and saving every part of the organisation, as
soon as it is rendered superfluous, without by any means causing some
other part to be largely developed in a corresponding degree. And,
conversely, that natural selection may perfectly well succeed in
largely developing any organ, without requiring as a necessary
compensation the reduction of some adjoining part.
It seems to be a rule, as remarked by Is. Geoffroy St Hilaire, both
in varieties and in species, that when any part or organ is repeated
many times in the structure of the same individual (as the vertebrae
in snakes, and the stamens in polyandrous flowers) the number is
variable; whereas the number of the same part or organ, when it occurs
in lesser numbers, is constant. The same author and some botanists
have further remarked that multiple parts are also very liable to
variation in structure. Inasmuch as this 'vegetative repetition,' to
use Prof. Owen's expression, seems to be a sign of low organisation;
the foregoing remark seems connected with the very general opinion of
naturalists, that beings low in the scale of nature are more variable
than those which are higher. I presume that lowness in this case means
that the several parts of the organisation have been but little
specialised for particular functions; and as long as the same part has
to perform diversified work, we can perhaps see why it should remain
variable, that is, why natural selection should have preserved or
rejected each little deviation of form less carefully than when the
part has to serve for one special purpose alone. In the same way that
a knife which has to cut all sorts of things may be of almost any
shape; whilst a tool for some particular object had better be of some
particular shape. Natural selection, it should never be forgotten, can
act on each part of each being, solely through and for its advantage.
Rudimentary parts, it has been stated by some authors, and I
believe with truth, are apt to be highly variable. We shall have to
recur to the general subject of rudimentary and aborted organs; and I
will here only add that their variability seems to be owing to their
uselessness, and therefore to natural selection having no power to
check deviations in their structure. Thus
rudimentary parts are left
to the free play of the various laws of growth, to the effects of
long-continued disuse, and to the tendency to reversion.
A part developed in any species in an
extraordinary degree or manner, in comparison with the same part in
allied species, tends to be highly variable.
Several years ago
I was much struck with a remark, nearly to the above effect, published
by Mr Waterhouse. I infer also from an observation made by Professor
Owen, with respect to the length of the arms of the ourang-outang,
that he has come to a nearly similar conclusion. It is hopeless to
attempt to convince any one of the truth of this proposition without
giving the long array of facts which I have collected, and which
cannot possibly be here introduced. I can only state my conviction
that it is a rule of high generality. I am aware of several causes of
error, but I hope that I have made due allowance for them. It should
be understood that the rule by no means applies to any part, however
unusually developed, unless it be unusually developed in comparison
with the same part in closely allied species. Thus, the bat's wing is
a most abnormal structure in the class mammalia; but the rule would
not here apply, because there is a whole group of bats having wings;
it would apply only if some one species of bat had its wings developed
in some remarkable manner in comparison with the other species of the
same genus. The rule applies very strongly in the case of secondary
sexual characters, when displayed in any unusual manner. The term,
secondary sexual characters, used by Hunter, applies to characters
which are attached to one sex, but are not
directly connected with the act of reproduction. The rule applies to
males and females; but as females more rarely offer remarkable
secondary sexual characters, it applies
more rarely to them. The rule
being so plainly applicable in the case of secondary
sexual characters, may be due to the great variability of these
characters, whether or not displayed in any unusual manner — of
which fact I think there can be little doubt. But that our rule is not
confined to secondary sexual characters is clearly shown in the case
of hermaphrodite cirripedes; and I may here add, that I particularly
attended to Mr. Waterhouse's remark, whilst investigating this Order,
and I am fully convinced that the rule almost invariably holds good
with cirripedes. I shall, in my future work, give a list of the more
remarkable cases; I will here only briefly give one, as it illustrates
the rule in its largest application. The opercular valves of sessile
cirripedes (rock barnacles) are, in every sense of the word, very
important structures, and they differ extremely little even in
different genera; but in the several species of one genus, Pyrgoma,
these valves present a marvellous amount of diversification: the
homologous valves in the different species being sometimes wholly
unlike in shape; and the amount of variation in the individuals of
several of the species is so great, that it is no exaggeration to
state that the varieties differ more from each other in the characters
of these important valves than do other species of distinct genera.
As birds within the same country vary in a remarkably small degree,
I have particularly attended to them, and
the rule seems to me certainly to hold good in this class. I cannot
make out that it applies to plants, and this would seriously have
shaken my belief in its truth, had not the great variability in plants
made it particularly difficult to compare their relative degrees of
variability.
When we see any part or organ developed in a remarkable degree or
manner in any species, the fair
presumption is that it is of high
importance to that species; nevertheless the part in this case is
eminently liable to variation. Why should this be so? On the view that
each species has been independently created, with all its parts as we
now see them, I can see no explanation. But on the view that groups of
species have descended from other species, and have been modified
through natural selection, I think we can obtain some light. In our
domestic animals, if any part, or the whole animal, be neglected and
no selection be applied, that part (for instance, the comb in the
Dorking fowl) or the whole breed will cease to have a
nearly uniform character. The breed will then be said to have
degenerated. In rudimentary organs, and in those which have been but
little specialized for any particular purpose, and perhaps in
polymorphic groups, we see a nearly parallel natural case; for in such
cases natural selection either has not or cannot come into full play,
and thus the organisation is left in a fluctuating condition. But what
here more especially concerns us is, that in our domestic animals
those points, which at the present time are undergoing rapid change by
continued selection, are also eminently liable to variation. Look at
the breeds of the pigeon; see what a prodigious amount of difference
there is in the beak of the different tumblers, in the beak and wattle
of the different carriers, in the carriage and tail of our fantails,
c., these being the points now mainly attended to by English
fanciers. Even in the sub-breeds, as in the short-faced tumbler, it is
notoriously difficult to breed them nearly to perfection, and
frequently individuals are born which depart widely from the standard.
There may be truly said to be a constant struggle going on between, on
the one hand, the tendency to reversion to a less modified state, as
well as an innate tendency to further
variability of all kinds, and,
on the other hand, the power of steady selection to keep the breed
true. In the long run selection gains the day, and we do not expect to
fail so far as to breed a bird as coarse as a common tumbler from a
good short-faced strain. But as long as selection is rapidly going on,
there may always be expected to be much variability in the structure
undergoing modification. It further deserves notice that these
variable characters, produced by man's selection, sometimes become
attached, from causes quite unknown to us, more to one sex than to the
other, generally to the male sex, as with the wattle of carriers and
the enlarged crop of pouters.
Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other
species of the same genus, we may conclude that this part has
undergone an extraordinary amount of modification, since the period
when the species branched off from the common progenitor of the genus.
This period will seldom be remote in any extreme degree,
as species very rarely endure for more than one geological period. An
extraordinary amount of modification implies an unusually large and
long-continued amount of variability, which has continually been
accumulated by natural selection for the benefit of the species. But
as the variability of the extraordinarily-developed part or organ has
been so great and long-continued within a period not excessively
remote, we might, as a general rule, expect still to find more
variability in such parts than in other parts of the organisation,
which have remained for a much longer period nearly constant. And
this, I am convinced, is the case. That the struggle between natural
selection on the one hand, and the tendency to reversion and
variability on the other hand, will in the
course of time cease; and
that the most abnormally developed organs may be made constant, I can
see no reason to doubt. Hence when an
organ, however abnormal it may be, has been transmitted in
approximately the same condition to many modified descendants, as in
the case of the wing of the bat, it must have existed, according to my
theory, for an immense period in nearly the same state; and thus it
comes to be no more variable than any other structure. It is only in
those cases in which the modification has been comparatively recent
and extraordinarily great that we ought to find the generative variability, as it may be called,
still present in a high degree. For in this case the variability will
seldom as yet have been fixed by the continued selection of the
individuals varying in the required manner and degree, and by the
continued rejection of those tending to revert to a former and less
modified condition.
The principle included in these remarks may be extended. It is
notorious that specific characters are more variable than generic.
To explain by a simple example what is
meant. If some species in a large genus of plants had blue flowers and
some had red, the colour would be only a specific character, and no
one would be surprised at one of the blue species varying into red, or
conversely; but if all the species had blue flowers, the colour would
become a generic character, and its variation would be a more unusual
circumstance. I have chosen this example because an explanation is not
in this case applicable, which most naturalists would
advance, namely, that specific characters are more variable than
generic, because they are taken from parts of less physiological
importance than those commonly used for classing genera. I believe
this explanation is partly, yet only indirectly, true; I shall,
however, have to return
to this subject in our chapter on
Classification. It would be almost superfluous to adduce evidence in
support of the above statement, that specific characters are more
variable than generic; but I have repeatedly noticed in works on
natural history, that when an author has remarked with surprise that
some important organ or part, which is
generally very constant throughout large groups of species, has differed considerably in closely-allied species,
that it has, also, been variable in the
individuals of some of the species. And this fact shows that a
character, which is generally of generic value, when it sinks in value
and becomes only of specific value, often becomes variable, though its
physiological importance may remain the same. Something of the same
kind applies to monstrosities: at least Is. Geoffroy
St. Hilaire seems to entertain no doubt, that the
more an organ normally differs in the different species of the same
group, the more subject it is to individual anomalies.
On the ordinary view of each species having been independently
created, why should that part of the structure, which differs from the
same part in other independently-created species of the same genus, be
more variable than those parts which are closely alike in the several
species? I do not see that any explanation can be given. But on the
view of species being only strongly marked and fixed varieties, we
might surely expect to find them still often continuing to vary in
those parts of their structure which have varied within a moderately
recent period, and which have thus come to differ. Or to state the
case in another manner: — the points in which all the species of
a genus resemble each other, and in which they differ from the species
of some other genus, are called generic characters; and these
characters in common I attribute to inheritance from a common
progenitor, for it can rarely have happened that natural selection
will have modified several species, fitted to more or less
widely-different habits, in exactly the same manner: and as these
so-called generic characters have been inherited from a remote period, since that period when the species first branched off
from their common progenitor, and subsequently have not varied or come
to differ in any degree, or only in a slight degree, it is not
probable that they should vary at the present day. On the other hand,
the points in which species differ from other species of the same
genus, are called specific characters; and as these specific
characters have varied and come to differ within the period of the
branching off of the species from a common progenitor, it is probable
that they should still often be in some degree variable, — at
least more variable than those parts of the organisation which have
for a very long period remained constant.
In connexion with the present subject, I will make only two other
remarks. I think it will be admitted, without my entering on details,
that secondary sexual characters are very variable; I think it also
will be admitted that species of the same group differ from each other
more widely in their secondary sexual characters, than in other parts
of their organisation; compare, for instance, the amount of difference
between the males of gallinaceous birds, in which secondary sexual
characters are strongly displayed, with the amount of difference
between their females; and the truth of this proposition will be
granted. The cause of the original variability of secondary sexual
characters is not manifest; but we can see why these characters should
not have been rendered as constant and uniform as other parts of the
organisation; for secondary sexual characters have been accumulated by
sexual selection, which
is less rigid in its action than ordinary
selection, as it does not entail death, but only gives fewer offspring
to the less favoured males. Whatever the cause may be of the
variability of secondary sexual characters, as they are highly
variable, sexual selection will have had a wide scope for action, and
may thus readily have succeeded in giving to the species of the same
group a greater amount of difference in their sexual characters, than
in other parts of their structure.
It is a remarkable fact, that the secondary sexual differences
between the two sexes of the same species are generally displayed in
the very same parts of the organisation in which the different species
of the same genus differ from each other. Of this fact I will give in
illustration two instances, the first which happen to
stand on my list; and as the differences in these cases are of a very
unusual nature, the relation can hardly be accidental. The same number
of joints in the tarsi is a character generally common to very large
groups of beetles, but in the Engidae, as Westwood has remarked, the
number varies greatly; and the number likewise differs in the two
sexes of the same species: again in fossorial hymenoptera, the manner
of neuration of the wings is a character of the highest importance,
because common to large groups; but in certain genera the neuration
differs in the different species, and likewise in the two sexes of the
same species. This relation has a clear meaning on my view of the
subject: I look at all the species of the same genus as having as
certainly descended from the same progenitor, as have the two sexes of
any one of the species. Consequently, whatever part of the structure
of the common progenitor, or of its early descendants, became
variable; variations of this part would it is highly probable, be
taken advantage of by natural and sexual selection, in
order to fit
the several species to their several places in the economy of nature,
and likewise to fit the two sexes of the same species to each other,
or to fit the males and females to different habits of life, or the
males to struggle with other males for the possession of the females.
Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which the species possess in common;
-- that the frequent extreme variability of any part which is
developed in a species in an extraordinary manner in comparison with
the same part in its congeners; and the not great degree of
variability in a part, however extraordinarily it may be developed, if
it be common to a whole group of species; — that the great
variability of secondary sexual characters, and the great amount of
difference in these same characters between closely allied species;
-- that secondary sexual and ordinary specific differences are
generally displayed in the same parts of the organisation, — are
all principles closely connected together. All being mainly due to the
species of the same group having descended from a common progenitor,
from whom they have inherited much in common, — to parts which
have recently and largely varied being more likely still
to go on varying than parts which have long been inherited and have
not varied, — to natural selection having more or less
completely, according to the lapse of time, overmastered the tendency
to reversion and to further variability, — to sexual selection
being less rigid than ordinary selection, — and to variations in
the same parts having been accumulated by natural and sexual
selection, and thus adapted for secondary sexual, and for ordinary
specific purposes.
Distinct species present analogous variations;
and a variety of one species often assumes some of the characters of
an allied species, or reverts to some of the characters of an early
progenitor.
These propositions will be most readily understood
by looking to our domestic races. The most distinct breeds of pigeons,
in countries most widely apart, present sub-varieties with reversed
feathers on the head and feathers on the feet, — characters not
possessed by the aboriginal rock-pigeon; these then are analogous
variations in two or more distinct races. The frequent presence of
fourteen or even sixteen tail-feathers in the pouter, may be
considered as a variation representing the normal structure of another
race, the fantail. I presume that no one will doubt that all such
analogous variations are due to the several
races of the pigeon having inherited from a common parent the same
constitution and tendency to variation, when acted on by similar
unknown influences. In the vegetable kingdom we have a case of
analogous variation, in the enlarged stems, or roots as commonly
called, of the Swedish turnip and Ruta baga, plants which several
botanists rank as varieties produced by cultivation from a common
parent: if this be not so, the case will then be one of analogous
variation in two so-called distinct species; and to these a third may
be added, namely, the common turnip. According to the ordinary view of
each species having been independently created, we should have to
attribute this similarity in the enlarged stems of these three plants,
not to the vera causa of community of
descent, and a consequent tendency to vary in a like manner, but to
three separate yet closely related acts of creation.
With pigeons, however, we have another case, namely, the occasional appearance in all the breeds, of slaty-blue birds
with two black bars on the wings, a white
rump, a bar at the end of
the tail, with the outer feathers externally edged near their bases
with white. As all these marks are characteristic of the parent
rock-pigeon, I presume that no one will doubt that this is a case of
reversion, and not of a new yet analogous variation appearing in the
several breeds. We may I think confidently come to this conclusion,
because, as we have seen, these coloured marks are eminently liable to
appear in the crossed offspring of two distinct and differently
coloured breeds; and in this case there is nothing in the external
conditions of life to cause the reappearance of the slaty-blue, with
the several marks, beyond the influence of the mere act of crossing on
the laws of inheritance.
No doubt it is a very surprising fact that characters should
reappear after having been lost for many, perhaps for hundreds of
generations. But when a breed has been crossed only once by some other
breed, the offspring occasionally show a tendency to revert in
character to the foreign breed for many generations — some say,
for a dozen or even a score of generations. After twelve generations,
the proportion of blood, to use a common expression, of any one
ancestor, is only 1 in 2048; and yet, as we see, it is generally
believed that a tendency to reversion is retained by this very small
proportion of foreign blood. In a breed which has not been crossed,
but in which both parents have lost some
character which their progenitor possessed, the tendency, whether
strong or weak, to reproduce the lost character might be, as was
formerly remarked, for all that we can see to the contrary,
transmitted for almost any number of generations. When a character
which has been lost in a breed, reappears after a great number of
generations, the most probable hypothesis is, not that the offspring
suddenly takes after an ancestor some hundred generations
distant, but
that in each successive generation there has been a tendency to
reproduce the character in question, which at last, under unknown
favourable conditions, gains an ascendancy. For instance, it is
probable that in each generation of the barb-pigeon, which produces
most rarely a blue and black-barred bird, there has been a tendency in
each generation in the plumage to assume this colour.
This view is hypothetical, but could be supported by some facts; and I
can see no more abstract improbability in a tendency to produce any
character being inherited for an endless number of generations, than
in quite useless or rudimentary organs being, as we all know them to
be, thus inherited. Indeed, we may sometimes observe a mere tendency
to produce a rudiment inherited: for instance, in the common
snapdragon (Antirrhinum) a rudiment of a fifth stamen so often
appears, that this plant must have an inherited tendency to produce
it.
As all the species of the same genus are supposed, on my theory, to
have descended from a common parent, it might be expected that they
would occasionally vary in an analogous manner; so that a variety of
one species would resemble in some of its characters another species;
this other species being on my view only a well-marked and permanent
variety. But characters thus gained would probably be of an
unimportant nature, for the presence of all important characters will
be governed by natural selection, in accordance with the diverse
habits of the species, and will not be left to the mutual action of
the conditions of life and of a similar inherited constitution. It
might further be expected that the species of the same genus would
occasionally exhibit reversions to lost ancestral characters. As,
however, we never know the exact character of the common ancestor of a
group, we could not distinguish these two
cases: if, for instance, we
did not know that the rock-pigeon was not feather-footed or
turn-crowned, we could not have told, whether these characters in our
domestic breeds were reversions or only analogous variations; but we
might have inferred that the blueness was a case of reversion, from
the number of the markings, which are correlated with the blue tint,
and which it does not appear probable would all appear together from
simple variation. More especially we might have inferred this, from
the blue colour and marks so often appearing when distinct breeds of
diverse colours are crossed. Hence, though under nature it must
generally be left doubtful, what cases are reversions to an anciently
existing character, and what are new but analogous variations, yet we
ought, on my theory, sometimes to find the varying offspring of a species assuming characters (either from reversion or
from analogous variation) which already occur in some members of the
same group. And this undoubtedly is the case in nature.
A considerable part of the difficulty in recognising a variable
species in our systematic works, is due to its varieties mocking, as
it were, come of the other species of the same genus. A considerable
catalogue, also, could be given of forms intermediate between two
other forms, which themselves must be doubtfully ranked as either
varieties or species, that the one in varying has assumed some of the
characters of the other, so as to produce the intermediate form. But
the best evidence is afforded by parts or organs of an important and
uniform nature occasionally varying so as to acquire, in some degree,
the character of the same part or organ in an allied species. I have
collected a long list of such cases; but
here, as before, I lie under
a great disadvantage in not being able to give them. I can only
repeat that such cases certainly do occur, and seem to me very
remarkable.
I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several
species of the same genus, partly under domestication and partly under
nature. It is a case apparently of reversion. The ass not rarely has
very distinct transverse bars on its legs, like those of a zebra: it
has been asserted that these are plainest in the foal, and from
inquiries which I have made, I believe this to be true. It has also
been asserted that the stripe on each shoulder is sometimes double.
The shoulder-stripe is certainly very variable in length and outline.
A white ass, but not an albino, has been
described without either spinal or shoulder-stripe; and these stripes
are sometimes very obscure, or actually quite lost, in dark-coloured
asses. The koulan of Pallas is said to have been seen with a double
shoulder-stripe; but traces of it, as stated by Mr Blyth and others,
occasionally appear: and I have been informed by Colonel Poole that
foals of this species are generally striped on the legs, and faintly
on the shoulder. The quagga, though so plainly barred
like a zebra over the body, is without bars on the legs; but Dr Gray
has figured one specimen with very distinct zebra-like bars on the
hocks.
With respect to the horse, I have collected cases in England of the
spinal stripe in horses of the most distinct breeds, and of all colours; transverse bars on the legs are not
rare in duns, mouse-duns, and in one instance in a chestnut: a faint
shoulder-stripe may sometimes be seen in duns, and I have seen a trace
in a
bay horse. My son made a careful examination and sketch for me of
a dun Belgian cart-horse with a double stripe on each shoulder and
with leg-stripes; and a man, whom I can implicitly
trust, has examined for me a small dun Welch pony
with three short parallel stripes on each
shoulder.
In the north-west part of India the Kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined
the breed for the Indian Government, a horse without stripes is not
considered as purely-bred. The spine is always striped; the legs are
generally barred; and the shoulder-stripe, which is sometimes double
and sometimes treble, is common; the side of the face, moreover, is
sometimes striped. The stripes are plainest in the foal; and sometimes
quite disappear in old horses. Colonel Poole has seen both gray and
bay Kattywar horses striped when first foaled. I have, also, reason to
suspect, from information given me by Mr. W. W. Edwards, that with the
English race-horse the spinal stripe is much commoner in the foal than
in the full-grown animal. Without here entering on further details, I
may state that I have collected cases of leg and shoulder stripes in
horses of very different breeds, in various countries from Britain to
Eastern China; and from Norway in the north to the Malay Archipelago
in the south. In all parts of the world these stripes occur far
oftenest in duns and mouse-duns; by the term dun a large range of
colour is included, from one between brown and black to a close
approach to cream-colour.
I am aware that Colonel Hamilton Smith, who has written on this
subject, believes that the several breeds of the horse have descended
from several aboriginal species — one of which, the dun, was
striped; and that the above-described appearances are all
due to ancient
crosses with the dun stock. But I am not at all
satisfied with this theory, and should be loth to apply it to breeds
so distinct as the heavy Belgian cart-horse, Welch ponies, cobs, the
lanky Kattywar race, c., inhabiting the most distant parts of the
world.
Now let us turn to the effects of crossing the several species of
the horse-genus. Rollin asserts, that the common mule from the
ass and horse is particularly apt to have bars on
its legs. I once saw a mule with its legs
so much striped that any one at first would have thought that it must
have been the product of a zebra; and Mr. W. C. Martin, in his
excellent treatise on the horse, has given a figure of a similar mule.
In four coloured drawings, which I have seen, of hybrids between the
ass and zebra, the legs were much more plainly barred than the rest of
the body; and in one of them there was a double shoulder-stripe. In
Lord Moreton's famous hybrid from a chestnut mare and male quagga, the
hybrid, and even the pure offspring subsequently produced from the
mare by a black Arabian sire, were much more plainly barred across the
legs than is even the pure quagga. Lastly, and this is another most
remarkable case, a hybrid has been figured by Dr Gray (and he informs
me that he knows of a second case) from the ass and the hemionus; and
this hybrid, though the ass seldom has stripes on its legs and the
hemionus has none and has not even a shoulder-stripe, nevertheless had
all four legs barred, and had three short shoulder-stripes, like those
on the dun Welch pony, and even had some zebra-like stripes on the
sides of its face. With respect to this
last fact, I was so convinced that not even a stripe of colour appears
from what would commonly be called an accident, that I was led solely
from the occurrence of the face-stripes on this hybrid from the ass
and hemionus,
to ask Colonel Poole whether such face-stripes ever
occur in the eminently striped Kattywar breed of horses, and was, as
we have seen, answered in the affirmative.
What now are we to say to these several facts? We see several very
distinct species of the horse-genus becoming, by simple variation,
striped on the legs like a zebra, or striped on the shoulders like an
ass. In the horse we see this tendency strong whenever a dun tint
appears — a tint which approaches to that of the
general colouring of the other species of the genus. The appearance of
the stripes is not accompanied by any change of form or by any other
new character. We see this tendency to become striped most strongly
displayed in hybrids from between several of the most distinct
species. Now observe the case of the several breeds of pigeons: they
are descended from a pigeon (including two or three sub-species or
geographical races) of a bluish colour, with certain bars and other
marks; and when any breed assumes by simple variation a bluish tint,
these bars and other marks invariably reappear; but without any
other change of form or character. When the oldest and truest breeds
of various colours are crossed, we see a strong tendency for the blue
tint and bars and marks to reappear in the mongrels. I have stated
that the most probable hypothesis to account for the reappearance of
very ancient characters, is — that there is a tendency in the young of each successive
generation to produce the long-lost character, and that this tendency,
from unknown causes, sometimes prevails. And we have just seen that in
several species of the horse-genus the stripes are either plainer or
appear more commonly in the young than in the old. Call the breeds of
pigeons, some of which have bred true for centuries, species; and how
exactly parallel is the case with that of the species of the
horse-genus!
For myself, I venture confidently to look back thousands
on thousands of generations, and I see an animal striped like a zebra,
but perhaps otherwise very differently constructed, the common parent
of our domestic horse, whether or not it be descended from one or more
wild stocks, of the ass, the hemionus, quagga, and zebra.
He who believes that each equine species was independently created,
will, I presume, assert that each species has been created with a
tendency to vary, both under nature and under domestication, in this
particular manner, so as often to become striped like other species of
the genus; and that each has been created with a strong tendency, when
crossed with species inhabiting distant quarters of the world, to
produce hybrids resembling in their stripes, not their own parents,
but other species of the genus. To admit this view is, as it seems to
me, to reject a real for an unreal, or at least for an unknown, cause.
It makes the works of God a mere mockery and deception; I
would almost as soon believe with the old and ignorant cosmogonists,
that fossil shells had never lived, but had been created in stone so
as to mock the shells now living on the sea-shore.
Summary.
Our ignorance of the laws of
variation is profound. Not in one case out of a hundred can we pretend
to assign any reason why this or that part differs, more or less, from
the same part in the parents. But whenever we have the means of
instituting a comparison, the same laws appear to have acted in
producing the lesser differences between varieties of the same
species, and the greater differences between species of the same
genus. The external conditions of life, as climate and food, c.,
seem to have induced some slight modifications. Habit in producing
constitutional differences, and use in strengthening, and disuse in
weakening and diminishing organs, seem to have been more potent in
their effects. Homologous parts tend to vary in the same way, and
homologous parts tend to cohere. Modifications in hard parts and in
external parts sometimes affect softer and internal parts. When one
part is largely developed, perhaps it tends to draw nourishment from
the adjoining parts; and every part of the structure which can be
saved without detriment to the individual, will be saved. Changes of
structure at an early age will generally affect parts subsequently
developed; and there are very many other correlations of growth, the
nature of which we are utterly unable to understand. Multiple parts
are variable in number and in structure, perhaps arising from such
parts not having been closely specialized to any particular function,
so that their modifications have not been closely checked by natural
selection. It is probably from this same cause that organic beings low
in the scale of nature are more variable than those which have their
whole organisation more specialized, and are higher in the scale.
Rudimentary organs, from being useless, will be disregarded by natural
selection, and hence probably are variable. Specific characters
-- that is, the characters which have come to differ since the
several species of the same genus branched off from a common parent
-- are more variable than generic characters, or those which have
long been inherited, and have not differed within this
same period. In these remarks we have referred to special parts or
organs being still variable, because they have recently varied and
thus come to differ; but we have also seen in the second Chapter that
the same principle applies to the whole individual; for in a district
where many species of any genus are found — that is, where there
has been much former
variation and differentiation, or where the
manufactory of new specific forms has been actively at work --
there, on an average, we now find most varieties or incipient species.
Secondary sexual characters are highly variable, and such characters
differ much in the species of the same group. Variability in the same
parts of the organisation has generally been taken advantage of in
giving secondary sexual differences to the sexes of the same species,
and specific differences to the several species of the same genus. Any
part or organ developed to an extraordinary size or in an
extraordinary manner, in comparison with the same part or organ in the
allied species, must have gone through an extraordinary amount of
modification since the genus arose; and thus we can understand why it
should often still be variable in a much higher degree than other
parts; for variation is a long-continued and slow process, and natural
selection will in such cases not as yet have had time to overcome the
tendency to further variability and to reversion to a less modified
state. But when a species with any extraordinarily-developed organ has
become the parent of many modified descendants — which on my
view must be a very slow process, requiring a long lapse of time
-- in this case, natural selection may readily have succeeded in
giving a fixed character to the organ, in however extraordinary a
manner it may be developed. Species inheriting nearly the same
constitution from a common parent and exposed to similar influences
will naturally tend to present analogous variations, and these same
species may occasionally revert to some of the characters of their
ancient progenitors. Although new and important modifications may not
arise from reversion and analogous variation, such modifications will
add to the beautiful and harmonious diversity of nature.
Whatever the cause may be of each slight difference in the
offspring from their parents — and a cause for each must exist
-- it is the steady accumulation, through natural
selection, of such differences, when beneficial to the individual,
that gives rise to all the more important modifications of structure,
by which the innumerable beings on the face of this earth are enabled
to struggle with each other, and the best adapted to survive.
DIFFICULTIES ON THEORY
- Difficulties on the theory of descent with modification
- Transitions
- Absence or rarity of transitional varieties
- Transitions in habits of life
- Diversified habits in the same species
- Species with habits widely different from those of
their allies
- Organs of extreme perfection
- Means of transition
- Cases of difficulty
- Natura non facit saltum
- Organs of small importance
- Organs not in all cases absolutely perfect
- The law of Unity of Type and of the Conditions of Existence
embraced by the theory of Natural Selection
LONG before having
arrived at this part of my work, a crowd of difficulties will have
occurred to the reader. Some of them are so grave that to this day I
can never reflect on them without being staggered; but, to the best of
my judgment, the greater number are only apparent, and those that are
real are not, I think, fatal to my theory.
These difficulties and objections may be classed under the
following heads:-Firstly, why, if species have descended from other
species by insensibly fine gradations, do we not everywhere see
innumerable transitional forms? Why is not all nature in confusion
instead of the species being, as we see them, well defined?
Secondly, is it possible that an animal having, for instance, the
structure and habits of a bat, could have been formed by the
modification of some animal with wholly different habits? Can we
believe that natural selection could produce, on the one hand, organs
of trifling importance, such as the tail of a giraffe, which serves as
a fly-flapper, and, on the other hand, organs of
such wonderful
structure, as the eye, of which we hardly as yet fully understand the
inimitable perfection?
Thirdly, can instincts be acquired and modified through natural
selection? What shall we say to so marvellous an instinct
as that which leads the bee to make cells, which have practically
anticipated the discoveries of profound mathematicians?
Fourthly, how can we account for species, when crossed, being
sterile and producing sterile offspring, whereas, when varieties are
crossed, their fertility is unimpaired?
The two first heads shall be here discussed — Instinct and
Hybridism in separate chapters.
On the absence or rarity of transitional
varieties.
As natural selection acts solely by the preservation
of profitable modifications, each new form will tend in a
fully-stocked country to take the place of, and finally to
exterminate, its own less improved parent or other less-favoured forms
with which it comes into competition. Thus extinction and natural
selection will, as we have seen, go hand in hand. Hence, if we look at
each species as descended from some other unknown form, both the
parent and all the transitional varieties will generally have been
exterminated by the very process of formation and perfection of the
new form.
But, as by this theory innumerable transitional forms must have
existed, why do we not find them embedded in countless numbers in the
crust of the earth? It will be much more convenient to discuss this
question in the chapter on the Imperfection of the geological record;
and I will here only state that I believe the answer mainly lies in
the record being incomparably less perfect than is generally supposed;
the imperfection of the record being chiefly due to organic beings not
inhabiting
profound depths of the sea, and to their remains being
embedded and preserved to a future age only in masses of sediment
sufficiently thick and extensive to withstand an enormous amount of
future degradation; and such fossiliferous masses can be accumulated
only where much sediment is deposited on the shallow bed of the sea,
whilst it slowly subsides. These contingencies will concur only
rarely, and after enormously long intervals. Whilst the bed of the sea
is stationary or is rising, or when very little sediment is being
deposited, there will be blanks in our geological history. The crust
of the earth is a vast museum; but the natural
collections have been made only at intervals of time immensely remote.
But it may be urged that when several closely-allied species
inhabit the same territory we surely ought to find at the present time
many transitional forms. Let us take a simple case: in travelling from
north to south over a continent, we generally meet at successive
intervals with closely allied or representative species, evidently
filling nearly the same place in the natural economy of the land.
These representative species often meet and interlock; and as the one
becomes rarer and rarer, the other becomes more and more frequent,
till the one replaces the other. But if we compare these species where
they intermingle, they are generally as absolutely distinct from each
other in every detail of structure as are specimens taken from the
metropolis inhabited by each. By my theory these allied species have
descended from a common parent; and during the process of
modification, each has become adapted to the conditions of life of its
own region, and has supplanted and exterminated its original parent
and all the transitional varieties between its past and present
states. Hence we ought not to expect at the
present time to meet with
numerous transitional varieties in each region, though they must have
existed there, and may be embedded there in a fossil condition. But in
the intermediate region, having intermediate conditions of life, why
do we not now find closely-linking intermediate varieties? This
difficulty for a long time quite confounded me. But I think it can be
in large part explained.
In the first place we should be extremely cautious in inferring,
because an area is now continuous, that it has been continuous during
a long period. Geology would lead us to believe that almost every
continent has been broken up into islands even during the later
tertiary periods; and in such islands distinct species might have been
separately formed without the possibility of intermediate varieties
existing in the intermediate zones. By changes in the form of the land
and of climate, marine areas now continuous must often have existed
within recent times in a far less continuous and uniform condition
than at present. But I will pass over this way of escaping from the
difficulty; for I believe that many perfectly defined
species have been formed on strictly continuous areas; though I do not
doubt that the formerly broken condition of areas now continuous has
played an important part in the formation of new species, more
especially with freely-crossing and wandering animals.
In looking at species as they are now distributed over a wide area,
we generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and
finally disappearing. Hence the neutral territory between two
representative species is generally narrow in comparison with the
territory proper to each. We see the same fact in ascending mountains,
and sometimes
it is quite remarkable how abruptly, as Alph. De
Candolle has observed, a common alpine species disappears. The same
fact has been noticed by Forbes in sounding the depths of the sea with
the dredge. To those who look at climate and the physical conditions
of life as the all-important elements of distribution, these facts
ought to cause surprise, as climate and height or depth graduate away
insensibly. But when we bear in mind that almost every species, even
in its metropolis, would increase immensely in numbers, were it not
for other competing species; that nearly all either prey on or serve
as prey for others; in short, that each organic being is either
directly or indirectly related in the most important manner to other
organic beings, we must see that the range of the inhabitants of any
country by no means exclusively depends on insensibly changing
physical conditions, but in large part on the presence of other
species, on which it depends, or by which it is destroyed, or with
which it comes into competition; and as these species are already
defined objects (however they may have become so), not blending one
into another by insensible gradations, the range of any one species,
depending as it does on the range of others, will tend to be sharply
defined. Moreover, each species on the confines of its range, where it
exists in lessened numbers, will, during fluctuations in the number of
its enemies or of its prey, or in the seasons, be extremely liable to
utter extermination; and thus its geographical range will come to be
still more sharply defined.
If I am right in believing that allied or representative species,
when inhabiting a continuous area, are generally so
distributed that each has a wide range, with a comparatively narrow
neutral territory between them, in which they become rather suddenly
rarer and rarer; then, as varieties do not essentially differ from
species,
the same rule will probably apply to both; and if we in
imagination adapt a varying species to a very large area, we shall
have to adapt two varieties to two large areas, and a third variety to
a narrow intermediate zone. The intermediate variety, consequently,
will exist in lesser numbers from inhabiting a narrow and lesser area;
and practically, as far as I can make out, this rule holds good with
varieties in a state of nature. I have met with striking instances of
the rule in the case of varieties intermediate between well-marked
varieties in the genus Balanus. And it would appear from information
given me by Mr Watson, Dr Asa Gray, and Mr Wollaston, that generally
when varieties intermediate between two other forms occur, they are
much rarer numerically than the forms which they connect. Now, if we
may trust these facts and inferences, and therefore conclude that
varieties linking two other varieties together have generally existed
in lesser numbers than the forms which they connect, then, I think, we
can understand why intermediate varieties should not endure for very
long periods; — why as a general rule they should be
exterminated and disappear, sooner than the forms which they
originally linked together.
For any form existing in lesser numbers would, as already remarked,
run a greater chance of being exterminated than one existing in large
numbers; and in this particular case the intermediate form would be
eminently liable to the inroads of closely allied forms existing on
both sides of it. But a far more important consideration, as I
believe, is that, during the process of further modification, by which
two varieties are supposed on my theory to be converted and perfected
into two distinct species, the two which exist in larger numbers from
inhabiting larger areas, will have a great advantage over the
intermediate variety, which exists
in smaller numbers in a narrow and
intermediate zone. For forms existing in larger numbers will always
have a better chance, within any given period, of presenting further
favourable variations for natural selection to seize on, than will the
rarer forms which exist in lesser numbers. Hence, the
more common forms, in the race for life, will tend to beat and
supplant the less common forms, for these will be more slowly modified
and improved. It is the same principle which, as I believe, accounts
for the common species in each country, as shown in the second
chapter, presenting on an average a greater number of well-marked
varieties than do the rarer species. I may illustrate what I mean by
supposing three varieties of sheep to be kept, one adapted to an
extensive mountainous region; a second to a comparatively narrow,
hilly tract; and a third to wide plains at the base; and that the
inhabitants are all trying with equal
steadiness and skill to improve their stocks by selection; the chances
in this case will be strongly in favour of the great holders on the
mountains or on the plains improving their breeds more quickly than
the small holders on the intermediate narrow, hilly tract; and
consequently the improved mountain or plain breed will soon take the
place of the less improved hill breed; and thus the two breeds, which
originally existed in greater numbers, will come into close contact
with each other, without the interposition of the supplanted,
intermediate hill-variety.
To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos
of varying and intermediate links: firstly, because new varieties are
very slowly formed, for variation is a very slow process, and natural
selection can do nothing until favourable variations chance to occur,
and until a place in the natural
polity of the country can be better
filled by some modification of some one or more of its inhabitants. And
such new places will depend on slow changes of climate, or on the
occasional immigration of new inhabitants, and, probably, in a still
more important degree, on some of the old inhabitants becoming slowly
modified, with the new forms thus produced and the old ones acting and
reacting on each other. So that, in any one region and at any one
time, we ought only to see a few species presenting slight
modifications of structure in some degree permanent; and this
assuredly we do see.
Secondly, areas now continuous must often have existed within the
recent period in isolated portions, in which many forms,
more especially amongst the classes which unite for each birth and
wander much, may have separately been rendered sufficiently distinct
to rank as representative species. In this case, intermediate
varieties between the several representative species and their common
parent, must formerly have existed in each broken portion of the land,
but these links will have been supplanted and exterminated during the
process of natural selection, so that they will no longer exist in a
living state.
Thirdly, when two or more varieties have been formed in different
portions of a strictly continuous area, intermediate varieties will,
it is probable, at first have been formed in the intermediate zones,
but they will generally have had a short duration. For these
intermediate varieties will, from reasons already assigned (namely
from what we know of the actual distribution of closely allied or
representative species, and likewise of acknowledged varieties), exist
in the intermediate zones in lesser numbers than the varieties which
they tend to connect. From this cause alone the intermediate
varieties
will be liable to accidental extermination; and during the process of
further modification through natural selection, they will almost
certainly be beaten and supplanted by the forms which they connect;
for these from existing in greater numbers will, in the aggregate,
present more variation, and thus be further improved through natural
selection and gain further advantages.
Lastly, looking not to any one time, but to all time, if my theory
be true, numberless intermediate varieties, linking most closely all
the species of the same group together, must assuredly have existed;
but the very process of natural selection constantly tends, as has
been so often remarked, to exterminate the parent forms and the
intermediate links. Consequently evidence of their former existence
could be found only amongst fossil remains, which are preserved, as we
shall in a future chapter attempt to show, in an extremely imperfect
and intermittent record.
On the origin and transitions of organic beings
with peculiar habits and structure.
It has been asked by the
opponents of such views as I hold, how, for instance, a land
carnivorous animal could have been converted into one with aquatic
habits; for how could the animal in its transitional
state have subsisted? It would be easy to show that within the same
group carnivorous animals exist having every intermediate grade
between truly aquatic and strictly terrestrial habits; and as each
exists by a struggle for life, it is clear that each is well adapted
in its habits to its place in nature. Look at the Mustela vison of
North America, which has webbed feet and which resembles an otter in
its fur, short legs, and form of tail; during summer this animal dives
for and preys on fish, but during the long winter
it leaves the frozen
waters, and preys like other polecats on mice and land animals. If a
different case had been taken, and it had been asked how an
insectivorous quadruped could possibly have been converted into a
flying bat, the question would have been far more difficult, and I
could have given no answer. Yet I think such difficulties have very
little weight.
Here, as on other occasions, I lie under a heavy disadvantage, for
out of the many striking cases which I have collected, I can give only
one or two instances of transitional habits and structures in closely
allied species of the same genus; and of diversified habits, either
constant or occasional, in the same species. And it seems to me that
nothing less than a long list of such cases is sufficient to lessen
the difficulty in any particular case like that of the bat.
Look at the family of squirrels; here we have the finest gradation
from animals with their tails only slightly flattened, and from
others, as Sir J. Richardson has remarked, with the posterior part of
their bodies rather wide and with the skin on their flanks rather
full, to the so-called flying squirrels; and flying squirrels have
their limbs and even the base of the tail united by a broad expanse of
skin, which serves as a parachute and allows them to glide through the
air to an astonishing distance from tree to tree. We cannot doubt that
each structure is of use to each kind of squirrel in its own country,
by enabling it to escape birds or beasts of prey, or to collect food
more quickly, or, as there is reason to believe, by lessening the
danger from occasional falls. But it does not follow from this fact
that the structure of each squirrel is the best that it is possible to
conceive under all natural conditions. Let the climate and vegetation
change, let other competing rodents or new beasts of prey
immigrate, or old ones
become modified, and all analogy would lead us
to believe that some at least of the squirrels would decrease in
numbers or become exterminated, unless they also became modified and
improved in structure in a corresponding manner. Therefore, I can see
no difficulty, more especially under changing conditions of life, in
the continued preservation of individuals with fuller and fuller
flank-membranes, each modification being useful, each being
propagated, until by the accumulated effects of this process of
natural selection, a perfect so-called flying squirrel was produced.
Now look at the Galeopithecus or flying lemur, which formerly was
falsely ranked amongst bats. It has an extremely wide flank-membrane,
stretching from the corners of the jaw to the tail, and including the
limbs and the elongated fingers: the flank membrane is, also,
furnished with an extensor muscle. Although no graduated links of
structure, fitted for gliding through the air, now connect the
Galeopithecus with the other Lemuridae, yet I can see no difficulty in
supposing that such links formerly existed, and that each had been
formed by the same steps as in the case of the less perfectly gliding
squirrels; and that each grade of structure had been useful to its
possessor. Nor can I see any insuperable difficulty in further
believing it possible that the membrane-connected fingers and fore-arm
of the Galeopithecus might be greatly lengthened by natural selection;
and this, as far as the organs of flight are concerned, would convert
it into a bat. In bats which have the wing-membrane extended from the
top of the shoulder to the tail, including the hind-legs, we perhaps
see traces of an apparatus originally constructed for gliding through
the air rather than for flight.
If about a dozen genera of birds had become extinct or were
unknown, who would have ventured to have
surmised that birds might
have existed which used their wings solely as flappers, like the
logger-headed duck (Micropterus of Eyton); as fins in the water and
front legs on the land, like the penguin; as sails, like the ostrich;
and functionally for no purpose, like the Apteryx. Yet the structure
of each of these birds is good for it, under the conditions of life to
which it is exposed, for each has to live by a struggle;
but it is not necessarily the best possible under all possible
conditions. It must not be inferred from these remarks that any of the
grades of wing-structure here alluded to, which perhaps may all have
resulted from disuse, indicate the natural steps by which birds have
acquired their perfect power of flight; but they serve, at least, to
show what diversified means of transition are possible.
Seeing that a few members of such water-breathing classes as the
Crustacea and Mollusca are adapted to live on the land, and seeing
that we have flying birds and mammals, flying insects of the most
diversified types, and formerly had flying reptiles, it is conceivable
that flying-fish, which now glide far through the air, slightly rising
and turning by the aid of their fluttering fins, might have been
modified into perfectly winged animals. If early transitional state
they had been inhabitants of the open ocean, and had used their
incipient organs of flight exclusively, as far as we know, to escape
being devoured by other fish?
When we see any structure highly perfected for any particular
habit, as the wings of a bird for flight, we should bear in mind that
animals displaying early transitional grades of the structure will
seldom continue to exist to the present day, for they will have been
supplanted by the very process of perfection through natural
selection. Furthermore, we may conclude that transitional
grades
between structures fitted for very different habits of life will
rarely have been developed at an early period in great numbers and
under many subordinate forms. Thus, to return to our imaginary
illustration of the flying-fish, it does not seem probable that fishes
capable of true flight would have been developed under many
subordinate forms, for taking prey of many kinds in many ways, on the
land and in the water, until their organs of flight had come to a high
stage of perfection, so as to have given them a decided advantage over
other animals in the battle for life. Hence the chance of discovering
species with transitional grades of structure in a fossil condition
will always be less, from their having existed in lesser numbers, than
in the case of species with fully developed structures.
I will now give two or three instances of diversified and of changed habits in the individuals of the same species. When
either case occurs, it would be easy for natural selection to fit the
animal, by some modification of its structure, for its changed habits,
or exclusively for one of its several different habits. But it is
difficult to tell, and immaterial for us, whether habits generally
change first and structure afterwards; or whether slight modifications
of structure lead to changed habits; both probably often change almost
simultaneously. Of cases of changed habits it will suffice merely to
allude to that of the many British insects which now feed on exotic
plants, or exclusively on artificial substances. Of diversified habits
innumerable instances could be given: I have often watched a tyrant
flycatcher (Saurophagus sulphuratus) in South America, hovering over
one spot and then proceeding to another, like a kestrel, and at other
times standing stationary on the margin of water, and then dashing
like a kingfisher at a fish. In our own country the larger titmouse
(Parus major) may be
seen climbing branches, almost like a creeper; it
often, like a shrike, kills small birds by blows on the head; and I
have many times seen and heard it hammering the seeds of the yew on a
branch, and thus breaking them like a nuthatch. In North America the
black bear was seen by Hearne swimming for hours with widely open
mouth, thus catching, like a whale, insects in the water. Even in so
extreme a case as this, if the supply of insects were constant, and if
better adapted competitors did not already exist in the country, I can
see no difficulty in a race of bears being rendered, by natural
selection, more and more aquatic in their structure and habits, with
larger and larger mouths, till a creature was produced as monstrous as
a whale.
As we sometimes see individuals of a species following habits
widely different from those both of their own species and of the other
species of the same genus, we might expect, on my theory, that such
individuals would occasionally have given rise to new species, having
anomalous habits, and with their structure either slightly or
considerably modified from that of their proper type. And such
instances do occur in nature. Can a more striking instance of
adaptation be given than that of a woodpecker for climbing trees and
for seizing insects in the chinks of the bark? Yet in
North America there are woodpeckers which feed largely on fruit, and
others with elongated wings which chase insects on the wing; and on
the plains of La Plata, where not a tree grows, there is a woodpecker,
which in every essential part of its organisation, even in its
colouring, in the harsh tone of its voice, and undulatory flight, told
me plainly of its close blood-relationship
to our common species; yet it is a woodpecker which never climbs a
tree!
Petrels are the most arial and oceanic of birds, yet in the quiet
Sounds of Tierra del Fuego, the Puffinuria
berardi, in its general
habits, in its astonishing power of diving, its manner of swimming,
and of flying when unwillingly it takes flight, would be mistaken by
any one for an auk or grebe; nevertheless, it is essentially a petrel,
but with many parts of its organisation profoundly modified. On the
other hand, the acutest observer by examining the dead body of the
water-ouzel would never have suspected its sub-aquatic habits; yet
this anomalous member of the strictly terrestrial thrush family wholly
subsists by diving, — grasping the stones with its feet and
using its wings under water.
He who believes that each being has been created as we now see it,
must occasionally have felt surprise when he has met with an animal
having habits and structure not at all in agreement. What can be
plainer than that the webbed feet of ducks and geese are formed for
swimming; yet there are upland geese with webbed feet which rarely or
never go near the water; and no one except Audubon has seen the
frigate-bird, which has all its four toes webbed, alight on the
surface of the sea. On the other hand, grebes and coots are eminently
aquatic, although their toes are only bordered by membrane. What seems
plainer than that the long toes of grallatores are formed for walking
over swamps and floating plants, yet the water-hen is nearly as
aquatic as the coot; and the landrail nearly as terrestrial as the
quail or partridge. In such cases, and many others could be given,
habits have changed without a corresponding change of structure. The
webbed feet of the upland goose may be said to have become rudimentary
in function, though not in structure. In the frigate-bird, the
deeply-scooped membrane between the toes shows that structure has
begun to change.
He who believes in separate and innumerable acts of creation will
say, that in these cases it has pleased the
Creator to cause a being
of one type to take the place of one of another type; but this seems
to me only restating the fact in dignified language. He who believes
in the struggle for existence and in the principle of natural
selection, will acknowledge that every organic being is constantly
endeavouring to increase in numbers; and that if any one being vary
ever so little, either in habits or structure, and thus gain an
advantage over some other inhabitant of the country, it will seize on
the place of that inhabitant, however different it may be from its own
place. Hence it will cause him no surprise that there should be geese
and frigate-birds with webbed feet, either living on the dry land or
most rarely alighting on the water; that there should be long-toed
corncrakes living in meadows instead of in swamps; that there should
be woodpeckers where not a tree grows; that there should be diving
thrushes, and petrels with the habits of auks.
Organs of extreme perfection and
complication.
To suppose that the eye, with all its inimitable
contrivances for adjusting the focus to different distances, for
admitting different amounts of light, and for the correction of
spherical and chromatic aberration, could have been formed by natural
selection, seems, I freely confess, absurd
in the highest possible degree. Yet reason tells
me, that if numerous gradations from a perfect and complex eye to one
very imperfect and simple, each grade being useful to its possessor,
can be shown to exist; if further, the eye does vary ever so slightly,
and the variations be inherited, which is certainly the case; and if
any variation or modification in the organ be ever useful to an animal
under changing conditions of life, then the difficulty of believing
that a perfect and complex eye could be formed by natural
selection,
though insuperable by our imagination, can hardly be considered real.
How a nerve comes to be sensitive to light, hardly concerns us more
than how life itself first originated; but I may remark that several
facts make me suspect that any sensitive nerve may be rendered
sensitive to light, and likewise to those coarser vibrations of the
air which produce sound.
In looking for the gradations by which an organ in any species has
been perfected, we ought to look exclusively to its lineal ancestors;
but this is scarcely ever possible, and we are forced in each case to
look to species of the same group, that is to the collateral
descendants from the same original parent-form, in order to see what
gradations are possible, and for the chance of some gradations having
been transmitted from the earlier stages of descent, in an unaltered
or little altered condition. Amongst existing Vertebrata, we find but
a small amount of gradation in the structure of the eye, and from
fossil species we can learn nothing on this head. In this great class
we should probably have to descend far beneath the lowest known
fossiliferous stratum to discover the earlier stages, by which the eye
has been perfected.
In the Articulata we can commence a series with an optic nerve
merely coated with pigment, and without any other mechanism; and from
this low stage, numerous gradations of structure, branching off in two
fundamentally different lines, can be shown to exist, until we reach a
moderately high stage of perfection. In certain crustaceans, for
instance, there is a double cornea, the inner one divided into facets,
within each of which there is a lens shaped swelling. In other
crustaceans the transparent cones which are coated by pigment, and
which properly act only by excluding lateral pencils of light, are
convex at their upper ends
and must act by convergence; and at their
lower ends there seems to be an imperfect vitreous substance. With
these facts, here far too briefly and
imperfectly given, which show that there is much graduated diversity
in the eyes of living crustaceans, and bearing in mind how small the
number of living animals is in proportion to those which have become
extinct, I can see no very great difficulty (not more than in the case
of many other structures) in believing that natural selection has
converted the simple apparatus of an optic nerve merely coated with
pigment and invested by transparent membrane, into an optical
instrument as perfect as is possessed by any member of the great
Articulate class.
He who will go thus far, if he find on
finishing this treatise that large bodies of facts, otherwise
inexplicable, can be explained by the theory of descent, ought not to
hesitate to go further, and to admit that a structure
even as perfect as the eye of an eagle might be formed by natural
selection, although in this case he does not know any of the
transitional grades. His reason ought to conquer his imagination;
though I have felt the difficulty far too
keenly to be surprised at any degree of hesitation in extending the
principle of natural selection to such startling lengths.
It is scarcely possible to avoid comparing the eye to a telescope.
We know that this instrument has been perfected by the long-continued
efforts of the highest human intellects; and we naturally infer that
the eye has been formed by a somewhat analogous process. But may not
this inference be presumptuous? Have we any right to assume that the
Creator works by intellectual powers like those of man? If we must
compare the eye to an optical instrument, we ought in imagination to
take a thick layer of transparent tissue, with a nerve sensitive to
light beneath, and then suppose every
part of this layer to be
continually changing slowly in density, so as to separate into layers
of different densities and thicknesses, placed at different distances
from each other, and with the surfaces of each layer slowly changing
in form. Further we must suppose that there is a power always intently
watching each slight accidental alteration in the transparent layers;
and carefully selecting each alteration which, under varied
circumstances, may in any way, or in any degree, tend to produce a
distincter image. We must suppose each new state of the instrument to
be multiplied by the million; and each to be preserved till a better
be produced, and then the old ones to be destroyed. In living bodies,
variation will cause the slight alterations, generation will multiply
them almost infinitely, and natural selection will pick out with
unerring skill each improvement. Let this process go on for millions
on millions of years; and during each year on millions of individuals
of many kinds; and may we not believe that a living optical instrument
might thus be formed as superior to one of glass, as the works of the
Creator are to those of man?
If it could be demonstrated that any complex organ existed, which
could not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can find
out no such case. No doubt many organs exist of which we
do not know the transitional grades, more especially if we look to
much-isolated species, round which, according to my theory, there has
been much extinction. Or again, if we look to an organ common to all
the members of a large class, for in this latter case the organ must
have been first formed at an extremely remote period, since which all
the many members of the class have been developed; and in order to
discover the early transitional grades through which the organ has
passed, we should have to look to very ancient ancestral forms, long
since become extinct.
We should be extremely cautious in concluding that an organ could
not have been formed by transitional gradations of some kind. Numerous
cases could be given amongst the lower animals of the same organ
performing at the same time wholly distinct functions; thus the
alimentary canal respires, digests, and excretes in the larva of the
dragon-fly and in the fish Cobites. In the Hydra, the animal may be
turned inside out, and the exterior surface will then digest and the
stomach respire. In such cases natural selection might easily
specialise, if any advantage were thus gained, a part or organ, which
had performed two functions, for one function alone, and thus wholly
change its nature by insensible steps. Two
distinct organs sometimes perform simultaneously the same function in
the same individual; to give one instance, there are fish with gills
or branchiae that breathe the air dissolved in the water, at the same
time that they breathe free air in their swimbladders, this latter
organ having a ductus pneumaticus for its supply, and being divided by
highly vascular partitions. In these cases, one of the two organs
might with ease be modified and perfected so as to perform all the
work by itself, being aided during the process of modification by the
other organ; and then this other organ might be modified for some
other and quite distinct purpose, or be quite obliterated.
The illustration of the swimbladder in
fishes is a good one, because it shows us clearly the highly important
fact that an organ originally constructed for one purpose, namely
flotation, may be converted into one for a wholly different purpose,
namely respiration. The swimbladder has, also, been worked in as an
accessory to the auditory organs of certain fish, or, for I do not know which
view is now generally held, a part of the auditory
apparatus has been worked in as a complement to the swimbladder. All
physiologists admit that the swimbladder is homologous, or 'ideally
similar,' in position and structure with the lungs of the higher
vertebrate animals: hence there seems to me to be no great difficulty
in believing that natural selection has actually converted a
swimbladder into a lung, or organ used exclusively for respiration.
I can, indeed, hardly doubt that all vertebrate animals having true
lungs have descended by ordinary generation from an ancient prototype,
of which we know nothing, furnished with a floating apparatus or
swimbladder. We can thus, as I infer from Professor Owen's interesting
description of these parts, understand the strange fact that every
particle of food and drink which we swallow has to pass over the
orifice of the trachea, with some risk of falling into the lungs,
notwithstanding the beautiful contrivance by which the glottis is
closed. In the higher Vertebrata the branchiae have wholly disappeared
-- the slits on the sides of the neck and the loop-like course of
the arteries still marking in the embryo their former position. But it
is conceivable that the now utterly lost branchiae might have been
gradually worked in by natural selection for some quite distinct
purpose: in the same manner as, on the view entertained by some
naturalists that the branchiae and dorsal scales of Annelids are
homologous with the wings and wing-covers of insects, it is probable
that organs which at a very ancient period served for respiration have
been actually converted into organs of flight.
In considering transitions of organs, it is so important to bear in
mind the probability of conversion from one function to another, that
I will give one more instance. Pedunculated cirripedes have two
minute folds of skin,
called by me the ovigerous frena, which serve,
through the means of a sticky secretion, to retain the eggs until they
are hatched within the sack. These cirripedes have no branchiae, the
whole surface of the body and sack, including the small frena, serving
for respiration. The Balanidae or sessile cirripedes, on the other
hand, have no ovigerous frena, the eggs lying loose at the bottom of
the sack, in the well-enclosed shell; but they have large folded
branchiae. Now I think no one will dispute that the
ovigerous frena in the one family are strictly homologous with the
branchiae of the other family; indeed, they graduate into each other.
Therefore I do not doubt that little folds of skin, which originally
served as ovigerous frena, but which, likewise, very slightly aided
the act of respiration, have been gradually converted by natural
selection into branchiae, simply through an increase in their size and
the obliteration of their adhesive glands. If all pedunculated
cirripedes had become extinct, and they have already suffered far more
extinction than have sessile cirripedes, who would ever have imagined
that the branchiae in this latter family had originally existed as
organs for preventing the ova from being washed out of the sack?
Although we must be extremely cautious in concluding that any organ
could not possibly have been produced by successive transitional
gradations, yet, undoubtedly, grave cases of difficulty occur, some of
which will be discussed in my future work.
One of the gravest is that of neuter insects, which are often very
differently constructed from either the males or fertile females; but
this case will be treated of in the next chapter. The electric organs
of fishes offer another case of special difficulty; it is impossible
to conceive by what steps these wondrous organs have been produced;
but, as Owen and others have remarked,
their intimate structure
closely resembles that of common muscle; and as it has lately been
shown that Rays have an organ closely analogous to the electric
apparatus, and yet do not, as Matteuchi asserts, discharge any
electricity, we must own that we are far
too ignorant to argue that no transition of any
kind is possible.
The electric organs offer another and even more serious difficulty;
for they occur in only about a dozen fishes, of which several are
widely remote in their affinities. Generally when the same organ
appears in several members of the same class, especially if in members
having very different habits of life, we may attribute its presence to
inheritance from a common ancestor; and its absence in some of the
members to its loss through disuse or natural selection. But if the
electric organs had been inherited from one ancient progenitor thus
provided, we might have expected that all electric fishes
would have been specially related to each other. Nor does geology at
all lead to the belief that formerly most fishes had electric organs,
which most of their modified descendants have lost. The presence of
luminous organs in a few insects, belonging to different families and
orders, offers a parallel case of difficulty. Other cases could be
given; for instance in plants, the very curious contrivance of a mass
of pollen-grains, borne on a foot-stalk with a sticky gland at the
end, is the same in Orchis and Asclepias, — genera almost as
remote as possible amongst flowering plants. In all these cases of two
very distinct species furnished with apparently the same anomalous
organ, it should be observed that, although the general appearance and
function of the organ may be the same, yet some fundamental difference
can generally be detected. I am inclined to believe that in nearly the
same way as two men have sometimes independently hit on
the very same
invention, so natural selection, working for the good of each being
and taking advantage of analogous variations, has sometimes modified
in very nearly the same manner two parts in two organic beings, which
owe but little of their structure in common to inheritance from the
same ancestor.
Although in many cases it is most difficult to conjecture by what
transitions an organ could have arrived at its present state; yet,
considering that the proportion of living and known forms to the
extinct and unknown is very small, I have been astonished how rarely
an organ can be named, towards which no transitional grade is known to
lead. The truth of this remark is indeed shown by that old canon in
natural history of 'Natura non facit saltum.' We meet with this
admission in the writings of almost every experienced naturalist; or,
as Milne Edwards has well expressed it, nature is prodigal in variety,
but niggard in innovation. Why, on the theory of Creation, should this
be so? Why should all the parts and organs of many independent beings,
each supposed to have been separately created for its proper place in
nature, be so invariably linked together by graduated steps? Why
should not Nature have taken a leap from structure to structure? On
the theory of natural selection, we can clearly understand why she
should not; for natural selection can act only by taking
advantage of slight successive variations; she can never take a leap,
but must advance by the shortest and slowest steps.
Organs of little apparent
importance.
As natural selection acts by life and death, — by the preservation
of individuals with any favourable variation, and by the destruction
of those with any unfavourable deviation of structure, — I have
sometimes felt much difficulty in
understanding the origin of simple
parts, of which the importance does not seem sufficient to cause the
preservation of successively varying individuals. I have sometimes
felt as much difficulty, though of a very different kind, on this
head, as in the case of an organ as perfect and complex as the eye.
In the first place, we are much too
ignorant in regard to the whole economy of any one organic being, to
say what slight modifications would be of importance or not. In a
former chapter I have given instances of most trifling characters,
such as the down on fruit and the colour of the flesh, which, from
determining the attacks of insects or from being correlated with
constitutional differences, might assuredly be acted on by natural
selection. The tail of the giraffe looks like an artificially
constructed fly-flapper; and it seems at first incredible that this
could have been adapted for its present
purpose by successive slight modifications, each better and better,
for so trifling an object as driving away flies; yet we should pause
before being too positive even in this case, for we know that the
distribution and existence of cattle and other animals in South
America absolutely depends on their power of resisting the attacks of
insects: so that individuals which could by any means defend
themselves from these small enemies, would be able to range into new
pastures and thus gain a great advantage. It is not that the larger
quadrupeds are actually destroyed (except in some rare cases) by the
flies, but they are incessantly harassed and their strength reduced,
so that they are more subject to disease, or not so well enabled in a
coming dearth to search for food, or to escape from beasts of prey.
Organs now of trifling importance have probably in some cases been
of high importance to an early progenitor, and, after
having been slowly perfected at a
former period, have been transmitted
in nearly the same state, although now become of very slight use; and
any actually injurious deviations in their structure will always have
been checked by natural selection. Seeing how important an organ of
locomotion the tail is in most aquatic animals, its general presence
and use for many purposes in so many land animals, which in their
lungs or modified swim-bladders betray their aquatic origin, may
perhaps be thus accounted for. A well-developed tail having been
formed in an aquatic animal, it might subsequently come to be worked
in for all sorts of purposes, as a fly-flapper, an organ of
prehension, or as an aid in turning, as with the dog, though the aid
must be slight, for the hare, with hardly any tail, can double quickly
enough.
In the second place, we may sometimes attribute importance to
characters which are really of very little importance, and which have
originated from quite secondary causes, independently of natural
selection. We should remember that climate, food, c., probably
have some little direct influence on the organisation; that characters
reappear from the law of reversion;, that correlation of growth will
have had a most important influence in modifying various structures;
and finally, that sexual selection will often have largely modified
the external characters of animals having a will, to give one male an
advantage in fighting with another or in charming the females.
Moreover when a modification of structure has primarily arisen from
the above or other unknown causes, it may at first have been of no
advantage to the species, but may subsequently have been taken
advantage of by the descendants of the species under new conditions of
life and with newly acquired habits.
To give a few instances to illustrate
these latter
remarks. If green woodpeckers alone had existed, and we
did not know that there were many black and pied kinds, I dare say
that we should have thought that the green colour was a beautiful
adaptation to hide this tree-frequenting bird from its enemies; and
consequently that it was a character of importance and might have been
acquired through natural selection; as it is, I have no doubt that the
colour is due to some quite distinct cause, probably to
sexual selection. A trailing bamboo in the Malay Archipelago climbs
the loftiest trees by the aid of exquisitely constructed
hooks clustered around the ends of the branches,
and this contrivance, no doubt, is of the highest service to the
plant; but as we see nearly similar hooks on many trees which are not
climbers the hooks on the bamboo may have
arisen from unknown laws of growth, and have been subsequently taken
advantage of by the plant undergoing further modification and becoming
a climber. The naked skin on the head of a vulture is generally looked
at as a direct adaptation for wallowing in putridity; and so it may
be, or it may possibly be due to the direct action of putrid matter;
but we should be very cautious in drawing any such inference, when we
see that the skin on the head of the clean-feeding male turkey is
likewise naked. The sutures in the skulls of young mammals have been
advanced as a beautiful adaptation for aiding parturition, and no
doubt they facilitate, or may be indispensable for this act; but as
sutures occur in the skulls of young birds and reptiles, which have
only to escape from a broken egg, we may infer that this structure has
arisen from the laws of growth, and has been taken advantage of in the
parturition of the higher animals.
We are profoundly ignorant of the causes producing slight and
unimportant variations; and we are immediately
made conscious of this
by reflecting on the differences in the breeds of our domesticated
animals in different countries, — more especially in the less
civilized countries where there has been but little artificial
selection. Careful observers are convinced that a damp climate affects
the growth of the hair, and that with the hair the horns are
correlated. Mountain breeds always differ from lowland breeds; and a
mountainous country would probably affect the hind limbs from
exercising them more, and possibly even the form of the pelvis; and
then by the law of homologous variation, the front limbs and even the
head would probably be affected. The shape, also, of the pelvis might
affect by pressure the shape of the head of the young in the womb. The
laborious breathing necessary in high regions would, we have some
reason to believe, increase the size of the chest; and again
correlation would come into play. Animals kept by savages in different
countries often have to struggle for their own
subsistence, and would be exposed to a certain extent to natural
selection, and individuals with slightly different constitutions would
succeed best under different climates; and there is reason to believe
that constitution and colour are correlated. A good observer, also,
states that in cattle susceptibility to the attacks of flies is
correlated with colour, as is the liability to be poisoned by certain
plants; so that colour would be thus subjected to the action of
natural selection. But we are far too
ignorant to speculate on the relative importance of the several known
and unknown laws of variation; and I have here alluded to them only to
show that, if we are unable to account for the characteristic
differences of our domestic breeds, which nevertheless we generally
admit to have arisen through ordinary generation, we ought not to lay
too much stress on our
ignorance of the
precise cause of the slight analogous differences between species. I
might have adduced for this same purpose the differences between the
races of man, which are so strongly marked; I may add that some little
light can apparently be thrown on the origin of these differences,
chiefly through sexual selection of a particular kind, but without
here entering on copious details my reasoning would appear frivolous.
The foregoing remarks lead me to say a few words on the protest
lately made by some naturalists, against the utilitarian doctrine that
every detail of structure has been produced for the good of its
possessor. They believe that very many structures have been created
for beauty in the eyes of man, or for mere variety. This doctrine, if
true, would be absolutely fatal to my theory. Yet I fully admit that
many structures are of no direct use to their possessors. Physical
conditions probably have had some little effect on structure, quite
independently of any good thus gained. Correlation of growth has no
doubt played a most important part, and a useful modification of one
part will often have entailed on other parts diversified changes of no
direct use. So again characters which
formerly were useful, or which formerly had arisen from correlation of
growth, or from other unknown cause, may reappear from the law of
reversion, though now of no direct use. The effects of sexual
selection, when displayed in beauty to charm the females,
can be called useful only in rather a forced sense. But by far the
most important consideration is that the chief part of the
organisation of every being is simply due to inheritance; and
consequently, though each being assuredly is well fitted for its place
in nature, many structures now have no direct relation to the habits
of life of each species. Thus, we can hardly believe that the webbed
feet of the upland
goose or of the frigate-bird are of special use to
these birds; we cannot believe that the same bones in the arm of the
monkey, in the fore leg of the horse, in the wing of the bat, and in
the flipper of the seal, are of special use to these animals. We may
safely attribute these structures to inheritance. But to the
progenitor of the upland goose and of the frigate-bird, webbed feet no
doubt were as useful as they now are to the most aquatic of existing
birds. So we may believe that the progenitor of the seal had not a
flipper, but a foot with five toes fitted for walking or grasping; and
we may further venture to believe that the several bones in the limbs
of the monkey, horse, and bat, which have been inherited from a common
progenitor, were formerly of more special use to that progenitor, or
its progenitors, than they now are to these animals having such widely
diversified habits. Therefore we may infer that these several bones
might have been acquired through natural selection, subjected
formerly, as now, to the several laws of inheritance, reversion,
correlation of growth, c. Hence every detail of structure in
every living creature (making some little allowance for the direct
action of physical conditions) may be viewed, either as having been of
special use to some ancestral form, or as being now of special use to
the descendants of this form — either directly, or indirectly
through the complex laws of growth.
Natural selection cannot possibly produce any modification in any
one species exclusively for the good of another species; though
throughout nature one species incessantly takes advantage of, and
profits by, the structure of another. But natural selection can and
does often produce structures for the direct injury of other species,
as we see in the fang of the adder, and in the ovipositor of the
ichneumon, by which its eggs are deposited
in the living bodies of
other insects. If it could be proved that any part of the
structure of any one species had been formed for the exclusive good of
another species, it would annihilate my theory, for such could not
have been produced through natural selection. Although many statements
may be found in works on natural history to this effect, I cannot find
even one which seems to me of any weight. It is admitted that the
rattlesnake has a poison-fang for its own defence and for the
destruction of its prey; but some authors suppose that at the same
time this snake is furnished with a rattle for its own injury, namely,
to warn its prey to escape. I would almost as soon believe that the
cat curls the end of its tail when preparing to spring, in order to
warn the doomed mouse. But I have not space here to enter on this and
other such cases.
Natural selection will never produce in a being anything injurious
to itself, for natural selection acts solely by and for the good of
each. No organ will be formed, as Paley has remarked, for the purpose
of causing pain or for doing an injury to its possessor. If a fair
balance be struck between the good and evil caused by each part, each
will be found on the whole advantageous. After the lapse of time,
under changing conditions of life, if any part comes to be injurious,
it will be modified; or if it be not so, the being will become
extinct, as myriads have become extinct.
Natural selection tends only to make each organic being as perfect
as, or slightly more perfect than, the other inhabitants of the same
country with which it has to struggle for existence. And we see that
this is the degree of perfection attained under nature. The endemic
productions of New Zealand, for instance, are perfect one compared
with another; but they are now rapidly yielding before the advancing
legions of plants
and animals introduced from Europe. Natural
selection will not produce absolute perfection, nor do we always meet,
as far as we can judge, with this high standard under nature. The
correction for the aberration of light is said, on high authority, not
to be perfect even in that most perfect organ, the eye. If our reason
leads us to admire with enthusiasm a multitude of inimitable
contrivances in nature, this same reason tells us, though we may
easily err on both sides, that some other contrivances are less perfect. Can we consider the sting of the wasp or of the bee
as perfect, which, when used against many attacking animals, cannot be
withdrawn, owing to the backward serratures, and so inevitably causes
the death of the insect by tearing out its viscera?
If we look at the sting of the bee, as having originally existed in
a remote progenitor as a boring and serrated instrument, like that in
so many members of the same great order,
and which has been modified but not perfected for its present purpose,
with the poison originally adapted to cause galls subsequently
intensified, we can perhaps understand how it is that the use of the
sting should so often cause the insect's own death: for if on the
whole the power of stinging be useful to the community, it will fulfil
all the requirements of natural selection, though it may cause the
death of some few members. If we admire the truly wonderful power of
scent by which the males of many insects find their females, can we
admire the production for this single purpose of thousands of drones,
which are utterly useless to the community for any other end, and
which are ultimately slaughtered by their industrious and sterile
sisters? It may be difficult, but we ought to admire the savage
instinctive hatred of the queen-bee, which urges her instantly to
destroy the
young queens her daughters as soon as born, or to perish
herself in the combat; for undoubtedly this is for the good of the
community; and maternal love or maternal hatred, though the latter
fortunately is most rare, is all the same to the inexorable principle
of natural selection. If we admire the several ingenious contrivances,
by which the flowers of the orchis and of many other plants are
fertilised through insect agency, can we consider as equally perfect
the elaboration by our fir-trees of dense clouds of pollen, in order
that a few granules may be wafted by a chance breeze on to the ovules?
Summary of Chapter.
We have in this
chapter discussed some of the difficulties and objections which may be
urged against my theory. Many of them are very grave; but I think that
in the discussion light has been thrown on several facts, which on the
theory of independent acts of creation are utterly obscure. We have seen that species at any one period are not indefinitely
variable, and are not linked together by a multitude of intermediate
gradations, partly because the process of natural selection will
always be very slow, and will act, at any one time, only on a very few
forms; and partly because the very process of natural selection almost
implies the continual supplanting and extinction of preceding and
intermediate gradations. Closely allied species, now living on a
continuous area, must often have been formed when the area was not
continuous, and when the conditions of life did not insensibly
graduate away from one part to another. When two varieties are formed
in two districts of a continuous area, an intermediate variety will
often be formed, fitted for an intermediate zone; but from reasons
assigned, the intermediate variety will usually exist in lesser
numbers than
the two forms which it connects; consequently the two
latter, during the course of further modification, from existing in
greater numbers, will have a great advantage over the less numerous
intermediate variety, and will thus generally succeed in supplanting
and exterminating it.
We have seen in this chapter how cautious we should be in
concluding that the most different habits of life could not graduate
into each other; that a bat, for instance, could not have been formed
by natural selection from an animal which at first could only glide
through the air.
We have seen that a species may under new conditions of life change
its habits, or have diversified habits, with some habits very unlike
those of its nearest congeners. Hence we can understand bearing in
mind that each organic being is trying to live wherever it can live,
how it has arisen that there are upland geese with webbed feet, ground
woodpeckers, diving thrushes, and petrels with the habits of auks.
Although the belief that an organ so perfect as the eye could have
been formed by natural selection, is more than enough to stagger any
one; yet in the case of any organ, if we know of a long series of
gradations in complexity, each good for its possessor, then, under
changing conditions of life, there is no logical impossibility in the
acquirement of any conceivable degree of perfection through natural
selection. In the cases in which we know of no
intermediate or transitional states, we should be very cautious in
concluding that none could have existed, for the homologies of many
organs and their intermediate states show that wonderful metamorphoses
in function are at least possible. For instance, a swim-bladder has
apparently been converted into an air-breathing lung. The same organ
having performed
simultaneously very different functions, and then
having been specialised for one function; and two very distinct organs
having performed at the same time the same function, the one having
been perfected whilst aided by the other, must often have largely
facilitated transitions.
We are far too ignorant, in almost every
case, to be enabled to assert that any part or organ is so unimportant
for the welfare of a species, that modifications in its structure
could not have been slowly accumulated by means of natural selection.
But we may confidently believe that many modifications, wholly due to
the laws of growth, and at first in no way advantageous to a species,
have been subsequently taken advantage of by the still further
modified descendants of this species. We may, also, believe that a
part formerly of high importance has often been retained (as the tail
of an aquatic animal by its terrestrial descendants), though it has
become of such small importance that it could not, in its present
state, have been acquired by natural selection, — a power which
acts solely by the preservation of profitable variations in the
struggle for life.
Natural selection will produce nothing in one species for the
exclusive good or injury of another; though it may well produce parts,
organs, and excretions highly useful or even indispensable, or highly
injurious to another species, but in all cases at the same time useful
to the owner. Natural selection in each well-stocked country, must act
chiefly through the competition of the inhabitants one with another,
and consequently will produce perfection, or strength in the battle
for life, only according to the standard of that country. Hence the
inhabitants of one country, generally the smaller one, will often
yield, as we see they do yield, to the inhabitants of another and
generally larger country. For in
the larger country there will have
existed more individuals, and more diversified forms, and the
competition will have been severer, and thus the standard
of perfection will have been rendered higher. Natural selection will
not necessarily produce absolute perfection; nor, as far as we can
judge by our limited faculties, can absolute perfection be everywhere
found.
On the theory of natural selection we can clearly understand the
full meaning of that old canon in natural
history, 'Natura non facit saltum.' This canon, if we look only to the
present inhabitants of the world, is not strictly correct, but if we
include all those of past times, it must by my theory be strictly
true.
It is generally acknowledged that all organic beings have been
formed on two great laws — Unity of Type, and the Conditions of
Existence. By unity of type is meant that fundamental agreement in
structure, which we see in organic beings of the same class, and which
is quite independent of their habits of life. On my theory, unity of
type is explained by unity of descent. The
expression of conditions of existence, so often insisted on by the
illustrious Cuvier, is fully embraced by the principle of natural
selection. For natural selection acts by either now adapting the
varying parts of each being to its organic and inorganic conditions of
life; or by having adapted them during long-past periods of time: the
adaptations being aided in some cases by use and disuse, being
slightly affected by the direct action of the external conditions of
life, and being in all cases subjected to the several laws of growth.
Hence, in fact, the law of the Conditions of Existence is the higher
law; as it includes, through the inheritance of former adaptations,
that of Unity of Type.
INSTINCT
- Instincts comparable with habits, but different in their origin
- Instincts graduated
- Aphides and ants
- Instincts variable
- Domestic instincts, their origin
- Natural instincts of the cuckoo, ostrich, and parasitic bees
- Slave-making ants
- Hive-bee, its cell-making instinct
- Difficulties on the theory
of the Natural Selection of instincts
- Neuter or sterile insects
- Summary
THE subject of instinct might have been
worked into the previous chapters; but I have thought that it would be
more convenient to treat the subject separately, especially as so
wonderful an instinct as that of the hive-bee making its cells will
probably have occurred to many readers, as a difficulty sufficient to
overthrow my whole theory. I must premise, that I have nothing to do
with the origin of the primary mental powers, any more than I have
with that of life itself. We are concerned only with the diversities
of instinct and of the other mental qualities of animals within the
same class.
I will not attempt any definition of instinct. It would be easy to
show that several distinct mental actions are commonly embraced by
this term; but every one understands what is meant, when it is said
that instinct impels the cuckoo to migrate and to lay her eggs in
other birds' nests. An action, which we ourselves should require
experience to enable us to perform, when performed by an animal, more
especially by a very young one, without any experience, and when
performed by many individuals in the same way, without their knowing
for what purpose it is performed, is usually said to be instinctive.
But I could show that none of these characters of instinct are
universal. A little dose, as Pierre Huber expresses it, of judgment or
reason, often comes into play, even in animals very low in the scale
of nature.
Frederick Cuvier and several of the older metaphysicians have
compared instinct with habit. This comparison gives, I think, a
remarkably accurate notion of the frame of mind under which an
instinctive action is performed, but not of its origin. How
unconsciously many habitual actions are performed, indeed not rarely
in direct opposition to our conscious will! yet they may be modified
by the will or reason. Habits easily become associated with other
habits, and with certain periods of time and states of the body. When
once acquired, they often remain constant throughout life. Several
other points of resemblance between instincts and habits could be
pointed out. As in repeating a well-known song, so in instincts, one
action follows another by a sort of rhythm; if a person be interrupted
in a song, or in repeating anything by rote, he is generally forced to
go back to recover the habitual train of thought: so P. Huber found it
was with a caterpillar, which makes a very complicated hammock; for if
he took a caterpillar which had completed its hammock up to, say, the
sixth stage of construction, and put it into a hammock completed up
only to the third stage, the caterpillar simply re-performed the
fourth, fifth, and sixth stages of construction. If, however, a
caterpillar were taken out of a hammock made up, for instance, to the
third stage, and were put into one finished up to the sixth stage, so
that much of its work was already done for it, far from feeling the
benefit of this, it was much embarrassed, and, in order to complete
its hammock, seemed forced to start from the third stage, where it had
left off, and thus tried to complete the already finished work.
If we suppose any habitual action to become inherited — and I
think it can be shown that this does sometimes happen — then the
resemblance between what originally was a habit and an instinct
becomes so close as not to be distinguished. If Mozart, instead of
playing the pianoforte at three years old with wonderfully little
practice, had played a tune with no practice at all, be might truly be
said to have done so instinctively. But it would be the most serious
error to suppose that the greater number of instincts have been
acquired by habit in one generation, and then transmitted by
inheritance to succeeding generations. It can be clearly shown that
the most wonderful instincts with which we are
acquainted, namely, those of the hive-bee and of many ants, could not
possibly have been thus acquired.
It will be universally admitted that instincts are as important as
corporeal structure for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to
a species; and if it can be shown that instincts do vary ever
so little, then I can see no difficulty in
natural selection preserving and continually accumulating variations
of instinct to any extent that may be profitable. It is thus, as I
believe, that all the most complex and wonderful instincts have
originated. As modifications of corporeal structure arise from, and
are increased by, use or habit, and are diminished or lost by disuse,
so I do not doubt it has been with instincts. But I believe that the
effects of habit are of quite subordinate importance to the effects of
the natural selection of what may be called accidental variations of
instincts; — that is of variations produced by the same unknown
causes which produce slight deviations of bodily structure.
No complex instinct can possibly be produced through
natural
selection, except by the slow and gradual accumulation of numerous,
slight, yet profitable, variations. Hence, as in the case of corporeal
structures, we ought to find in nature, not the actual transitional
gradations by which each complex instinct has been acquired --
for these could be found only in the lineal ancestors of each species
-- but we ought to find in the collateral lines of descent some
evidence of such gradations; or we ought at least to be able to show
that gradations of some kind are possible; and this we certainly can
do. I have been surprised to find, making allowance for the instincts
of animals having been but little observed except in Europe and North
America, and for no instinct being known amongst extinct species, how
very generally gradations, leading to the most complex instincts, can
be discovered. The canon of 'Natura non facit saltum' applies with
almost equal force to instincts as to bodily organs. Changes of
instinct may sometimes be facilitated by the same species having
different instincts at different periods of life, or at different
seasons of the year, or when placed under different circumstances, c.; in which case either one or the other instinct might
be preserved by natural selection. And such instances of diversity of
instinct in the same species can be shown to occur in nature.
Again as in the case of corporeal structure, and conformably with
my theory, the instinct of each species is good for itself, but has
never, as far as we can judge, been produced for the exclusive good of
others. One of the strongest instances of
an animal apparently performing an action for the sole good of
another, with which I am acquainted, is that of aphides voluntarily
yielding their sweet excretion to ants: that they do so voluntarily,
the following facts show. I removed all the ants from a group of about
a dozen aphides on a dock-plant,
and prevented their attendance during
several hours. After this interval, I felt sure that the aphides would
want to excrete. I watched them for some time through a lens, but not
one excreted; I then tickled and stroked them with a hair in the same
manner, as well as I could, as the ants do with their antennae; but
not one excreted. Afterwards I allowed an ant to visit them, and it
immediately seemed, by its eager way of running about, to be well
aware what a rich flock it had discovered; it then began to play with
its antennae on the abdomen first of one aphis and then of another;
and each aphis, as soon as it felt the antennae, immediately lifted up
its abdomen and excreted a limpid drop of sweet juice, which was
eagerly devoured by the ant. Even the quite young aphides behaved in
this manner, showing that the action was instinctive, and not the
result of experience. But as the excretion is extremely viscid, it is
probably a convenience to the aphides to have it removed; and
therefore probably the aphides do not instinctively excrete for the
sole good of the ants. Although I do not believe that any animal in
the world performs an action for the exclusive good of another of a
distinct species, yet each species tries to take advantage of the
instincts of others, as each takes advantage of the weaker bodily
structure of others. So again, in some few
cases, certain instincts cannot be considered as absolutely perfect;
but as details on this and other such points are not indispensable,
they may be here passed over.
As some degree of variation in instincts under a state of nature, and the inheritance of such variations, are
indispensable for the action of natural selection, as many instances
as possible ought to have been here given; but want of space prevents
me. I can only assert, that instincts certainly do vary — for
instance,
the migratory instinct, both in extent and direction, and in
its total loss. So it is with the nests of
birds, which vary partly in dependence on the situations chosen, and
on the nature and temperature of the country inhabited, but often from
causes wholly unknown to us: Audubon has given several remarkable
cases of differences in nests of the same species in the northern and
southern United States. Fear of any particular enemy is certainly an
instinctive quality, as may be seen in nestling birds, though it is
strengthened by experience, and by the sight of fear of the same enemy
in other animals. But fear of man is slowly acquired, as I have
elsewhere shown, by various animals inhabiting desert islands; and we
may see an instance of this, even in England, in the greater wildness
of all our large birds than of our small birds; for the large birds
have been most persecuted by man. We may safely attribute the greater
wildness of our large birds to this cause; for in uninhabited islands
large birds are not more fearful than small; and the magpie, so wary
in England, is tame in Norway, as is the hooded crow in Egypt.
That the general disposition of individuals of the same species,
born in a state of nature, is extremely diversified, can be shown by a
multitude of facts. Several cases also, could be given, of occasional
and strange habits in certain species, which might, if advantageous to
the species, give rise, through natural selection, to quite new
instincts. But I am well aware that these general statements, without
facts given in detail, can produce but a feeble effect on the reader's
mind. I can only repeat my assurance, that I do not speak without good
evidence.
The possibility, or even probability, of inherited variations of
instinct in a state of nature will be strengthened by briefly
considering a few cases under
domestication. We shall thus also be
enabled to see the respective parts which habit and the selection of
so-called accidental variations have played in modifying the mental
qualities of our domestic animals. A number of curious
and authentic instances could be given of the inheritance of all
shades of disposition and tastes, and likewise of the oddest tricks,
associated with certain frames of mind or periods of time. But let us
look to the familiar case of the several breeds of dogs: it cannot be
doubted that young pointers (I have myself seen a striking instance)
will sometimes point and even back other dogs the very first time that
they are taken out; retrieving is certainly in some degree inherited
by retrievers; and a tendency to run round, instead of at, a flock of
sheep, by shepherd-dogs. I cannot see that these actions, performed
without experience by the young, and in nearly the same manner by each
individual, performed with eager delight by each breed, and without
the end being known, — for the young pointer can no more know
that he points to aid his master, than the white butterfly knows why
she lays her eggs on the leaf of the cabbage, — I cannot see
that these actions differ essentially from true instincts. If we were
to see one kind of wolf, when young and without any training, as soon
as it scented its prey, stand motionless like a statue, and then
slowly crawl forward with a peculiar gait; and another kind of wolf
rushing round, instead of at, a herd of deer, and driving them to a
distant point, we should assuredly call these actions instinctive.
Domestic instincts, as they may be called, are certify far less fixed
or invariable than natural instincts; but they have been acted on by
far less rigorous selection, and have been transmitted for an
incomparably shorter period, under less fixed conditions of life.
How strongly these domestic instincts, habits, and dispositions
are
inherited, and how curiously they become mingled, is well shown when
different breeds of dogs are crossed. Thus it is known that a cross
with a bull-dog has affected for many generations the courage and
obstinacy of greyhounds; and a cross with a greyhound has given to a
whole family of shepherd-dogs a tendency to hunt hares. These
domestic instincts, when thus tested by crossing, resemble natural
instincts, which in a like manner become curiously blended together,
and for a long period exhibit traces of the instincts of either
parent: for example, Le Roy describes a dog, whose great-grandfather
was a wolf, and this dog showed a trace of its wild parentage only in
one way, by not coming in a straight line to his master
when called.
Domestic instincts are sometimes spoken of as actions which have
become inherited solely from long-continued and compulsory habit, but
this, I think, is not true. No one would ever have thought of
teaching, or probably could have taught, the tumbler-pigeon to tumble,
-- an action which, as I have witnessed, is performed by young
birds, that have never seen a pigeon tumble. We may believe that some
one pigeon showed a slight tendency to this strange habit, and that
the long-continued selection of the best individuals in successive
generations made tumblers what they now are; and near Glasgow there
are house-tumblers, as I hear from Mr Brent, which cannot fly eighteen
inches high without going head over heels. It may be doubted whether
any one would have thought of training a dog to point, had not some
one dog naturally shown a tendency in this line; and this is known
occasionally to happen, as I once saw in a pure terrier. When the
first tendency was once displayed, methodical selection and the
inherited effects of compulsory training in each successive generation
would soon complete the work; and unconscious
selection is still at
work, as each man tries to procure, without intending to improve the
breed, dogs which will stand and hunt best. On the other hand, habit
alone in some cases has sufficed; no animal is more difficult to tame
than the young of the wild rabbit; scarcely any animal is tamer than
the young of the tame rabbit; but I do not suppose that domestic
rabbits have ever been selected for tameness; and I presume that we
must attribute the whole of the inherited change from extreme wildness
to extreme tameness, simply to habit and long-continued close
confinement.
Natural instincts are lost under domestication: a remarkable
instance of this is seen in those breeds of fowls which very rarely or
never become 'broody,' that is, never wish to sit on their eggs.
Familiarity alone prevents our seeing how universally and largely the
minds of our domestic animals have been modified by domestication. It
is scarcely possible to doubt that the love of man has become
instinctive in the dog. All wolves, foxes, jackals, and species of the
cat genus, when kept tame, are most eager to attack poultry, sheep,
and pigs; and this tendency has been found incurable in
dogs which have been brought home as puppies from countries, such as
Tierra del Fuego and Australia, where the savages do not keep these
domestic animals. How rarely, on the other hand, do our civilised
dogs, even when quite young, require to be taught not to attack
poultry, sheep, and pigs! No doubt they occasionally do make an
attack, and are then beaten; and if not cured, they are destroyed; so
that habit, with some degree of selection, has probably concurred in
civilising by inheritance our dogs. On the other hand, young chickens
have lost, wholly by habit, that fear of the dog and cat which no
doubt was originally instinctive in them, in the same way as it is so
plainly instinctive in
young pheasants, though reared under a hen. It
is not that chickens have lost all fear, but fear only of dogs and
cats, for if the hen gives the danger-chuckle, they will run (more
especially young turkeys) from under her, and conceal themselves in
the surrounding grass or thickets; and this is evidently done for the
instinctive purpose of allowing, as we see in wild ground-birds, their
mother to fly away. But this instinct retained by our chickens has
become useless under domestication, for the mother-hen has almost lost
by disuse the power of flight.
Hence, we may conclude, that domestic instincts have been acquired
and natural instincts have been lost partly by habit, and partly by
man selecting and accumulating during successive generations, peculiar
mental habits and actions, which at first appeared from what we must
in our ignorance call an accident. In some cases compulsory habit
alone has sufficed to produce such inherited mental changes; in other
cases compulsory habit has done nothing, and all has been the result
of selection, pursued both methodically and unconsciously; but in most
cases, probably, habit and selection have acted together.
We shall, perhaps, best understand how instincts in a state of
nature have become modified by selection, by considering a few cases.
I will select only three, out of the several which I shall have to
discuss in my future work, — namely, the instinct which leads
the cuckoo to lay her eggs in other birds' nests; the slave-making
instinct of certain ants; and the comb-making power of the hive-bee:
these two latter instincts have generally, and most
justly, been ranked by naturalists as the most wonderful of all known
instincts.
It is now commonly admitted that the more immediate and final cause
of the cuckoo's instinct is, that
she lays her eggs, not daily, but at
intervals of two or three days; so that, if she were to make her own
nest and sit on her own eggs, those first laid would have to be left
for some time unincubated, or there would be eggs and young birds of
different ages in the same nest. If this were the case, the process of
laying and hatching might be inconveniently long, more especially as
she has to migrate at a very early period; and the first hatched young
would probably have to be fed by the male alone. But the American
cuckoo is in this predicament; for she makes her own nest and has eggs
and young successively hatched, all at the same time. It has been
asserted that the American cuckoo occasionally lays her eggs in other
birds' nests; but I hear on the high authority of Dr. Brewer, that this
is a mistake. Nevertheless, I could give several instances of various
birds which have been known occasionally to lay their eggs in other
birds' nests. Now let us suppose that the ancient progenitor of our
European cuckoo had the habits of the American cuckoo; but that
occasionally she laid an egg in another bird's nest. If the old bird
profited by this occasional habit, or if the young were made more
vigorous by advantage having been taken of the mistaken maternal
instinct of another bird, than by their own mother's care, encumbered
as she can hardly fail to be by having eggs and young of different
ages at the same time; then the old birds or the fostered young would
gain an advantage. And analogy would lead me to believe, that the
young thus reared would be apt to follow by inheritance the occasional
and aberrant habit of their mother, and in their turn would be apt to
lay their eggs in other birds' nests, and thus be successful in
rearing their young. By a continued process of this nature, I believe
that the strange instinct of our cuckoo could be, and has been,
generated. I may add that, according to Dr. Gray and to some other
observers, the European cuckoo has not utterly lost all maternal love
and care for her own offspring.
The occasional habit of birds laying their eggs in other birds'
nests, either of the same or of a distinct species, is not very uncommon with the Gallinaceae; and this perhaps explains the
origin of a singular instinct in the allied group of ostriches. For
several hen ostriches, at least in the case of the American species,
unite and lay first a few eggs in one nest and then in another; and
these are hatched by the males. This instinct may probably be
accounted for by the fact of the hens laying a large number of eggs;
but, as in the case of the cuckoo, at intervals of two or three days.
This instinct, however, of the American ostrich has not as yet been
perfected; for a surprising number of eggs lie strewed over the
plains, so that in one day's hunting I picked up no less than twenty
lost and wasted eggs.
Many bees are parasitic, and always lay their eggs in the nests of
bees of other kinds. This case is more remarkable than that of the
cuckoo; for these bees have not only their instincts but their
structure modified in accordance with their parasitic habits; for they
do not possess the pollen-collecting apparatus which would be
necessary if they had to store food for their own young. Some species,
likewise, of Sphegidae (wasp-like insects) are parasitic on other
species; and M. Fabre has lately shown good reason for believing that
although the Tachytes nigra generally makes its own burrow and stores
it with paralysed prey for its own larvae to feed on, yet that when
this insect finds a burrow already made and stored by another sphex,
it takes advantage of the prize, and becomes for the occasion
parasitic. In this case, as with the supposed case of the cuckoo, I
can
see no difficulty in natural selection making an occasional habit
permanent, if of advantage to the species, and if the insect whose
nest and stored food are thus feloniously appropriated, be not thus
exterminated.
Slave-making instinct.
This remarkable
instinct was first discovered in the Formica (Polyerges) rufescens by
Pierre Huber, a better observer even than his celebrated father. This
ant is absolutely dependent on its slaves; without their aid, the
species would certainly become extinct in a single year. The males and
fertile females do no work. The workers or sterile females, though
most energetic and courageous in capturing slaves,
do no other work. They are incapable of making
their own nests, or of feeding their own larvae. When the old nest is
found inconvenient, and they have to migrate, it is the slaves which
determine the migration, and actually carry their masters in their
jaws. So utterly helpless are the masters, that when Huber shut up
thirty of them without a slave, but with plenty of the food which they
like best, and with their larvae and pupae to stimulate them to work,
they did nothing; they could not even feed themselves, and many
perished of hunger. Huber then introduced a single slave (F. fusca),
and she instantly set to work, fed and saved the survivors; made some
cells and tended the larvae, and put all to rights. What can be more
extraordinary than these well-ascertained facts? If we had not known
of any other slave-making ant, it would have been hopeless to have
speculated how so wonderful an instinct could have been perfected.
Formica sanguinea was likewise first discovered by P. Huber to be a
slave-making ant. This species is found in the southern parts of
England, and its habits have been attended to by Mr. F. Smith, of the
British
Museum, to whom I am much indebted for information on this and
other subjects. Although fully trusting to the statements of Huber and
Mr. Smith, I tried to approach the subject in a sceptical frame of
mind, as any one may well be excused for doubting the truth of so
extraordinary and odious an instinct as that of making slaves. Hence I
will give the observations which I have made myself made, in some
little detail. I opened fourteen nests of F. sanguinea, and found a few
slaves in all. Males and fertile females of the slave-species are
found only in their own proper communities, and have never been
observed in the nests of F. sanguinea. The slaves are black and not
above half the size of their red masters, so that the contrast in
their appearance is very great. When the nest is slightly disturbed,
the slaves occasionally come out, and like their masters are much
agitated and defend their nest: when the nest is much disturbed and
the larvae and pupae are exposed, the slaves work energetically with
their masters in carrying them away to a place of safety. Hence, it is
clear, that the slaves feel quite at home. During the months of June
and July, on three successive years, I have watched for many hours
several nests in Surrey and Sussex, and never saw a slave either leave
or enter a nest. As, during these months, the slaves are very few in
number, I thought that they might behave differently when more
numerous; but Mr. Smith informs me that he has watched the nests at
various hours during May, June and August, both in Surrey and
Hampshire, and has never seen the slaves, though present in large
numbers in August, either leave or enter the nest. Hence he considers
them as strictly household slaves. The masters, on the other hand, may
be constantly seen bringing in materials for the nest, and food of all
kinds. During the present year, however, in the month
of July, I came
across a community with an unusually large stock of slaves, and I
observed a few slaves mingled with their masters leaving the nest, and
marching along the same road to a tall Scotch-fir-tree, twenty-five
yards distant, which they ascended together, probably in search of
aphides or cocci. According to Huber, who had ample opportunities for
observation, in Switzerland the slaves habitually work with their
masters in making the nest, and they alone open and close the doors in
the morning and evening; and, as Huber expressly states, their
principal office is to search for aphides. This difference in the
usual habits of the masters and slaves in the two countries, probably
depends merely on the slaves being captured in greater numbers in
Switzerland than in England.
One day I fortunately chanced to witness a migration from one nest
to another, and it was a most interesting spectacle to behold the
masters carefully carrying, as Huber has described, their slaves in
their jaws. Another day my attention was struck by about a score of
the slave-makers haunting the same spot, and evidently not in search
of food; they approached and were vigorously repulsed by an
independent community of the slave species (F. fusca); sometimes as
many as three of these ants clinging to the legs of the slave-making
F. sanguinea. The latter ruthlessly killed their small opponents, and
carried their dead bodies as food to their nest, twenty-nine yards
distant; but they were prevented from getting any pupae to rear as
slaves. I then dug up a small parcel of the pupae of F. fusca from
another nest, and put them down on a bare spot near the place of
combat; they were eagerly seized, and carried off by the tyrants, who
perhaps fancied that, after all,
they had been victorious in their late combat.
At the same time I laid on the same place a small parcel of the
pupae of another species, F. flava, with a few of these little yellow
ants still clinging to the fragments of the nest. This species is
sometimes, though rarely, made into slaves, as has been described by
Mr Smith. Although so small a species, it is very courageous, and I
have seen it ferociously attack other ants. In one instance I found to
my surprise an independent community of F. flava under a stone beneath
a nest of the slave-making F. sanguinea; and when I had accidentally
disturbed both nests, the little ants attacked their big neighbours
with surprising courage. Now I was curious to ascertain whether F.
sanguinea could distinguish the pupae of F. fusca, which they
habitually make into slaves, from those of the little and furious F.
flava, which they rarely capture, and it was evident that they did at
once distinguish them: for we have seen that they eagerly and
instantly seized the pupae of F. fusca, whereas they were much
terrified when they came across the pupae, or even the earth from the
nest of F. flava, and quickly ran away; but in about a quarter of an
hour, shortly after all the little yellow ants had crawled away, they
took heart and carried off the pupae.
One evening I visited another community of F. sanguinea, and found
a number of these ants entering their nest, carrying the dead bodies
of F. fusca (showing that it was not a migration) and numerous pupae.
I traced the returning file burthened with booty, for about forty
yards, to a very thick clump of heath. whence I saw the last
individual of F. sanguinea emerge, carrying a pupa; but I was not
able to find the desolated nest in the thick heath. The nest, however,
must have been close at hand, for two or three individuals of F. fusca
were rushing about in the greatest agitation, and one was
perched
motionless with its own pupa in its mouth on the top of a spray of
heath over its ravaged home.
Such are the facts, though they did not need confirmation by me, in
regard to the wonderful instinct of making slaves. Let it be observed
what a contrast the instinctive habits of F. sanguinea present with
those of the F. rufescens. The latter does
not build its own nest, does not determine its own
migrations, does not collect food for itself or its young, and cannot
even feed itself: it is absolutely dependent on its numerous slaves.
Formica sanguinea, on the other hand, possesses much fewer slaves, and
in the early part of the summer extremely few. The masters determine
when and where a new nest shall be formed, and when they migrate, the
masters carry the slaves. Both in Switzerland and England the slaves
seem to have the exclusive care of the larvae, and the masters alone
go on slave-making expeditions. In Switzerland the slaves and masters
work together, making and bringing materials for the nest: both, but
chiefly the slaves, tend, and milk as it may be called, their aphides;
and thus both collect food for the community. In England the masters
alone usually leave the nest to collect building materials and food
for themselves, their slaves and larvae. So
that the masters in this country receive much less service from their
slaves than they do in Switzerland.
By what steps the instinct of F. sanguinea originated I will not
pretend to conjecture. But as ants, which are not slave-makers, will,
as I have seen, carry off pupae of other species, if scattered near
their nests, it is possible that pupae originally stored as food might
become developed; and the ants thus unintentionally reared would then
follow their proper instincts, and do what work they could. If their
presence proved useful to the species which had seized them — if
it were more advantageous
to this species to capture workers than to
procreate them — the habit of collecting pupae originally for
food might by natural selection be strengthened and rendered permanent
for the very different purpose of raising slaves. When the instinct
was once acquired, if carried out to a much less extent even than in
our British F. sanguinea, which, as we have seen, is less aided by its
slaves than the same species in Switzerland, I can see no difficulty
in natural selection increasing and modifying the instinct --
always supposing each modification to be of use to the species --
until an ant was formed as abjectly dependent on its slaves as is the
Formica rufescens.
Cell-making instinct of the
Hive-Bee.
I
will not here enter on minute details on this subject, but will merely
give an outline of the conclusions at which I have arrived. He must be
a dull man who can examine the exquisite structure of a comb, so
beautifully adapted to its end, without enthusiastic admiration. We
hear from mathematicians that bees have practically solved a recondite
problem, and have made their cells of the proper shape to hold the
greatest possible amount of honey, with the least possible consumption
of previous wax in their construction. It has been remarked that a
skilful workman, with fitting tools and measures, would find it very
difficult to make cells of wax of the true form, though this is
perfectly effected by a crowd of bees working in a dark hive. Grant
whatever instincts you please, and it seems at first quite
inconceivable how they can make all the necessary angles and planes,
or even perceive when they are correctly made. But the difficulty
is not nearly so great as it at first appears: all this beautiful work
can be shown, I think, to follow from a few very simple instincts.
I was led to investigate this subject by Mr. Waterhouse, who has
shown that the form of the cell stands in close relation to the
presence of adjoining cells; and the following view may, perhaps, be
considered only as a modification of this theory. Let us look to the
great principle of gradation, and see whether Nature does not reveal
to us her method of work. At one end of a short series we have
humble-bees, which use their old cocoons to hold honey, sometimes
adding to them short tubes of wax, and likewise making separate and
very irregular rounded cells of wax. At the other end of the series we
have the cells of the hive-bee, placed in a double layer: each cell,
as is well know, is an hexagonal prism, with the basal edges of its
six sides bevelled so as to join on to a pyramid, formed of three
rhombs. These rhombs have certain angles, and the three which form the
pyramidal base of a single cell on one side of the comb, enter into
the composition of the bases of three adjoining cells on the opposite
side. In the series between the extreme perfection of the cells of the
hive-bee and the simplicity of those of the humble-bee, we have the
cells of the Mexican Melipona domestica, carefully described and
figured by Pierre Huber. The Melipona itself is intermediate in
structure between the hive and humble bee, but more
nearly related to the latter: it
forms a nearly regular waxen comb of cylindrical cells, in which the
young are hatched, and, in addition, some large cells of wax for
holding honey. These latter cells are nearly spherical and of nearly
equal sizes, and are aggregated into an irregular mass. But the
important point to notice, is that these cells are always made at that
degree of nearness to each other, that they would have intersected or
broken into each other, if the spheres had been completed; but this is
never permitted, the bees building perfectly flat walls of wax between
the spheres
which thus tend to intersect. Hence each cell consists of
an outer spherical portion and of two, three, or more perfectly flat
surfaces, according as the cell adjoins two, three or more other
cells. When one cell comes into contact with three other cells, which,
from the spheres being nearly of the same size, is very frequently and
necessarily the case, the three flat surfaces are united into a
pyramid; and this pyramid, as Huber has remarked, is manifestly a
gross imitation of the three-sided pyramidal basis of the cell of the
hive-bee. As in the cells of the hive-bee, so here, the three plane
surfaces in any one cell necessarily enter into the construction of
three adjoining cells. It is obvious that the Melipona saves wax by
this manner of building; for the flat walls between the adjoining
cells are not double, but are of the same thickness as the outer
spherical portions, and yet each flat portion forms a part of two
cells.
Reflecting on this case, it occurred to me that if the Melipona had
made its spheres at some given distance from each other, and had made
them of equal sizes and had arranged them symmetrically in a double
layer, the resulting structure would probably have been as perfect as
the comb of the hive-bee. Accordingly I wrote to Professor Miller, of
Cambridge, and this geometer has kindly read over the following
statement, drawn up from his information, and tells me that it is
strictly correct:-
If a number of equal spheres be described with their centres placed
in two parallel layers; with the centre of each sphere at the distance
of radius X /sqrt[2] or radius X 1.41421 (or at some lesser distance),
from the centres of the six surrounding spheres in the same layer; and
at the same distance from the centres of the adjoining spheres in the
other and parallel layer; then, if planes of intersection
between the several spheres in
both layers be formed, there will
result a double layer of hexagonal prisms united together by pyramidal
bases formed of three rhombs; and the rhombs and the sides of the
hexagonal prisms will have every angle identically the same with the
best measurements which have been made of the cells of the hive-bee.
Hence we may safely conclude that if we could slightly modify the
instincts already possessed by the Melipona, and in themselves not
very wonderful, this bee would make a structure as wonderfully perfect
as that of the hive-bee. We must suppose the Melipona to make her
cells truly spherical, and of equal sizes; and this would not be very
surprising, seeing that she already does so to a certain extent, and
seeing what perfectly cylindrical burrows in wood many insects can
make, apparently by turning round on a fixed point. We must suppose
the Melipona to arrange her cells in level layers, as she already does
her cylindrical cells; and we must further suppose, and this is the
greatest difficulty, that she can somehow judge accurately at what
distance to stand from her fellow-labourers when several are making
their spheres; but she is already so far enabled to judge of distance,
that she always describes her spheres so as to intersect largely; and
then she unites the points of intersection by perfectly flat surfaces.
We have further to suppose, but this is no difficulty, that after
hexagonal prisms have been formed by the intersection of adjoining
spheres in the same layer, she can prolong the hexagon to any length
requisite to hold the stock of honey; in the same way as the rude
humble-bee adds cylinders of wax to the circular mouths of her old
cocoons. By such modifications of instincts in themselves not very
wonderful, — hardly more wonderful than those which guide a bird
to make its nest, — I believe that the hive-bee
has acquired,
through natural selection, her inimitable architectural powers.
But this theory can be tested by experiment. Following the example
of Mr Tegetmeier, I separated two combs, and put between them a long,
thick, square strip of wax: the bees instantly began to excavate
minute circular pits in it; and as they deepened these little pits,
they made them wider and wider until they were converted into shallow
basins, appearing to the eye perfectly true or parts of a
sphere, and of about the diameter of a cell. It was most interesting
to me to observe that wherever several bees had begun to excavate
these basins near together, they had begun their work at such a
distance from each other, that by the time the basins had acquired the
above stated width (i.e. about the width of
an ordinary cell), and were in depth about one sixth of the diameter
of the sphere of which they formed a part, the rims of the basins
intersected or broke into each other. As soon as this occurred, the
bees ceased to excavate, and began to build up flat walls of wax on
the lines of intersection between the basins, so that each hexagonal
prism was built upon the festooned edge of a smooth basin, instead of
on the straight edges of a three-sided pyramid as in the case of
ordinary cells.
I then put into the hive, instead of a thick, square piece of wax,
a thin and narrow, knife-edged ridge, coloured with vermilion. The
bees instantly began on both sides to excavate little basins near to
each other, in the same way as before; but the ridge of wax was so
thin, that the bottoms of the basins, if they had been excavated to
the same depth as in the former experiment, would have broken into
each other from the opposite sides. The
bees, however, did not suffer this to happen, and they stopped their
excavations in due
time; so that the basins, as soon as they had been
a little deepened, came to have flat bottoms; and these flat bottoms,
formed by thin little plates of the vermilion wax having been left
ungnawed, were situated, as far as the eye could judge, exactly along
the planes of imaginary intersection between the basins on the
opposite sides of the ridge of wax. In parts, only little bits, in
other parts, large portions of a rhombic plate had been left between
the opposed basins, but the work, from the unnatural state of things,
had not been neatly performed. The bees must have worked at very
nearly the same rate on the opposite side of the ridge of vermilion
wax, as they circularly gnawed away and deepened the basins on both
sides, in order to have succeeded in thus leaving flat plates between
the basins, by stopping work along the intermediate planes or planes
of intersection.
Considering how flexible thin wax is, I
do not see that there is any
difficulty in the bees, whilst at work on the two sides of a strip of
wax, perceiving when they have gnawed the wax away to the proper
thinness, and then stopping their work. In ordinary combs it has
appeared to me that the bees do not always succeed in working at
exactly the same rate from the opposite sides; for I have noticed
half-completed rhombs at the base of a just-commenced cell, which were
slightly concave on one side, where I suppose that the bees had
excavated too quickly, and convex on the
opposed side, where the bees had worked less quickly. In one
well-marked instance, I put the comb back into the hive and allowed
the bees to go on working for a short time and again examined the
cell, and I found that the rhombic plate had been completed, and had
become perfectly flat: it was absolutely
impossible, from the extreme thinness of the little rhombic plate,
that they could have affected
this by gnawing away the convex side;
and I suspect that the bees in such cases stand in the opposed cells
and push and bend the ductile and warm wax (which as I have tried is
easily done) into its proper intermediate plane, and thus flatten it.
From the experiment of the ridge of vermilion wax, we can clearly
see that if the bees were to build for themselves a thin wall of wax,
they could make their cells of the proper shape, by standing at the
proper distance from each other, by excavating at the same rate, and
by endeavouring to make equal spherical hollows, but never allowing
the spheres to break into each other. Now bees, as may be clearly seen
by examining the edge of a growing comb, do make a rough,
circumferential wall or rim all round the comb; and they gnaw into
this from the opposite sides, always working circularly as they deepen
each cell. They do not make the whole three-sided pyramidal base of
any one cell at the same time, but only the one rhombic plate which
stands on the extreme growing margin, or the two plates, as the case
may be; and they never complete the upper edges of the rhombic plates,
until the hexagonal walls are commenced. Some of these statements
differ from those made by the justly celebrated elder Huber, but I am
convinced of their accuracy; and if I had space, I could show that
they are conformable with my theory.
Huber's statement that the very first cell is excavated out of a
little parallel-sided wall of wax, is not, as far as I have seen,
strictly correct; the first commencement having always been a little
hood of wax; but I will not here enter on these details. We see how
important a part excavation plays in the construction of the cells;
but it would be a great error to suppose that the bees cannot build up
a rough wall of wax in the proper
position — that is, along the
plane of intersection between two adjoining spheres. I have several
specimens showing clearly that they can do this. Even in the rude
circumferential rim or wall of wax round a growing comb, flexures may
sometimes be observed, corresponding in position to the planes of the
rhombic basal plates of future cells. But the rough wall of wax has in
every case to be finished off, by being largely gnawed away on both
sides. The manner in which the bees build is curious; they always make
the first rough wall from ten to twenty times thicker than the
excessively thin finished wall of the cell, which will ultimately be
left. We shall understand how they work, by supposing masons first to
pile up a broad ridge of cement, and then to begin cutting it away
equally on both sides near the ground, till a smooth, very thin wall
is left in the middle; the masons always piling up the cut-away
cement, and adding fresh cement, on the summit of the ridge. We shall
thus have a thin wall steadily growing upward; but always crowned by a
gigantic coping. From all the cells, both those just commenced and
those completed, being thus crowned by a strong coping of wax, the
bees can cluster and crawl over the comb without injuring the delicate
hexagonal walls, which are only about one four-hundredth of an inch in
thickness; the plates of the pyramidal basis being about twice as
thick. By this singular manner of building, strength is continually
given to the comb, with the utmost ultimate economy of wax.
It seems at first to add to the difficulty of understanding how the
cells are made, that a multitude of bees all work together; one bee
after working a short time at one cell going to another, so that, as
Huber has stated, a score of individuals work even at the commencement
of the first cell. I was able practically to show this fact, by
covering the edges of the hexagonal walls
of a single
cell, or the extreme margin of the circumferential rim of a growing
comb, with an extremely thin layer of melted vermilion wax; and I
invariably found that the colour was most delicately diffused by the
bees — as delicately as a painter could have done with his brush
-- by atoms of the coloured wax having been taken from the spot
on which it had been placed, and worked into the growing edges of the
cells all round. The work of construction seems to be a sort of
balance struck between many bees, all instinctively standing at the
same relative distance from each other, all trying to sweep equal
spheres, and then building up, or leaving ungnawed, the planes of
intersection between these spheres. It was really curious to note in
cases of difficulty, as when two pieces of comb met at an angle, how
often the bees would entirely pull down and rebuild in different ways
the same cell, sometimes recurring to a shape which they had at first
rejected.
When bees have a place on which they can stand in their proper
positions for working, — for instance, on a slip of wood, placed
directly under the middle of a comb growing downwards so that the comb
has to be built over one face of the slip — in this case the
bees can lay the foundations of one wall of a new hexagon, in its
strictly proper place, projecting beyond the other completed cells. It
suffices that the bees should be enabled to stand at their proper
relative distances from each other and from the walls of the last
completed cells, and then, by striking imaginary spheres, they can
build up a wall intermediate between two adjoining spheres; but, as
far as I have seen, they never gnaw away and finish off the angles of
a cell till a large part both of that cell and of the adjoining cells
has been built. This capacity in bees of laying down under certain
circumstances a rough wall in its proper place between two
just-commenced
cells, is important, as it bears on a fact, which seems
at first quite subversive of the foregoing theory; namely, that the
cells on the extreme margin of wasp-combs are sometimes strictly
hexagonal; but I have not space here to enter on this subject. Nor
does there seem to me any great difficulty in a single insect (as in
the case of a queen-wasp) making hexagonal cells, if she work
alternately on the inside and outside of two or three
cells commenced at the same time, always standing at the proper
relative distance from the parts of the cells just begun, sweeping
spheres or cylinders, and building up intermediate planes. It is even
conceivable that an insect might, by fixing on a point at which to
commence a cell, and then moving outside, first to one point, and
then to five other points, at the proper relative distances from the
central point and from each other, strike the planes of intersection,
and so make an isolated hexagon: but I am not aware that any such case
has been observed; nor would any good be derived from a single hexagon
being built, as in its construction more materials would be required
than for a cylinder.
As natural selection acts only by the accumulation of slight
modifications of structure or instinct, each profitable to the
individual under its conditions of life, it may reasonably be asked,
how a long and graduated succession of modified architectural
instincts, all tending towards the present perfect plan of
construction, could have profited the progenitors of the hive-bee? I
think the answer is not difficult: it is known that bees are often
hard pressed to get sufficient nectar; and I am informed by Mr.
Tegetmeier that it has been experimentally found that no less than
from twelve to fifteen pounds of dry sugar are consumed by a hive of
bees for the secretion of each pound of wax; so that a prodigious
quantity of fluid nectar must be collected and consumed by the bees in
a hive for
the secretion of the wax necessary for the construction of
their combs. Moreover, many bees have to remain idle for many days
during the process of secretion. A large store of honey is
indispensable to support a large stock of bees during the winter; and
the security of the hive is known mainly to depend on a large number
of bees being supported. Hence the saving of wax by largely saving
honey must be a most important element of success in any family of
bees. Of course the success of any species of bee may be dependent on
the number of its parasites or other enemies, or on quite distinct
causes, and so be altogether independent of the quantity of honey
which the bees could collect. But let us suppose that this latter
circumstance determined, as it probably often does determine, the
numbers of a humble-bee which could exist in a country;
and let us further suppose that the community lived throughout the
winter, and consequently required a store of honey: there can in this
case be no doubt that it would be an advantage to our humble-bee, if a
slight modification of her instinct led her to make her waxen cells
near together, so as to intersect a little; for a wall in common even
to two adjoining cells, would save some little wax. Hence it would
continually be more and more advantageous to our humble-bee, if she
were to make her cells more and more regular, nearer together, and
aggregated into a mass, like the cells of the Melipona; for in this
case a large part of the bounding surface of each cell would serve to
bound other cells, and much wax would be saved. Again, from the same
cause, it would be advantageous to the Melipona, if she were to make
her cells closer together, and more regular in every way than at
present; for then, as we have seen, the spherical surfaces would
wholly disappear, and would all be replaced by plane surfaces; and the
Melipona
would make a comb as perfect as that of the hive-bee. Beyond
this stage of perfection in architecture, natural selection could not
lead; for the comb of the hive-bee, as far as we can see, is
absolutely perfect in economising wax.
Thus, as I believe, the most wonderful of all known instincts, that
of the hive-bee, can be explained by natural selection having taken
advantage of numerous, successive, slight modifications of simpler
instincts; natural selection having by slow degrees, more and more
perfectly, led the bees to sweep equal spheres at a given distance
from each other in a double layer, and to build up and excavate the
wax along the planes of intersection. The bees, of course, no more
knowing that they swept their spheres at one particular distance from
each other, than they know what are the several angles of the
hexagonal prisms and of the basal rhombic plates. The motive power of
the process of natural selection having been economy of wax; that
individual swarm which wasted least honey in the secretion of wax,
having succeeded best, and having transmitted by inheritance its newly
acquired economical instinct to new swarms, which in their turn will
have had the best chance of succeeding in the struggle for existence.
No doubt many instincts of very difficult explanation could be
opposed to the theory of natural selection, — cases, in which we
cannot see how an instinct could possibly have originated; cases, in
which no intermediate gradations are known to exist; cases of instinct
of apparently such trifling importance, that they could hardly have
been acted on by natural selection; cases of instincts almost
identically the same in animals so remote in the scale of nature, that
we cannot account
for their similarity by inheritance from a common
parent, and must therefore believe that they have been acquired by
independent acts of natural selection. I will not here enter on these
several cases, but will confine myself to one special difficulty,
which at first appeared to me insuperable, and actually fatal to my
whole theory. I allude to the neuters or sterile females in
insect-communities: for these neuters often differ widely in instinct
and in structure from both the males and fertile females, and yet,
from being sterile, they cannot propagate their kind.
The subject well deserves to be discussed at great length, but I
will here take only a single case, that of working or sterile ants.
How the workers have been rendered sterile is a difficulty; but not
much greater than that of any other striking modification of
structure; for it can be shown that some insects and other articulate
animals in a state of nature occasionally become sterile; and if such
insects had been social, and it had been profitable to the community
that a number should have been annually born capable of work, but
incapable of procreation, I can see no very great difficulty in this
being effected by natural selection. But I must pass over this
preliminary difficulty. The great difficulty lies in the working ants
differing widely from both the males and the fertile females in
structure, as in the shape of the thorax and in being destitute of
wings and sometimes of eyes, and in instinct. As far as instinct
alone is concerned, the prodigious difference in this respect between
the workers and the perfect females, would have been far better
exemplified by the hive-bee. If a working ant or other neuter insect
had been an animal in the ordinary state, I should have unhesitatingly
assumed that all its characters had been slowly acquired through
natural selection; namely, by an individual
having been born with some slight profitable modification of structure, this
being inherited by its offspring, which again varied and were again
selected, and so onwards. But with the working ant we have an insect
differing greatly from its parents, yet absolutely sterile; so that it
could never have transmitted successively acquired modifications of
structure or instinct to its progeny. It may well be asked how is it
possible to reconcile this case with the theory of natural selection?
First, let it be remembered that we have innumerable instances,
both in our domestic productions and in those in a state of nature, of
all sorts of differences of structure which have become correlated to
certain ages, and to either sex. We have differences correlated not
only to one sex, but to that short period alone when the reproductive
system is active, as in the nuptial plumage of many birds, and in the
hooked jaws of the male salmon. We have even slight differences in the
horns of different breeds of cattle in relation to an artificially
imperfect state of the male sex; for oxen of certain breeds have
longer horns than in other breeds, in comparison with the horns of the
bulls or cows of these same breeds. Hence I can see no real difficulty
in any character having become correlated with the sterile condition
of certain members of insect-communities: the difficulty lies in
understanding how such correlated modifications of structure could
have been slowly accumulated by natural selection.
This difficulty, though appearing insuperable, is lessened, or, as
I believe, disappears, when it is remembered that selection may be
applied to the family, as well as to the individual, and may thus gain
the desired end. Thus, a well-flavoured vegetable is cooked, and the
individual is destroyed; but the horticulturist sows seeds of the same
stock, and confidently expects to
get nearly the same variety;
breeders of cattle wish the flesh and fat to be well marbled together;
the animal has been slaughtered, but the breeder goes with confidence
to the same family. I have such faith in the powers of selection, that
I do not doubt that a breed of cattle, always yielding oxen with
extraordinarily long horns, could be slowly formed by carefully
watching which individual bulls and cows, when matched, produced oxen
with the longest horns; and yet no one ox could ever have
propagated its kind. Thus I believe it has been with social insects: a
slight modification of structure, or instinct, correlated with the
sterile condition of certain members of the community, has been
advantageous to the community: consequently the fertile males and
females of the same community flourished, and transmitted to their
fertile offspring a tendency to produce sterile members having the
same modification. And I believe that this process has been repeated,
until that prodigious amount of difference between the fertile and
sterile females of the same species has been produced, which we see in
many social insects.
But we have not as yet touched on the climax of the difficulty;
namely, the fact that the neuters of several ants differ, not only
from the fertile females and males, but from each other, sometimes to
an almost incredible degree, and are thus divided into two or even
three castes. The castes, moreover, do not generally graduate into
each other, but are perfectly well defined; being as distinct from
each other, as are any two species of the same genus, or rather as any
two genera of the same family. Thus in Eciton, there are working and
soldier neuters, with jaws and instincts extraordinarily different: in
Cryptocerus, the workers of one caste alone carry a wonderful sort of
shield on their heads, the use of which is quite unknown: in the
Mexican Myrmecocystus,
the workers of one caste never leave the nest;
they are fed by the workers of another caste, and they have an
enormously developed abdomen which secretes a sort of honey, supplying
the place of that excreted by the aphides, or the domestic cattle as
they may be called, which our European ants guard or imprison.
It will indeed be thought that I have an overweening confidence in
the principle of natural selection, when I do not admit that such
wonderful and well-established facts at once annihilate my theory. In
the simpler case of neuter insects all of one caste or of the same
kind, which have been rendered by natural selection, as I believe to
be quite possible, different from the fertile males and females,
-- in this case, we may safely conclude from the analogy of
ordinary variations, that each successive, slight, profitable
modification did not probably at first appear in all the
individual neuters in the same nest, but in a few alone; and that by
the long-continued selection of the fertile parents which produced
most neuters with the profitable modification, all the neuters
ultimately came to have the desired character. On this view we ought
occasionally to find neuter-insects of the same species, in the same
nest, presenting gradations of structure; and this we do find, even
often, considering how few neuter-insects out of Europe have been
carefully examined. Mr F. Smith has shown how surprisingly the
neuters of several British ants differ from each other in size and
sometimes in colour; and that the extreme forms can sometimes be
perfectly linked together by individuals taken out of the same nest: I
have myself compared perfect gradations of this kind. It often happens
that the larger or the smaller sized workers are the most numerous; or
that both large and small are numerous, with those of an intermediate
size scanty in numbers. Formica flava has larger and
smaller workers,
with some of intermediate size; and, in this species, as Mr F. Smith
has observed, the larger workers have simple eyes (ocelli), which
though small can be plainly distinguished, whereas the smaller workers
have their ocelli rudimentary. Having carefully dissected several
specimens of these workers, I can affirm that the eyes are far more
rudimentary in the smaller workers than can be accounted for merely by
their proportionally lesser size; and I fully believe, though I dare
not assert so positively, that the workers of intermediate size have
their ocelli in an exactly intermediate condition. So that we here
have two bodies of sterile workers in the same nest, differing not
only in size, but in their organs of vision, yet connected by some few
members in an intermediate condition. I may digress by adding, that if
the smaller workers had been the most useful to the community, and
those males and females had been continually selected, which produced
more and more of the smaller workers, until all the workers had come
to be in this condition; we should then have had a species of ant with
neuters very nearly in the same condition with those of Myrmica. For
the workers of Myrmica have not even rudiments of ocelli, though the
male and female ants of this genus have well-developed ocelli.
I may give one other case: so confidently did I expect to find
gradations in important points of structure between the different
castes of neuters in the same species, that I gladly availed myself of
Mr F. Smith's offer of numerous specimens from the same nest of the
driver ant (Anomma) of West Africa. The reader will perhaps best
appreciate the amount of difference in these workers, by my giving not
the actual measurements, but a strictly accurate illustration: the
difference was the same as if we were to see a set of workmen building
a house of whom many were five feet four inches high, and many sixteen
feet high; but we must suppose that the larger workmen had heads four
instead of three times as big as those of the smaller men, and jaws
nearly five times as big. The jaws, moreover, of the working ants of
the several sizes differed wonderfully in shape, and in the form and
number of the teeth. But the important fact for us is, that though the
workers can be grouped into castes of different sizes, yet they
graduate insensibly into each other, as does the widely-different
structure of their jaws. I speak confidently on this latter point, as
Mr Lubbock made drawings for me with the camera lucida of the jaws
which I had dissected from the workers of the several sizes.
With these facts before me, I believe that natural selection, by
acting on the fertile parents, could form a species which should
regularly produce neuters, either all of large size with one form of
jaw, or all of small size with jaws having a widely different
structure; or lastly, and this is our climax of difficulty, one set of
workers of one size and structure, and simultaneously another set of
workers of a different size and structure; — a graduated series
having been first formed, as in the case of the driver ant, and then
the extreme forms, from being the most useful to the community, having
been produced in greater and greater numbers through the natural
selection of the parents which generated them; until none with an
intermediate structure were produced.
Thus, as I believe, the wonderful fact of two distinctly defined
castes of sterile workers existing in the same nest, both widely
different from each other and from their parents, has originated. We
can see how useful their production may have been to a
social community of insects, on the same principle that the division
of
labour is useful to civilised man. As ants work by inherited
instincts and by inherited tools or weapons, and not by acquired
knowledge and manufactured instruments, a perfect division of labour
could be effected with them only by the workers being sterile; for had
they been fertile, they would have intercrossed, and their instincts
and structure would have become blended. And nature has, as I believe,
effected this admirable division of labour in the communities of ants,
by the means of natural selection. But I am bound to confess, that,
with all my faith in this principle, I should never have anticipated
that natural selection could have been efficient in so high a degree,
had not the case of these neuter insects convinced me of the fact. I
have, therefore, discussed this case, at some little but wholly
insufficient length, in order to show the power of natural selection,
and likewise because this is by far the most serious special
difficulty, which my theory has encountered. The case, also, is very
interesting, as it proves that with animals, as with plants, any
amount of modification in structure can be effected by the
accumulation of numerous, slight, and as we must call them accidental,
variations, which are in any manner profitable, without exercise or
habit having come into play. For no amount of exercise, or habit, or
volition, in the utterly sterile members of a community could possibly
have affected the structure or instincts of the fertile members, which
alone leave descendants. I am surprised that no one has advanced this
demonstrative case of neuter insects, against the well-known doctrine
of Lamarck.
Summary.
I have endeavoured briefly in this chapter to show that
the mental qualities of our domestic animals vary, and that the
variations are inherited. Still more briefly I have attempted to show
that instincts vary slightly in a state of nature. No one will dispute
that instincts are of the highest importance to each animal. Therefore
I can see no difficulty, under changing conditions of life, in natural
selection accumulating slight modifications of instinct to any extent,
in any useful direction. In some cases habit or use and disuse have
probably come into play. I do not pretend that the facts given in this
chapter strengthen in any great degree my theory; but
none of the cases of difficulty, to the best of my judgment,
annihilate it. On the other hand, the fact that instincts are not
always absolutely perfect and are liable to mistakes; — that no
instinct has been produced for the exclusive good of other animals,
but that each animal takes advantage of the instincts of others;
-- that the canon in natural history, of 'natura non facit
saltum' is applicable to instincts as well as to corporeal structure,
and is plainly explicable on the foregoing views, but is otherwise
inexplicable, — all tend to corroborate the theory of natural
selection.
This theory is, also, strengthened by some few other facts in
regard to instincts; as by that common case of closely allied, but
certainly distinct, species, when inhabiting distant parts of the
world and living under considerably different conditions of life, yet
often retaining nearly the same instincts. For instance, we can
understand on the principle of inheritance, how it is that the thrush
of South America lines its nest with mud, in the same peculiar manner
as does our British thrush: how it is that the male wrens
(Troglodytes) of North America, build 'cock-nests,' to roost in, like
the males of our distinct Kitty-wrens, — a habit wholly unlike
that of any other known bird. Finally, it may not be a logical
deduction, but to my imagination it is far more satisfactory to look
at such instincts as the young
cuckoo ejecting its foster-brothers,
-- ants making slaves, — the larvae of ichneumonidae
feeding within the live bodies of caterpillars, — not as
specially endowed or created instincts, but as small consequences of
one general law, leading to the advancement of all organic beings,
namely, multiply, vary, let the strongest live and the weakest die.
HYBRIDISM
- Distinction between the sterility of first crosses and of hybrids
- Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication
- Laws governing the sterility of hybrids
- Sterility not a special endowment,
but incidental on other differences
- Causes of the sterility of first crosses and of hybrids
- Parallelism between the effects of changed conditions of life
and crossing
- Fertility of varieties when crossed and of their mongrel offspring
not universal
- Hybrids and mongrels compared independently of their fertility
- Summary
THE view generally entertained by naturalists is
that species, when intercrossed, have been specially endowed with the
quality of sterility, in order to prevent the confusion of all organic
forms. This view certainly seems at first probable, for species within
the same country could hardly have kept distinct had they been capable
of crossing freely. The importance of the fact that hybrids are very
generally sterile, has, I think, been much underrated by some late
writers. On the theory of natural selection the case is especially
important, inasmuch as the sterility of hybrids could not possibly be
of any advantage to them, and therefore could not have been acquired
by the continued preservation of successive profitable degrees of
sterility. I hope, however, to be able to show that sterility is not
a specially acquired or endowed quality, but is incidental on other
acquired differences.
In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded together;
namely, the sterility of two
species when first crossed, and the
sterility of the hybrids produced from them.
Pure species have of course their organs of reproduction in a
perfect condition, yet when intercrossed they produce either few or no
offspring. Hybrids, on the other hand, have their reproductive organs functionally impotent, as may be clearly seen in the
state of the male element in both plants and animals; though the
organs themselves are perfect in structure, as far as the microscope
reveals. In the first case the two sexual elements which go to form
the embryo are perfect; in the second case they are either not at all
developed, or are imperfectly developed. This distinction is
important, when the cause of the sterility, which is common to the two
cases, has to be considered. The distinction has probably been slurred
over, owing to the sterility in both cases being looked on as a
special endowment, beyond the province of our reasoning powers.
The fertility of varieties, that is of the forms known or believed
to have descended from common parents, when intercrossed, and
likewise the fertility of their mongrel offspring, is, on my theory, of
equal importance with the sterility of species; for it seems to make a
broad and clear distinction between varieties and species.
First, for the sterility of species when crossed and of their
hybrid offspring. It is impossible to study the several memoirs and
works of those two conscientious and admirable observers,
Klreuter and Grtner, who almost devoted their lives to
this subject, without being deeply impressed with the high generality
of some degree of sterility. Klreuter makes the rule universal;
but then he cuts the knot, for in ten cases in which he found two
forms, considered by most authors as distinct species, quite fertile
together, he
unhesitatingly ranks them as varieties. Grtner,
also, makes the rule equally universal; and he disputes the entire
fertility of Klreuter's ten cases. But in these and in many
other cases, Grtner is obliged carefully to count the seeds, in
order to show that there is any degree of sterility. He always
compares the maximum number of seeds produced by two species when
crossed and by their hybrid offspring, with the average number
produced by both pure parent-species in a state of nature. But a
serious cause of error seems to me to be here introduced: a plant to
be hybridised must be castrated, and, what is often more important,
must be secluded in order to prevent pollen being brought to it by
insects from other plants. Nearly all the plants experimentised on by
Grtner were potted, and apparently were kept in a chamber in his
house. That these processes are often injurious to the
fertility of a plant cannot be doubted; for Grtner gives in his
table about a score of cases of plants which he castrated, and
artificially fertilised with their own pollen, and (excluding all
cases such as the Leguminosae, in which there is an acknowledged
difficulty in the manipulation) half of these twenty plants had their
fertility in some degree impaired. Moreover, as Grtner during
several years repeatedly crossed the primrose and cowslip, which we
have such good reason to believe to be varieties, and only once or
twice succeeded in getting fertile seed; as he found the common red
and blue pimpernels (Anagallis arvensis and coerulea), which the best
botanists rank as varieties, absolutely sterile together; and as he
came to the same conclusion in several other analogous cases; it seems
to me that we may well be permitted to doubt whether many other
species are really so sterile, when intercrossed, as Grtner
believes.
It is certain, on the one hand, that the sterility of various
species when crossed is so different in degree and graduates away so
insensibly, and, on the other hand, that the fertility of pure species
is so easily affected by various circumstances, that for all practical
purposes it is most difficult to say where perfect fertility ends and
sterility begins. I think no better evidence of this can be required
than that the two most experienced observers who have ever lived,
namely, Klreuter and Grtner, should have arrived at
diametrically opposite conclusions in regard to the very same species.
It is also most instructive to compare — but I have not space
here to enter on details — the evidence advanced by our best
botanists on the question whether certain doubtful forms should be
ranked as species or varieties, with the evidence from fertility
adduced by different hybridisers, or by the same author, from
experiments made during different years. It can thus be shown that
neither sterility nor fertility affords any clear distinction between
species and varieties; but that the evidence from this source
graduates away, and is doubtful in the same degree as is the evidence
derived from other constitutional and structural differences.
In regard to the sterility of hybrids in successive generations;
though Grtner was enabled to rear some hybrids, carefully guarding them from a cross with either pure parent, for six
or seven, and in one case for ten generations, yet he asserts
positively that their fertility never increased, but generally greatly
decreased. I do not doubt that this is usually the case, and that the
fertility often suddenly decreases in the first few generations.
Nevertheless I believe that in all these experiments the fertility has
been diminished by an independent cause, namely, from close
interbreeding. I have collected so large a body of facts, showing
that
close interbreeding lessens fertility, and, on the other hand, that an
occasional cross with a distinct individual or variety increases
fertility, that I cannot doubt the correctness of this almost
universal belief amongst breeders. Hybrids are seldom raised by
experimentalists in great numbers; and as the parent-species, or other
allied hybrids, generally grow in the same garden, the visits of
insects must be carefully prevented during the flowering season: hence
hybrids will generally be fertilised during each generation by their
own individual pollen; and I am convinced that this would be injurious
to their fertility, already lessened by their hybrid origin. I am
strengthened in this conviction by a remarkable statement repeatedly
made by Grtner, namely, that if even the less fertile hybrids be
artificially fertilised with hybrid pollen of the same kind, their
fertility, notwithstanding the frequent ill effects of manipulation,
sometimes decidedly increases, and goes on increasing. Now, in
artificial fertilisation pollen is as often taken by chance (as I know
from my own experience) from the anthers of another flower, as from
the anthers of the flower itself which is to be fertilised; so that a
cross between two flowers, though probably on the same plant, would be
thus effected. Moreover, whenever complicated experiments are in
progress, so careful an observer as Grtner would have castrated
his hybrids, and this would have insured in each generation a cross
with the pollen from a distinct flower, either from the same plant or
from another plant of the same hybrid nature. And thus, the strange
fact of the increase of fertility in the successive generations of
artificially fertilised hybrids may, I
believe, be accounted for by close interbreeding having been avoided.
Now let us turn to the results arrived at by the third most experienced hybridiser, namely, the Hon. and
Rev. W.
Herbert. He is as emphatic in his conclusion that some hybrids are
perfectly fertile — as fertile as the pure parent-species
-- as are Klreuter and Grtner that some degree of
sterility between distinct species is a universal law of nature. He
experimentised on some of the very same species as did Grtner.
The difference in their results may, I think, be in part accounted for
by Herbert's great horticultural skill, and by his having hothouses at
his command. Of his many important statements I will here give only a
single one as an example, namely, that 'every ovule in a pod of Crinum
capense fertilised by C. revolutum produced a plant, which (he says)
I never saw to occur in a case of its natural fecundation.' So that we
here have perfect, or even more than commonly perfect, fertility in a
first cross between two distinct species.
This case of the Crinum leads me to refer to a most singular fact,
namely, that there are individual plants, as with certain species of
Lobelia, and with all the species of the genus Hippeastrum, which can
be far more easily fertilised by the pollen of another and distinct
species, than by their own pollen. For these plants have been found to
yield seed to the pollen of a distinct species, though quite sterile
with their own pollen, notwithstanding that their own pollen was found
to be perfectly good, for it fertilised distinct species. So that
certain individual plants and all the individuals of certain species
can actually be hybridised much more readily than they can be
self-fertilised! For instance, a bulb of Hippeastrum aulicum produced
four flowers; three were fertilised by Herbert with their own pollen,
and the fourth was subsequently fertilised by the pollen of a compound
hybrid descended from three other and distinct species: the result was
that 'the ovaries of the three first flowers soon ceased to grow, and
after a
few days perished entirely, whereas the pod impregnated by the
pollen of the hybrid made vigorous growth and rapid progress to
maturity, and bore good seed, which vegetated freely.' In a letter to
me, in 1839, Mr Herbert told me that he had then tried the experiment
during five years, and he continued to try it during several
subsequent years, and always with the same result. This result
has, also, been confirmed by other observers in the case
of Hippeastrum with its sub-genera, and in the case of some other
genera, as Lobelia, Passiflora and Verbascum. Although the plants in
these experiments appeared perfectly healthy, and although both the
ovules and pollen of the same flower were perfectly good with respect
to other species, yet as they were functionally imperfect in their
mutual self-action, we must infer that the plants were in an unnatural
state. Nevertheless these facts show on what slight and mysterious
causes the lesser or greater fertility of species when crossed, in
comparison with the same species when self-fertilised, sometimes
depends.
The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, c., have been crossed, yet many of these
hybrids seed freely. For instance, Herbert asserts that a hybrid from
Calceolaria integrifolia and plantaginea, species most widely
dissimilar in general habit, 'reproduced itself as perfectly as if it
had been a natural species from the mountains of Chile.' I have taken
some pains to ascertain the degree of fertility of some of the complex
crosses of Rhododendrons, and I am assured that many of them are
perfectly fertile. Mr C. Noble, for instance, informs me that he
raises stocks for grafting from a hybrid
between Rhod. Ponticum and
Catawbiense, and that this hybrid 'seeds as freely as it is possible
to imagine.' Had hybrids, when fairly treated, gone on decreasing in
fertility in each successive generation, as Grtner believes to
be the case, the fact would have been notorious to nurserymen.
Horticulturists raise large beds of the same hybrids, and such alone
are fairly treated, for by insect agency the several individuals of
the same hybrid variety are allowed to freely cross with each other,
and the injurious influence of close interbreeding is thus prevented.
Any one may readily convince himself of the efficiency of
insect-agency by examining the flowers of the more sterile kinds of
hybrid rhododendrons, which produce no pollen, for he will find on
their stigmas plenty of pollen brought from other flowers.
In regard to animals, much fewer experiments have been
carefully tried than with plants. If our systematic arrangements can
be trusted, that is if the genera of animals are as distinct from each
other, as are the genera of plants, then we may infer that animals
more widely separated in the scale of nature can be more easily
crossed than in the case of plants; but the hybrids themselves are, I
think, more sterile. I doubt whether any case of a perfectly fertile
hybrid animal can be considered as thoroughly well authenticated. It
should, however, be borne in mind that, owing to few animals breeding
freely under confinement, few experiments have been fairly tried: for
instance, the canary-bird has been crossed with nine other finches,
but as not one of these nine species breeds freely in confinement, we
have no right to expect that the first crosses between them and the
canary, or that their hybrids, should be perfectly fertile. Again, with
respect to the fertility in successive generations of the more fertile
hybrid animals, I hardly know of an instance in which two families of
the same hybrid have been raised at the same time from different
parents, so as to avoid the ill effects of close interbreeding. On the
contrary, brothers and sisters have usually been crossed in each
successive generation, in opposition to the constantly repeated
admonition of every breeder. And in this case, it is not at all
surprising that the inherent sterility in the hybrids should have gone
on increasing. If we were to act thus, and pair brothers and sisters
in the case of any pure animal, which from any cause had the least
tendency to sterility, the breed would assuredly be lost in a very few
generations.
Although I do not know of any thoroughly well-authenticated cases
of perfectly fertile hybrid animals, I have some reason to believe
that the hybrids from Cervulus vaginalis and Reevesii, and from
Phasianus colchicus with p. torquatus and with p. versicolor are
perfectly fertile. The hybrids from the common and Chinese geese (A.
cygnoides), species which are so different that they are generally
ranked in distinct genera, have often bred in this country with either
pure parent, and in one single instance they have bred inter se. This was effected by Mr Eyton, who
raised two hybrids from the same parents but from different hatches;
and from these two birds he raised no less than eight
hybrids (grandchildren of the pure geese) from one nest. In India,
however, these cross-bred geese must be far more fertile; for I am
assured by two eminently capable judges, namely Mr Blyth and Capt.
Hutton, that whole flocks of these crossed geese are kept in various
parts of the country; and as they are kept for profit, where neither
pure parent-species exists, they must certainly be highly fertile.
A doctrine which originated with Pallas, has been
largely accepted
by modern naturalists; namely, that most of our domestic animals have
descended from two or more aboriginal species, since commingled by
intercrossing. On this view, the aboriginal species must either at
first have produced quite fertile hybrids, or the hybrids must have
become in subsequent generations quite fertile under domestication.
This latter alternative seems to me the most probable, and I am
inclined to believe in its truth, although its rests on no direct
evidence. I believe, for instance, that our dogs have descended from
several wild stocks; yet, with perhaps the exception of certain
indigenous domestic dogs of South America, all are quite fertile
together; and analogy makes me greatly doubt, whether the several
aboriginal species would at first have freely bred together and have
produced quite fertile hybrids. So again there is reason to believe
that our European and the humped Indian cattle are quite fertile
together; but from facts communicated to me by Mr Blyth, I think they
must be considered as distinct species. On this view of the origin of
many of our domestic animals, we must either give up the belief of the
almost universal sterility of distinct species of animals when
crossed; or we must look at sterility, not as an indelible
characteristic, but as one capable of being removed by domestication.
Finally, looking to all the ascertained facts on the intercrossing
of plants and animals, it may be concluded that some degree of
sterility, both in first crosses and in hybrids, is an extremely
general result; but that it cannot, under our present state of
knowledge, be considered as absolutely universal.
Laws governing the Sterility of first Crosses
and of Hybrids.
We will now consider a little more in detail the
circumstances and rules governing the sterility of first crosses
and of hybrids. Our chief object will be to see whether
or not the rules indicate that species have specially been endowed
with this quality, in order to prevent their crossing and blending
together in utter confusion. The following rules and conclusions are
chiefly drawn up from Grtner's admirable work on the
hybridisation of plants. I have taken much pains to ascertain how far
the rules apply to animals, and considering how scanty our knowledge
is in regard to hybrid animals, I have been surprised to find how
generally the same rules apply to both kingdoms.
It has been already remarked, that the degree of fertility, both of
first crosses and of hybrids, graduates from zero to perfect
fertility. It is surprising in how many curious ways this gradation
can be shown to exist; but only the barest outline of the facts can
here be given. When pollen from a plant of one family is placed on the
stigma of a plant of a distinct family, it exerts no more influence
than so much inorganic dust. From this absolute zero of fertility, the
pollen of different species of the same genus applied to the stigma of
some one species, yields a perfect gradation in the number of seeds
produced, up to nearly complete or even quite complete fertility; and,
as we have seen, in certain abnormal cases, even to an excess of
fertility, beyond that which the plant's own pollen will produce. So
in hybrids themselves, there are some which never have produced, and
probably never would produce, even with the pollen of either pure
parent, a single fertile seed: but in some of these cases a first
trace of fertility may be detected, by the pollen of one of the pure
parent-species causing the flower of the hybrid to wither earlier than
it otherwise would have done; and the early withering of the flower is
well known to be a sign
of incipient fertilisation. From this extreme
degree of sterility we have self-fertilised hybrids producing a
greater and greater number of seeds up to perfect fertility.
Hybrids from two species which are very difficult to cross, and
which rarely produce any offspring, are generally very sterile; but
the parallelism between the difficulty of making a first cross, and
the sterility of the hybrids thus produced — two classes of
facts which are generally confounded together — is by no means
strict. There are many cases, in which two pure species can be united with unusual facility, and produce numerous
hybrid-offspring, yet these hybrids are remarkably sterile. On the
other hand, there are species which can be crossed very rarely, or
with extreme difficulty, but the hybrids, when at last produced, are
very fertile. Even within the limits of the same genus, for instance
in Dianthus, these two opposite cases occur.
The fertility, both of first crosses and of hybrids, is more easily
affected by unfavourable conditions, than is the fertility of pure
species. But the degree of fertility is likewise innately variable;
for it is not always the same when the same two species are crossed
under the same circumstances, but depends in part upon the
constitution of the individuals which happen to have been chosen for
the experiment. So it is with hybrids, for their degree of fertility
is often found to differ greatly in the several individuals raised
from seed out of the same capsule and exposed to exactly the same
conditions.
By the term systematic affinity is meant, the resemblance between
species in structure and in constitution, more especially in the
structure of parts which are of high physiological importance and
which differ little in the allied species. Now the fertility of first
crosses
between species, and of the hybrids produced from them, is
largely governed by their systematic affinity. This is clearly shown
by hybrids never having been raised between species ranked by
systematists in distinct families; and on the other hand, by very
closely allied species generally uniting with facility. But the
correspondence between systematic affinity and the facility of
crossing is by no means strict. A multitude of cases could be given of
very closely allied species which will not unite, or only with extreme
difficulty; and on the other hand of very distinct species which unite
with the utmost facility. In the same family there may be a genus, as
Dianthus, in which very many species can most readily be crossed; and
another genus, as Silene, in which the most persevering efforts have
failed to produce between extremely close species a single hybrid.
Even within the limits of the same genus, we meet with this same
difference; for instance, the many species of Nicotiana have been more
largely crossed than the species of almost any other genus; but
Grtner found that N. acuminata, which is not a
particularly distinct species, obstinately failed to fertilise, or to
be fertilised by, no less than eight other species of Nicotiana. Very
many analogous facts could be given.
No one has been able to point out what kind, or what amount, of
difference in any recognisable character is sufficient to prevent two
species crossing. It can be shown that plants most widely different in
habit and general appearance, and having strongly marked differences
in every part of the flower, even in the pollen, in the fruit, and in
the cotyledons, can be crossed. Annual and perennial plants, deciduous
and evergreen trees, plants inhabiting different stations and fitted
for extremely different climates, can often be crossed with ease.
By a reciprocal cross between two species, I mean the case, for
instance, of a stallion-horse being first crossed with a female-ass,
and then a male-ass with a mare: these two species may then be said to
have been reciprocally crossed. There is often the widest possible
difference in the facility of making reciprocal crosses. Such cases
are highly important, for they prove that the capacity in any two
species to cross is often completely independent of their systematic
affinity, or of any recognisable difference in their whole
organisation. On the other hand, these cases clearly show that the
capacity for crossing is connected with constitutional differences
imperceptible by us, and confined to the reproductive system. This
difference in the result of reciprocal crosses between the same two
species was long ago observed by Klreuter. To give an instance:
Mirabilis jalappa can easily be fertilised by the pollen of M.
longiflora, and the hybrids thus produced are sufficiently fertile; but
Klreuter tried more than two hundred times, during eight
following years, to fertilise reciprocally M. longiflora with the
pollen of M. jalappa, and utterly failed. Several other equally
striking cases could be given. Thuret has observed the same fact with
certain sea-weeds or Fuci. Grtner, moreover, found that this
difference of facility in making reciprocal crosses is extremely
common in a lesser degree. He has observed it even between forms so
closely related (as Matthiola annua and glabra) that many botanists
rank them only as varieties. It is also a remarkable fact, that
hybrids raised from reciprocal crosses, though of course compounded of
the very same two species, the one species having first
been used as the father and then as the mother, generally differ
in fertility in a small, and occasionally in a high degree.
Several other singular rules could be given from
Grtner: for
instance, some species have a remarkable power of crossing with other
species; other species of the same genus have a remarkable power of
impressing their likeness on their hybrid offspring; but these two
powers do not at all necessarily go together. There are certain
hybrids which instead of having, as is usual, an intermediate
character between their two parents, always closely resemble one of
them; and such hybrids, though externally so like one of their pure
parent-species, are with rare exceptions extremely sterile. So again
amongst hybrids which are usually intermediate in structure between
their parents, exceptional and abnormal individuals sometimes are
born, which closely resemble one of their pure parents; and these
hybrids are almost always utterly sterile, even when the other hybrids
raised from seed from the same capsule have a considerable degree of
fertility. These facts show how completely fertility in the hybrid is
independent of its external resemblance to either pure parent.
Considering the several rules now given, which govern the fertility
of first crosses and of hybrids, we see that when forms, which must be
considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility, or even to fertility under
certain conditions in excess. That their fertility, besides being
eminently susceptible to favourable and unfavourable conditions, is
innately variable. That it is by no means always the same in degree in
the first cross and in the hybrids produced from this cross. That the
fertility of hybrids is not related to the degree in which they
resemble in external appearance either parent. And lastly, that the
facility of making a first cross between any two species is not always
governed by their systematic affinity or
degree of resemblance to each
other. This latter statement is clearly proved by reciprocal crosses
between the same two species, for according as the one species or the
other is used as the father or the mother, there is generally some
difference, and occasionally the widest possible difference, in the facility of effecting an union. The hybrids,
moreover, produced from reciprocal crosses often differ in fertility.
Now do these complex and singular rules indicate that species have
been endowed with sterility simply to prevent their becoming
confounded in nature? I think not. For why should the sterility be so
extremely different in degree, when various species are crossed, all
of which we must suppose it would be equally important to keep from
blending together? Why should the degree of sterility be innately
variable in the individuals of the same species? Why should some
species cross with facility, and yet produce very sterile hybrids; and
other species cross with extreme difficulty, and yet produce fairly
fertile hybrids? Why should there often be so great a difference in
the result of a reciprocal cross between the same two species? Why, it
may even be asked, has the production of hybrids been permitted? To
grant to species the special power of producing hybrids, and then to
stop their further propagation by different degrees of sterility, not
strictly related to the facility of the first union between their
parents, seems to be a strange arrangement.
The foregoing rules and facts, on the other hand, appear to me
clearly to indicate that the sterility both of first crosses and of
hybrids is simply incidental or dependent on unknown differences,
chiefly in the reproductive systems, of the species which are crossed.
The differences being of so peculiar and limited a nature,
that, in
reciprocal crosses between two species the male sexual element of the
one will often freely act on the female sexual element of the other,
but not in a reversed direction. It will be advisable to explain a
little more fully by an example what I mean by sterility being
incidental on other differences, and not a specially endowed quality.
As the capacity of one plant to be grafted or budded on another is so
entirely unimportant for its welfare in a state of nature, I presume
that no one will suppose that this capacity is a specially endowed quality, but will admit that it
is incidental on differences in the laws of growth of the two plants.
We can sometimes see the reason why one tree will not take on another,
from differences in their rate of growth, in the hardness of their
wood, in the period of the flow or nature of their sap, c.; but
in a multitude of cases we can assign no reason whatever.
Great diversity in the size of two plants, one being woody and the
other herbaceous, one being evergreen and the other deciduous, and
adaptation to widely different climates, does not always prevent the
two grafting together. As in hybridisation, so with grafting, the
capacity is limited by systematic affinity, for no one has been able
to graft trees together belonging to quite distinct families; and, on
the other hand, closely allied species, and varieties of the same
species, can usually, but not invariably, be grafted with ease. But
this capacity, as in hybridisation, is by no means absolutely governed
by systematic affinity. Although many distinct genera within the same
family have been grafted together, in other cases species of the same
genus will not take on each other. The pear can be grafted far more
readily on the quince, which is ranked as a distinct genus, than on
the apple, which is a member of the same genus. Even different
varieties of the pear take
with different degrees of facility on the
quince; so do different varieties of the apricot and peach on certain
varieties of the plum.
As Grtner found that there was sometimes an innate difference
in different individuals of the same two
species in crossing; so Sagaret believes this to be the case with
different individuals of the same two species in being grafted
together. As in reciprocal crosses, the facility of effecting an union
is often very far from equal, so it sometimes is in grafting; the
common gooseberry, for instance, cannot be grafted on the currant,
whereas the currant will take, though with difficulty, on the
gooseberry.
We have seen that the sterility of hybrids, which have their
reproductive organs in an imperfect condition, is a very different
case from the difficulty of uniting two pure species, which have their
reproductive organs perfect; yet these two distinct cases run to a
certain extent parallel. Something analogous occurs in grafting; for
Thouin found that three species of Robinia, which seeded freely on
their own roots, and which could be grafted with no great difficulty
on another species, when thus grafted were rendered barren. On the
other hand, certain species of Sorbus, when grafted on other species,
yielded twice as much fruit as when on their own roots. We are reminded by this latter fact of the extraordinary case of
Hippeastrum, Lobelia, c., which seeded much more freely when
fertilised with the pollen of distinct species, than when
self-fertilised with their own pollen.
We thus see, that although there is a clear and fundamental
difference between the mere adhesion of grafted stocks, and the union
of the male and female elements in the act of reproduction, yet that
there is a rude degree of parallelism in the results of grafting and
of crossing distinct species. And as we must look at the curious and
complex laws governing the facility with which trees can be grafted on
each other as incidental on unknown differences in their vegetative
systems, so I believe that the still more complex laws governing the
facility of first crosses, are incidental on unknown differences,
chiefly in their reproductive systems. These differences, in both
cases, follow to a certain extent, as might have been expected,
systematic affinity, by which every kind of resemblance and
dissimilarity between organic beings is attempted to be expressed. The
facts by no means seem to me to indicate that the greater or lesser
difficulty of either grafting or crossing together various species has
been a special endowment; although in the case of crossing, the
difficulty is as important for the endurance and stability of specific
forms, as in the case of grafting it is unimportant for their welfare.
Causes of the Sterility of first Crosses and of
Hybrids.
We may now look a little closer at the probable causes
of the sterility of first crosses and of hybrids. These two cases are
fundamentally different, for, as just remarked, in the union of two
pure species the male and female sexual elements are perfect, whereas
in hybrids they are imperfect. Even in first crosses, the greater or
lesser difficulty in effecting a union apparently depends on several
distinct causes. There must sometimes be a physical impossibility in
the male element reaching the ovule, as would be the case with a plant
having a pistil too long for the pollen-tubes to reach the ovarium. It
has also been observed that when pollen of one species is placed on
the stigma of a distantly allied species, though the pollen-tubes
protrude, they do not penetrate the stigmatic surface.
Again, the
male element may reach the female element, but be incapable
of causing an embryo to be developed, as seems to have been the case
with some of Thuret's experiments on Fuci. No explanation can be given
of these facts, any more than why certain trees cannot be grafted on
others. Lastly, an embryo may be developed, and then perish at an early
period. This latter alternative has not been sufficiently attended to;
but I believe, from observations communicated to me by Mr. Hewitt, who
has had great experience in hybridising gallinaceous birds, that the
early death of the embryo is a very frequent cause of sterility in
first crosses. I was at first very unwilling to believe in this view;
as hybrids, when once born, are generally healthy and long-lived, as
we see in the case of the common mule. Hybrids, however, are
differently circumstanced before and after birth: when born and living
in a country where their two parents can live, they are generally
placed under suitable conditions of life. But a hybrid partakes of
only half of the nature and constitution of its mother, and therefore
before birth, as long as it is nourished within its mother's womb or
within the egg or seed produced by the mother, it may be exposed to
conditions in some degree unsuitable, and consequently be liable to
perish at an early period; more especially as all very young beings
seem eminently sensitive to injurious or unnatural conditions of life.
In regard to the sterility of hybrids, in which the sexual elements
are imperfectly developed, the case is very different. I have more
than once alluded to a large body of facts, which I have collected,
showing that when animals and plants are removed from their natural
conditions, they are extremely liable to have their reproductive
systems seriously affected. This, in fact, is
the great bar to the
domestication of animals. Between the sterility thus superinduced and
that of hybrids, there are many points of similarity. In both cases
the sterility is independent of general health, and is often
accompanied by excess of size or great luxuriance. In both cases, the
sterility occurs in various degrees; in both, the male element is the
most liable to be affected; but sometimes the female more than the
male. In both, the tendency goes to a certain extent with systematic
affinity, or whole groups of animals and plants are rendered impotent by the same unnatural conditions; and whole groups
of species tend to produce sterile hybrids. On the other hand, one
species in a group will sometimes resist great changes of conditions
with unimpaired fertility; and certain species in a group will produce
unusually fertile hybrids. No one can tell, till he tries, whether any
particular animal will breed under confinement or any plant seed
freely under culture; nor can he tell, till he tries, whether any two
species of a genus will produce more or less sterile hybrids. Lastly,
when organic beings are placed during several generations under
conditions not natural to them, they are extremely liable to vary,
which is due, as I believe, to their reproductive systems having been
specially affected, though in a lesser degree than when sterility
ensues. So it is with hybrids, for hybrids in successive generations
are eminently liable to vary, as every experimentalist has observed.
Thus we see that when organic beings are placed under new and
unnatural conditions, and when hybrids are produced by the unnatural
crossing of two species, the reproductive system, independently of the
general state of health, is affected by sterility in a very similar
manner. In the one case, the conditions of life have been disturbed,
though often in so slight a degree as to
be inappreciable by us; in
the other case, or that of hybrids,the external conditions have
remained the same, but the organisation has been disturbed by two
different structures and constitutions having been blended into one.
For it is scarcely possible that two organisations should be
compounded into one, without some disturbance occurring in the
development, or periodical action, or mutual relation of the different
parts and organs one to another, or to the conditions of life. When
hybrids are able to breed inter se, they
transmit to their offspring from generation to generation the same
compounded organisation, and hence we need not be surprised that their
sterility, though in some degree variable, rarely diminishes.
It must, however, be confessed that we cannot understand, excepting
on vague hypotheses, several facts with respect to the sterility of
hybrids; for instance, the unequal fertility of hybrids produced from
reciprocal crosses; or the increased sterility in those hybrids which
occasionally and exceptionally resemble closely either
pure parent. Nor do I pretend that the foregoing remarks go to the
root of the matter: no explanation is offered why an organism, when
placed under unnatural conditions, is rendered sterile. All that I
have attempted to show, is that in two cases, in some respects allied,
sterility is the common result, — in the one case from the
conditions of life having been disturbed, in the other case from the
organisation having been disturbed by two organisations having been
compounded into one.
It may seem fanciful, but I suspect that a similar parallelism
extends to an allied yet very different class of facts. It is an old
and almost universal belief, founded, I think, on a considerable body
of evidence, that slight changes in the conditions of life are
beneficial to all living things. We see this acted on by
farmers and
gardeners in their frequent exchanges of seed, tubers, c., from
one soil or climate to another, and back again. During the
convalescence of animals, we plainly see that great benefit is derived
from almost any change in the habits of life. Again, both with plants
and animals, there is abundant evidence, that a cross between very
distinct individuals of the same species, that is between members of
different strains or sub-breeds, gives vigour and fertility to the
offspring. I believe, indeed, from the facts alluded to in our fourth
chapter, that a certain amount of crossing is indispensable even with
hermaphrodites; and that close interbreeding continued during several
generations between the nearest relations, especially if these be kept
under the same conditions of life, always induces weakness and
sterility in the progeny.
Hence it seems that, on the one hand, slight changes in the
conditions of life benefit all organic beings, and on the other hand,
that slight crosses, that is crosses between the males and females of
the same species which have varied and become slightly different, give
vigour and fertility to the offspring. But we have seen that greater
changes, or changes of a particular nature, often render organic
beings in some degree sterile; and that greater crosses, that is
crosses between males and females which have become widely or
specifically different, produce hybrids which are generally sterile in
some degree. I cannot persuade myself that this
parallelism is an accident or an illusion. Both series of facts seem to
be connected together by some common but unknown bond, which is
essentially related to the principle of life.
Fertility of Varieties when crossed, and of
their Mongrel off-spring.
It may be urged, as a most forcible
argument,
that there must be some essential distinction between
species and varieties, and that there must be some error in all the
foregoing remarks, inasmuch as varieties, however much they may differ
from each other in external appearance, cross with perfect facility,
and yield perfectly fertile offspring. I fully admit that this is
almost invariably the case. But if we look to varieties produced under
nature, we are immediately involved in hopeless difficulties; for if
two hitherto reputed varieties be found in any degree sterile
together, they are at once ranked by most naturalists as species. For
instance, the blue and red pimpernel, the primrose and cowslip, which
are considered by many of our best botanists as varieties, are said by
Grtner not to be quite fertile when crossed, and he
consequently ranks them as undoubted species. If we thus argue in a
circle, the fertility of all varieties produced under nature will
assuredly have to be granted.
If we turn to varieties, produced, or supposed to have been
produced, under domestication, we are still involved in doubt. For when
it is stated, for instance, that the German Spitz dog unites more
easily than other dogs with foxes, or that certain South American
indigenous domestic dogs do not readily cross with European dogs, the
explanation which will occur to everyone, and probably the true one,
is that these dogs have descended from several aboriginally distinct
species. Nevertheless the perfect fertility of so many domestic
varieties, differing widely from each other in appearance, for
instance of the pigeon or of the cabbage, is a remarkable fact; more
especially when we reflect how many species there are, which, though
resembling each other most closely, are utterly sterile when
intercrossed. Several considerations, however, render the fertility of
domestic varieties less remarkable than
at first appears. It can, in
the first place, be clearly shown that mere external dissimilarity
between two species does not determine their greater or
lesser degree of sterility when crossed; and we may apply the same
rule to domestic varieties. In the second place, some eminent
naturalists believe that a long course of domestication tends to
eliminate sterility in the successive generations of hybrids, which
were at first only slightly sterile; and if this be so, we surely
ought not to expect to find sterility both appearing and disappearing
under nearly the same conditions of life. Lastly, and this seems to me
by far the most important consideration, new races of animals and
plants are produced under domestication by man's methodical and
unconscious power of selection, for his own use and pleasure: he
neither wishes to select, nor could select, slight differences in the
reproductive system, or other constitutional difference correlated
with the reproductive system. He supplies his several varieties with
the same food; treats them in nearly the same manner, and does not
wish to alter their general habits of life. Nature acts uniformly and
slowly during vast periods of time on the whole organization, in any
way which may be for each creature's own good; and thus she may,
either directly, or more probably indirectly, through correlation,
modify the reproductive system in the several descendants from any one
species. Seeing this difference in the process of selection, as carried
on by man and nature, we need not be surprised at some difference in
the result.
I have as yet spoken as if the varieties of the same species were
invariably fertile when intercrossed. But it seems to me impossible to
resist the evidence of the existence of a certain amount of sterility
in the few following cases, which I will briefly abstract. The
evidence is at least as good as that from which we believe
in the
sterility of a multitude of species. The evidence is, also, derived
from hostile witnesses, who in all other cases consider fertility and
sterility as safe criterions of specific distinction. Grtner
kept during several years a dwarf kind of maize with yellow seeds, and
a tall variety with red seeds, growing near each other in his garden;
and although these plants have separated sexes, they never naturally
crossed. He then fertilized thirteen flowers of the one with the
pollen of the other; but only a single head produced any seed, and
this one head produced only five grains. Manipulation in
this case could not have been injurious, as the plants have separated
sexes. No one, I believe, has suspected that these varieties of maize
are distinct species; and it is important to notice that the hybrid
plants thus raised were themselves perfectly fertile; so that even Grtner did
not venture to consider the two varieties as specifically distinct.
Girou de Buzareingues crossed three varieties of gourd, which like
the maize has separated sexes, and he asserts that their mutual
fertilization is by so much the less easy as their differences are
greater. How far these experiments may be trusted, I know not; but the
forms experimentised on, are ranked by Sagaret, who mainly founds his
classification by the test of infertility, as varieties.
The following case is far more remarkable, and seems at first quite
incredible; but it is the result of an astonishing number of
experiments made during many years on nine species of Verbascum, by so
good an observer and so hostile a witness, as Grtner: namely,
that yellow and white varieties of the same species of Verbascum when
intercrossed produce less seed, than do either coloured varieties when
fertilized with pollen from their own coloured flowers. Moreover, he
asserts that when
yellow and white varieties of one species are
crossed with yellow and white varieties of a distinct species, more seed is produced by the
crosses between the same coloured flowers, than between those which
are differently coloured. Yet these varieties of Verbascum present no
other difference besides the mere colour of the flower; and one
variety can sometimes be raised from the seed of the other.
From observations which I have made on certain varieties of
hollyhock, I am inclined to suspect that they present analogous facts.
Klreuter, whose accuracy has been confirmed by every
subsequent observer, has proved the remarkable fact, that one variety
of the common tobacco is more fertile, when crossed with a widely
distinct species, than are the other varieties. He experimentised on
five forms, which are commonly reputed to be varieties, and which he
tested by the severest trial, namely, by reciprocal crosses, and he
found their mongrel offspring perfectly fertile. But one
of these five varieties, when used either as father or mother, and
crossed with the Nicotiana glutinosa, always yielded hybrids not so
sterile as those which were produced from the four other varieties
when crossed with N. glutinosa. Hence the reproductive system of this
one variety must have been in some manner and in some degree modified.
From these facts; from the great difficulty of ascertaining the
infertility of varieties in a state of nature, for a supposed variety
if infertile in any degree would generally be ranked as species; from
man selecting only external characters in the production of the most
distinct domestic varieties, and from not wishing or being able to
produce recondite and functional differences in the reproductive
system; from these several considerations and facts, I do not think
that the very general
fertility of varieties can be proved to be of
universal occurrence, or to form a fundamental distinction between
varieties and species. The general fertility of varieties does not
seem to me sufficient to overthrow the view which I have taken with
respect to the very general, but not invariable, sterility of first
crosses and of hybrids, namely, that it is not a special endowment,
but is incidental on slowly acquired modifications, more especially in
the reproductive systems of the forms which are crossed.
Hybrids and Mongrels compared, independently of
their fertility.
Independently of the question of fertility,
the offspring of species when crossed and of varieties when crossed
may be compared in several other respects. Grtner, whose strong
wish was to draw a marked line of distinction between species and
varieties, could find very few and, as it seems to me, quite
unimportant differences between the so-called hybrid offspring of
species, and the so-called mongrel offspring of varieties. And, on the
other hand, they agree most closely in very many important respects.
I shall here discuss this subject with extreme brevity. The most
important distinction is, that in the first generation mongrels are
more variable than hybrids; but Grtner admits that hybrids from
species which have long been cultivated are often variable in the
first generation; and I have myself seen striking
instances of this fact. Grtner further admits that hybrids
between very closely allied species are more variable than those from
very distinct species; and this shows that the difference in the
degree of variability graduates away. When mongrels and the more
fertile hybrids are propagated for several generations an extreme
amount of variability in their offspring is notorious;
but some few
cases both of hybrids and mongrels long retaining uniformity of
character could be given. The variability, however, in the successive
generations of mongrels is, perhaps, greater than in hybrids.
This greater variability of mongrels than of hybrids does not seem
to me at all surprising. For the parents of mongrels are varieties,
and mostly domestic varieties (very few experiments having been tried
on natural varieties), and this implies in most cases that there has
been recent variability; and therefore we might expect that such
variability would often continue and be super-added to that arising
from the mere act of crossing. The slight degree of variability in
hybrids from the first cross or in the first generation, in contrast
with their extreme variability in the succeeding generations, is a
curious fact and deserves attention. For it bears on and corroborates
the view which I have taken on the cause of ordinary variability;
namely, that it is due to the reproductive system being eminently
sensitive to any change in the conditions of life, being thus often
rendered either impotent or at least incapable of its proper function
of producing offspring identical with the parent-form. Now hybrids in
the first generation are descended from species (excluding those long
cultivated) which have not had their reproductive systems in any way
affected, and they are not variable; but hybrids themselves have their
reproductive systems seriously affected, and their descendants are
highly variable.
But to return to our comparison of mongrels and hybrids:
Grtner states that mongrels are more liable than hybrids to
revert to either parent-form; but this, if it be true, is certainly
only a difference in degree.
Grtner further insists that when any two species, although most
closely allied to each other, are
crossed with a third species, the
hybrids are widely different from each other; whereas if two very
distinct varieties of one species are crossed with another species,
the hybrids do not differ much. But this conclusion, as far as I can
make out, is founded on a single experiment; and seems
directly opposed to the results of several experiments made by
Klreuter.
These alone are the unimportant differences, which Grtner is
able to point out, between hybrid and mongrel plants. On the other
hand, the resemblance in mongrels and in hybrids to their respective
parents, more especially in hybrids produced from nearly related
species, follows according to Grtner the same laws. When two
species are crossed, one has sometimes a prepotent power of impressing
its likeness on the hybrid; and so I believe it to be with varieties
of plants. With animals one variety certainly often has this prepotent
power over another variety. Hybrid plants produced from a reciprocal
cross, generally resemble each other closely; and so it is with
mongrels from a reciprocal cross. Both hybrids and mongrels can be
reduced to either pure parent-form, by repeated crosses in successive
generations with either parent.
These several remarks are apparently applicable to animals; but the
subject is here excessively complicated, partly owing to the existence
of secondary sexual characters; but more especially owing to
prepotency in transmitting likeness running more strongly in one sex
than in the other, both when one species is crossed with another, and
when one variety is crossed with another variety. For instance, I
think those authors are right, who maintain that the ass has a
prepotent power over the horse, so that both the mule and the hinny
more resemble the ass than the horse; but that the prepotency runs
more strongly in the male-ass than in
the female, so that the mule,
which is the offspring of the male-ass and mare, is more like an ass,
than is the hinny, which is the offspring of the female-ass and
stallion.
Much stress has been laid by some authors on the supposed fact,
that mongrel animals alone are born closely like one of their parents;
but it can be shown that this does sometimes occur with hybrids; yet I
grant much less frequently with hybrids than with mongrels. Looking to
the cases which I have collected of cross-bred animals closely
resembling one parent, the resemblances seem chiefly confined to
characters almost monstrous in their nature, and which have suddenly
appeared — such as albinism, melanism, deficiency of tail or
horns, or additional fingers and toes; and do not relate
to characters which have been slowly acquired by selection.
Consequently, sudden reversions to the perfect character of either
parent would be more likely to occur with mongrels, which are
descended from varieties often suddenly produced and semi-monstrous in
character, than with hybrids, which are descended from species slowly
and naturally produced. On the whole I entirely agree with Dr Prosper
Lucas, who, after arranging an enormous body of facts with respect to
animals, comes to the conclusion, that the laws of resemblance of the
child to its parents are the same, whether the two parents differ much
or little from each other, namely in the union of individuals of the
same variety, or of different varieties, or of distinct species.
Laying aside the question of fertility and sterility, in all other
respects there seems to be a general and close similarity in the
offspring of crossed species, and of crossed varieties. If we look at
species as having been specially created, and at varieties as having
been produced by secondary laws, this similarity would be an
astonishing fact. But it harmonizes perfectly with the view that there
is no essential distinction between species and varieties.
Summary of Chapter.
First crosses
between forms sufficiently distinct to be ranked as species, and their
hybrids, are very generally, but not universally, sterile. The
sterility is of all degrees, and is often so slight that the two most
careful experimentalists who have ever lived, have come to
diametrically opposite conclusions in ranking forms by this test. The
sterility is innately variable in individuals of the same species, and
is eminently susceptible of favourable and unfavourable conditions.
The degree of sterility does not strictly follow systematic affinity,
but is governed by several curious and complex laws. It is generally
different, and sometimes widely different, in reciprocal crosses
between the same two species. It is not always equal in degree in a
first cross and in the hybrid produced from this cross.
In the same manner as in grafting trees, the capacity of one
species or variety to take on another, is incidental on generally unknown differences in their vegetative systems, so in
crossing, the greater or less facility of one species to unite with
another, is incidental on unknown differences in their reproductive
systems. There is no more reason to think that species have been
specially endowed with various degrees of sterility to prevent them
crossing and blending in nature, than to think that trees have been
specially endowed with various and somewhat analogous degrees of
difficulty in being grafted together in order to prevent them becoming
inarched in our forests.
The sterility of first crosses between pure species, which have
their reproductive systems perfect, seems
to depend on several
circumstances; in some cases largely on the early death of the embryo.
The sterility of hybrids, which have their reproductive systems
imperfect, and which have had this system and their whole organisation
disturbed by being compounded of two distinct species, seems closely
allied to that sterility which so frequently affects pure species,
when their natural conditions of life have been disturbed. This view
is supported by a parallelism of another kind; — namely, that
the crossing of forms only slightly different is favourable to the
vigour and fertility of their offspring; and that slight changes in
the conditions of life are apparently favourable to the vigour and
fertility of all organic beings. It is not surprising that the degree
of difficulty in uniting two species, and the degree of sterility of
their hybrid-offspring should generally correspond, though due to
distinct causes; for both depend on the amount of difference of some
kind between the species which are crossed. Nor is it surprising that
the facility of effecting a first cross, the fertility of the hybrids
produced, and the capacity of being grafted together — though
this latter capacity evidently depends on widely different
circumstances — should all run, to a certain extent, parallel
with the systematic affinity of the forms which are subjected to
experiment; for systematic affinity attempts to express all kinds of
resemblance between all species.
First crosses between forms known to be varieties, or sufficiently
alike to be considered as varieties, and their mongrel offspring, are
very generally, but not quite universally, fertile. Nor is this nearly
general and perfect fertility surprising, when we
remember how liable we are to argue in a circle with respect to
varieties in a state of nature; and when we remember that the greater
number of varieties have been produced under domestication
by the
selection of mere external differences, and not of differences in the
reproductive system. In all other respects, excluding fertility, there
is a close general resemblance between hybrids and mongrels. Finally,
then, the facts briefly given in this chapter do not seem to me
opposed to, but even rather to support the view, that there is no
fundamental distinction between species and varieties.
ON THE IMPERFECTION OF THE
GEOLOGICAL RECORD
- On the absence of intermediate varieties at the present day
- On the nature of extinct intermediate varieties; on their number
- On the vast lapse of time, as inferred from the rate of
deposition and of denudation
- On the poorness of our palaeontological collections
- On the intermittence of geological formations
- On the absence of intermediate varieties in any one formation
- On their sudden appearance in the lowest known fossiliferous
strata
IN the sixth chapter I
enumerated the chief objections which might be justly urged against
the views maintained in this volume. Most of them have now been
discussed. One, namely the distinctness of specific forms, and their
not being blended together by innumerable transitional links, is a
very obvious difficulty. I assigned reasons why such links do not
commonly occur at the present day, under the circumstances apparently
most favourable for their presence, namely on an extensive and
continuous area with graduated physical conditions. I endeavoured to
show, that the life of each species depends in a more important manner
on the presence of other already defined organic forms, than on
climate; and, therefore, that the really governing conditions of life
do not graduate away quite insensibly like heat or moisture. I
endeavoured, also, to show that intermediate varieties, from existing
in lesser numbers than the forms which they connect, will generally
be beaten out and exterminated during the course of further
modification and improvement. The main cause, however, of innumerable
intermediate links not now occurring everywhere throughout nature
depends
on the very process of natural selection, through which new
varieties continually take the places of and exterminate their
parent-forms. But just in proportion as this process of
extermination has acted on an enormous scale, so must the number of
intermediate varieties, which have formerly existed on the earth, be
truly enormous. Why then is not every geological formation and every
stratum full of such intermediate links? Geology assuredly does not
reveal any such finely graduated organic chain; and this, perhaps, is
the most obvious and gravest objection which can be urged against my
theory. The explanation lies, as I believe, in the extreme
imperfection of the geological record.
In the first place it should always be borne in mind what sort of
intermediate forms must, on my theory, have formerly existed. I have
found it difficult, when looking at any two species, to avoid
picturing to myself, forms directly
intermediate between them. But this is a wholly false view; we should
always look for forms intermediate between each species and a common
but unknown progenitor; and the progenitor will generally have
differed in some respects from all its modified descendants. To give a
simple illustration: the fantail and pouter pigeons have both
descended from the rock-pigeon; if we possessed all the intermediate
varieties which have ever existed, we should have an extremely close
series between both and the rock-pigeon; but we should have no
varieties directly intermediate between the fantail and pouter; none,
for instance, combining a tail somewhat expanded with a crop somewhat
enlarged, the characteristic features of these two breeds. These two
breeds, moreover, have become so much modified, that if we had no
historical or indirect evidence regarding their origin, it would not
have been possible to have
determined from a mere comparison of their
structure with that of the rock-pigeon, whether they had descended
from this species or from some other allied species, such as C. oenas.
So with natural species, if we look to forms very distinct, for
instance to the horse and tapir, we have no
reason to suppose that links ever existed directly intermediate
between them, but between each and an unknown common parent. The
common parent will have had in its whole organisation much general
resemblance to the tapir and to the horse; but in some points of
structure may have differed considerably from both, even perhaps more than they differ from each other. Hence in all such
cases, we should be unable to recognise the parent-form of any two or
more species, even if we closely compared the structure of the parent
with that of its modified descendants, unless at the same time we had
a nearly perfect chain of the intermediate links.
It is just possible by my theory, that one of two living forms
might have descended from the other; for instance, a horse from a
tapir; and in this case direct intermediate
links will have existed between them. But such a case would imply that
one form had remained for a very long period unaltered, whilst its
descendants had undergone a vast amount of change; and the principle
of competition between organism and organism, between child and
parent, will render this a very rare event; for in all cases the new
and improved forms of life will tend to supplant the old and unimproved.
By the theory of natural selection all living species have been
connected with the parent-species of each genus, by differences not
greater than we see between the varieties of the same species at the
present
day; and these parent-species, now generally extinct, have in
their turn been similarly connected with more ancient species; and so
on backwards, always converging to the
common ancestor of each great class. So that the number of intermediate and
transitional links, between all living and extinct species, must have
been inconceivably great. But assuredly, if this theory be true, such
have lived upon this earth.
On the lapse of Time.
Independently of
our not finding fossil remains of such infinitely numerous connecting
links, it may be objected, that time will not have sufficed for so
great an amount of organic change, all changes having been effected
very slowly through natural selection. It is hardly possible for me
even to recall to the reader, who may not be a practical geologist,
the facts leading the mind feebly to comprehend the lapse of time. He
who can read Sir Charles Lyell's grand work on the Principles of
Geology, which the future historian will recognise as having produced
a revolution in natural science, yet does not admit how
incomprehensibly vast have been the past periods of time, may at once
close this volume. Not that it suffices to study the
Principles of Geology, or to read special treatises by different
observers on separate formations, and to mark how each author attempts
to give an inadequate idea of the duration of each formation
or even each stratum. A man must for years
examine for himself great piles of superimposed strata, and watch the
sea at work grinding down old rocks and making fresh sediment, before
he can hope to comprehend anything of the lapse of time, the monuments
of which we see around us.
It is good to wander along lines of
sea-coast, when formed of moderately hard rocks, and mark the
process
of degradation. The tides in most cases reach the cliffs only for a
short time twice a day, and the waves eat into them only when they are
charged with sand or pebbles; for there is reason to believe that pure
water can effect little or nothing in wearing away rock. At last the
base of the cliff is undermined, huge fragments fall down, and these
remaining fixed, have to be worn away, atom by atom, until reduced in
size they can be rolled about by the waves, and then are more quickly
ground into pebbles, sand, or mud. But how often do we see along the
bases of retreating cliffs rounded boulders, all thickly clothed by
marine productions, showing how little they are abraded and how seldom
they are rolled about! Moreover, if we follow for a few miles any
line of rocky cliff, which is undergoing degradation, we find that it
is only here and there, along a short length or round a promontory,
that the cliffs are at the present time suffering. The appearance of
the surface and the vegetation show that elsewhere years have elapsed
since the waters washed their base.
He who most closely studies the action of the sea on
our shores, will, I believe, be most deeply
impressed with the slowness with which rocky coasts are worn away. The
observations on this head by Hugh Miller, and by that excellent
observer Mr. Smith of Jordan Hill, are most impressive. With the mind
thus impressed, let any one examine beds of conglomerate many thousand
feet in thickness, which, though probably formed at a quicker rate
than many other deposits, yet, from being formed of worn and rounded
pebbles, each of which bears the stamp of time, are good to show how
slowly the mass has been accumulated. Let him remember Lyell's
profound remark, that the
thickness and extent of sedimentary formations
are the result
and measure of the degradation which the earth's crust has elsewhere
suffered. And what an amount of degradation is implied by the
sedimentary deposits of many countries! Professor Ramsay has given me
the maximum thickness, in most cases from actual measurement, in a few
cases from estimate, of each formation in different parts of Great
Britain; and this is the result:-
Feet
Palaeozoic strata (not including igneous beds) 57,154
Secondary strata 13,190
Tertiary strata 2,240
-- making altogether 72,584 feet; that
is, very nearly thirteen and three-quarters British miles. Some of
these formations, which are represented in England by thin beds, are
thousands of feet in thickness on the Continent. Moreover, between
each successive formation, we have, in the opinion of most geologists,
enormously long blank periods. So that the lofty pile of sedimentary
rocks in Britain, gives but an inadequate idea of the time which has
elapsed during their accumulation; yet what time this must have
consumed! Good observers have estimated that sediment is deposited by
the great Mississippi river at the rate of only
600 feet in a hundred thousand years. This estimate may be
quite erroneous; yet, considering over what wide spaces very fine
sediment is transported by the currents of the sea, the process of
accumulation in any one area must be extremely slow.
But the amount of denudation which the strata have in many places
suffered, independently of the rate of accumulation of the degraded
matter, probably offers the best evidence of the lapse of time. I
remember having been much struck with the evidence of denudation, when
viewing volcanic islands, which have been
worn by the waves and pared
all round into perpendicular cliffs of one
or two thousand feet in height; for the gentle
slope of the lava-streams, due to their formerly liquid state, showed
at a glance how far the hard, rocky beds had once extended into the
open ocean. The same story is still more plainly told by faults,
-- those great cracks along which the strata have been upheaved on
one side, or thrown down on the other, to the height or depth of
thousands of feet; for since the crust cracked, the surface of the land has been so completely planed down by the action
of the sea, that no trace of these vast dislocations is externally
visible.
The Craven fault, for instance, extends for upwards of 30 miles,
and along this line the vertical displacement of the strata has varied
from 600 to 3000 feet. Prof. Ramsay has published an account of a
downthrow in Anglesea of 2300 feet; and he informs me that he fully
believes there is one in Merionethshire of 12,000 feet; yet in these
cases there is nothing on the surface to show such prodigious
movements; the pile of rocks on the one or other side having been
smoothly swept away. The consideration of these facts impresses my
mind almost in the same manner as does the vain endeavour to grapple
with the idea of eternity.
I am tempted to give one other case, the well-known one of the
denudation of the Weald. Though it must be admitted that the
denudation of the Weald has been a mere trifle, in comparison with
that which has removed masses of our Palaeozoic strata, in parts ten
thousand feet in thickness, as shown in Prof. Ramsay's masterly memoir
on this subject. Yet it is an admirable lesson to stand on the North
Downs and to look at the distant South Downs; for, remembering that at
no great distance to the west the northern and southern escarpments
meet and close, one can safely picture to
oneself the great dome of
rocks which must have covered up the Weald within so limited a period
as since the latter part of the Chalk formation. The distance from the
northern to the southern Downs is about 22 miles, and the thickness of
the several formations is on an average about 1100 feet, as I am
informed by Prof. Ramsay. But if, as some geologists suppose, a range
of older rocks underlies the Weald, on the flanks of which the
overlying sedimentary deposits might have accumulated in thinner
masses than elsewhere, the above estimate would be erroneous; but this
source of doubt probably would not greatly affect the estimate as
applied to the western extremity of the
district. If, then, we knew the rate at which the sea commonly wears
away a line of cliff of any given height, we could measure the time
requisite to have denuded the Weald. This, of course, cannot be done;
but we may, in order to form some crude
notion on the
subject, assume that the sea would eat into cliffs 500 feet in height
at the rate of one inch in a century. This will at first appear much
too small an allowance; but it is the same
as if we were to assume a cliff one yard in height to be eaten back
along a whole line of coast at the rate of one yard in nearly every
twenty-two years. I doubt whether any rock, even as soft as chalk,
would yield at this rate excepting on the most exposed coasts; though
no doubt the degradation of a lofty cliff would be more rapid from the
breakage of the fallen fragments. On the other hand, I do not believe
that any line of coast, ten or twenty miles in length, ever suffers
degradation at the same time along its whole indented length; and we
must remember that almost all strata contain harder layers or nodules,
which from long resisting attrition form a breakwater at the base.
Hence, under ordinary circumstances, I conclude that for a cliff
500 feet in height, a denudation
of one inch per
century for the whole length would be an ample allowance. At this
rate, on the above data, the denudation of the Weald must have
required 306,662,400 years; or say three
hundred million years.
The action of fresh water on the gently inclined Wealden district,
when upraised, could hardly have been great, but it would somewhat
reduce the above estimate. On the other hand, during oscillations of
level, which we know this area has undergone, the surface may have
existed for millions of years as land, and thus have escaped the
action of the sea: when deeply submerged for perhaps equally long
periods, it would, likewise, have escaped the action of the
coast-waves. So that in all probability a
far longer period than 300 million years
has elapsed since the latter part of the Secondary period.
I have made these few remarks because it is highly important for us
to gain some notion, however imperfect, of the lapse of years. During
each of these years, over the whole world, the land and the water has
been peopled by hosts of living forms. What an infinite number of
generations, which the mind cannot grasp, must have succeeded each
other in the long roll of years! Now turn to our richest geological
museums, and what a paltry display we behold!
On the poorness of our
Palaeontological collections.
That our Palaeontological collections are very imperfect, is admitted
by every one. The remark of that admirable Palaeontologist, the late
Edward Forbes, should not be forgotten, namely, that numbers of our
fossil species are known and named from single and often broken
specimens, or from a few specimens collected on some one spot. Only a
small portion of the surface of the earth has been geologically
explored, and no part with
sufficient care, as the important
discoveries made every year in Europe prove. No organism wholly soft
can be preserved. Shells and bones will decay and disappear when left
on the bottom of the sea, where sediment is not accumulating. I
believe we are continually taking a most erroneous view, when we
tacitly admit to ourselves that sediment is being deposited over
nearly the whole bed of the sea, at a rate sufficiently quick to embed
and preserve fossil remains. Throughout an enormously large proportion
of the ocean, the bright blue tint of the water bespeaks its purity.
The many cases on record of a formation conformably covered, after an
enormous interval of time, by another and later formation, without the
underlying bed having suffered in the interval any wear and tear, seem
explicable only on the view of the bottom of the sea not rarely lying
for ages in an unaltered condition. The remains which do become
embedded, if in sand or gravel, will when the beds are upraised
generally be dissolved by the percolation of rain-water. I suspect
that but few of the very many animals which live on the beach between
high and low watermark are preserved. For instance, the several
species of the Chthamalinae (a sub-family of sessile cirripedes) coat
the rocks all over the world in infinite numbers: they are all
strictly littoral, with the exception of a single Mediterranean
species, which inhabits deep water and has been found fossil in
Sicily, whereas not one other species has hitherto been found in any
tertiary formation: yet it is now known that the genus Chthamalus
existed during the chalk period. The molluscan genus Chiton offers a
partially analogous case.
With respect to the terrestrial productions which lived during the
Secondary and Palaeozoic periods, it is superfluous to state that our
evidence from fossil
remains is fragmentary in an extreme degree. For
instance, not a land shell is known belonging
to either of these vast periods,
with one exception discovered by Sir C. Lyell in the carboniferous
strata of North America. I n regard to mammiferous remains, a single
glance at the historical table published in the Supplement to Lyell's
Manual, will bring home the truth, how accidental and rare is their
preservation, far better than pages of detail. Nor is their rarity
surprising, when we remember how large a proportion of the bones of
tertiary mammals have been discovered either in caves or in lacustrine
deposits; and that not a cave or true lacustrine bed is known
belonging to the age of our secondary or palaeozoic formations.
But the imperfection in the geological record mainly results from
another and more important cause than any of the foregoing; namely,
from the several formations being separated from each other by wide
intervals of time. When we see the formations tabulated in written
works, or when we follow them in nature, it is difficult to avoid
believing that they are closely consecutive. But we know, for
instance, from Sir R. Murchison's great work on Russia, what wide gaps
there are in that country between the superimposed formations; so it
is in North America, and in many other parts of the world. The most
skilful geologist, if his attention had been exclusively confined to
these large territories, would never have suspected that during the
periods which were blank and barren in his own country, great piles of
sediment, charged with new and peculiar forms of life, had elsewhere
been accumulated. And if in each separate territory, hardly any idea
can be formed of the length of time which has elapsed between the
consecutive formations, we may infer that this could nowhere be
ascertained. The frequent
and great changes in the mineralogical
composition of consecutive formations, generally implying great
changes in the geography of the surrounding lands, whence the sediment
has been derived, accords with the belief of vast intervals of time
having elapsed between each formation.
But we can, I think, see why the geological formations of each
region are almost invariably intermittent; that is, have not followed
each other in close sequence. Scarcely any fact struck me more when
examining many hundred miles of the South American coasts, which have
been upraised several hundred feet within the recent period, than the
absence of any recent deposits sufficiently extensive
to last for even a short geological period. Along the whole west
coast, which is inhabited by a peculiar marine fauna, tertiary beds
are so scantily developed, that no record of several successive and
peculiar marine faunas will probably be preserved to a distant age. A
little reflection will explain why along the rising coast of the
western side of South America, no extensive formations with recent or
tertiary remains can anywhere be found, though the supply of sediment
must for ages have been great, from the enormous degradation of the
coast-rocks and from muddy streams entering the sea. The explanation,
no doubt, is, that the littoral and sub-littoral deposits are
continually worn away, as soon as they are brought up by the slow and
gradual rising of the land within the grinding action of the
coast-waves.
We may, I think, safely conclude that sediment must be accumulated
in extremely thick, solid, or extensive masses, in order to withstand
the incessant action of the waves, when first upraised and during
subsequent oscillations of level. Such thick and extensive
accumulations of sediment may be formed in two ways; either,
in
profound depths of the sea, in which case, judging from the researches
of E. Forbes, we may conclude that the
bottom will be inhabited by extremely few
animals, and the mass when upraised will give a most imperfect record
of the forms of life which then existed; or, sediment may be
accumulated to any thickness and extent over a shallow bottom, if it
continue slowly to subside. In this latter case, as long as the rate
of subsidence and supply of sediment nearly balance each other, the
sea will remain shallow and favourable for life, and thus a
fossiliferous formation thick enough, when upraised, to resist any
amount of degradation, may be formed.
I am convinced that all our ancient formations, which are rich in
fossils, have thus been formed during subsidence. Since publishing my
views on this subject in 1845, I have watched the progress of Geology,
and have been surprised to note how author after author, in treating
of this or that great formation, has come to the conclusion that it
was accumulated during subsidence. I may add, that the only ancient
tertiary formation on the west
coast of South America, which has been bulky enough
to resist such degradation as it has as yet suffered, but which will
hardly last to a distant geological age, was certainly deposited
during a downward oscillation of level, and thus gained considerable
thickness.
All geological facts tell us plainly that each area has undergone
numerous slow oscillations of level, and apparently these oscillations
have affected wide spaces. Consequently formations rich in fossils
and sufficiently thick and extensive to resist subsequent degradation,
may have been formed over wide spaces during periods of subsidence,
but only where the supply of sediment was sufficient to keep the sea
shallow and to embed and
preserve the remains before they had time to
decay. On the other hand, as long as the bed of the sea remained
stationary, thick deposits could not have been accumulated in the
shallow parts, which are the most favourable to life. Still less could
this have happened during the alternate periods of elevation; or, to
speak more accurately, the beds which were then accumulated will have
been destroyed by being upraised and brought within the limits of the
coast-action.
Thus the geological record will almost necessarily be rendered
intermittent. I feel much confidence in the truth of these views, for
they are in strict accordance with the general principles inculcated
by Sir C. Lyell; and E. Forbes independently arrived at a similar
conclusion.
One remark is here worth a passing notice. During periods of
elevation the area of the land and of the adjoining shoal parts of the
sea will be increased, and new stations will often be formed; --
all circumstances most favourable, as previously explained, for the
formation of new varieties and species; but during such periods there
will generally be a blank in the geological record. On the other hand,
during subsidence, the inhabited area and number of inhabitants will
decrease (excepting the productions on the shores of a continent when
first broken up into an archipelago), and consequently during
subsidence, though there will be much extinction, fewer new varieties
or species will be formed; and it is during these very periods of
subsidence, that our great deposits rich in fossils have been
accumulated. Nature may almost be said to have guarded
against the frequent discovery of her transitional or linking forms.
From the foregoing considerations it cannot be doubted that the
geological record, viewed as a whole, is extremely imperfect; but if
we confine our attention to any one formation, it becomes more
difficult to understand,
why we do not therein find closely graduated
varieties between the allied species which lived at its commencement
and at its close. Some cases are on record of the same species
presenting distinct varieties in the upper and lower parts of the same
formation, but, as they are rare, they may be here passed over.
Although each formation has indisputably required a vast number of
years for its deposition, I can see several reasons why each should
not include a graduated series of links between the species which then
lived; but I can by no means pretend to assign due proportional weight
to the following considerations.
Although each formation may mark a very long lapse of years, each
perhaps is short compared with the period requisite
to change one species into another. I am aware
that two palaeontologists, whose opinions are worthy of much
deference, namely Bronn and Woodward, have concluded that the average
duration of each formation is twice or thrice as long as the average
duration of specific forms. But insuperable difficulties, as it seems
to me, prevent us coming to any just conclusion on this head. When we
see a species first appearing in the middle of any formation, it would
be rash in the extreme to infer that it had not elsewhere previously
existed. So again when we find a species disappearing before the
uppermost layers have been deposited, it would be equally rash
to suppose that it then became wholly extinct. We
forget how small the area of Europe is compared with the rest of the
world; nor have the several stages of the same formation throughout
Europe been correlated with perfect accuracy.
With marine animals of all kinds, we may safely infer a large
amount of migration during climatal and other changes; and when we see
a species first appearing in any formation, the probability is that it
only then first immigrated into that area. It is well known, for
instance, that several species appeared somewhat earlier
in the palaeozoic beds of North America than in those of Europe; time
having apparently been required for their migration from the American
to the European seas. In examining the latest deposits of various
quarters of the world, it has everywhere been noted, that some few
still existing species are common in the deposit, but have become
extinct in the immediately surrounding sea; or, conversely, that some
are now abundant in the neighbouring sea, but are rare or absent in
this particular deposit. It is an excellent lesson to reflect on the
ascertained amount of migration of the inhabitants of Europe during
the Glacial period, which forms only a part of one whole geological
period; and likewise to reflect on the great changes of level, on the
inordinately great change of climate, on the prodigious lapse of time,
all included within this same glacial period. Yet it may be doubted
whether in any quarter of the world, sedimentary deposits,
including fossil remains, have gone on
accumulating within the same area during the whole of this period. It
is not, for instance, probable that sediment was deposited during the
whole of the glacial period near the mouth of the Mississippi, within
that limit of depth at which marine animals can flourish; for we know
what vast geographical changes occurred in other parts of America
during this space of time. When such beds as were deposited in shallow
water near the mouth of the Mississippi during some part of the
glacial period shall have been upraised, organic remains will probably
first appear and disappear at different levels, owing to the migration
of species and to geographical changes. And in the distant future, a
geologist examining these beds, might be tempted to conclude that the
average duration of life
of the embedded fossils had been less than
that of the glacial period, instead of having been really far greater,
that is extending from before the glacial epoch to the present day.
In order to get a perfect gradation between two forms in the upper
and lower parts of the same formation, the deposit must have gone on
accumulating for a very long period, in order to have given sufficient
time for the slow process of variation; hence the deposit will
generally have to be a very thick one; and the species undergoing
modification will have had to live on the same area
throughout this whole time. But we have seen that a thick
fossiliferous formation can only be accumulated during a period of
subsidence; and to keep the depth approximately the same, which is
necessary in order to enable the same species to live on the same
space, the supply of sediment must nearly have counterbalanced the
amount of subsidence. But this same movement of subsidence will often
tend to sink the area whence the sediment is derived, and thus
diminish the supply whilst the downward movement continues. In fact,
this nearly exact balancing between the supply of sediment and the
amount of subsidence is probably a rare contingency; for it has been
observed by more than one palaeontologist, that very thick deposits
are usually barren of organic remains, except near their upper or
lower limits.
It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation
composed of beds of different mineralogical composition, we may
reasonably suspect that the process of deposition has been much
interrupted, as a change in the currents of the sea and a supply of
sediment of a different nature will
generally have been due to
geographical changes requiring much time. Nor will the closest
inspection of a formation give any idea of the time which its
deposition has consumed. Many instances could be given of beds only a
few feet in thickness, representing formations, elsewhere thousands of
feet in thickness, and which must have required an enormous period for
their accumulation; yet no one ignorant of this fact would have
suspected the vast lapse of time represented by the thinner formation.
Many cases could be given of the lower beds of a formation having been
upraised, denuded, submerged, and then re-covered by the upper beds of
the same formation, — facts, showing what wide, yet easily
overlooked, intervals have occurred in its accumulation. In other
cases we have the plainest evidence in great fossilised trees, still
standing upright as they grew, of many long intervals of time and
changes of level during the process of deposition, which would never
even have been suspected, had not the trees chanced to have been
preserved: thus, Messrs Lyell and Dawson found
carboniferous beds 1400 feet thick in Nova Scotia, with ancient
root-bearing strata, one above the other, at no less than sixty-eight
different levels. Hence, when the same species occur at the bottom,
middle, and top of a formation, the probability is that they have not
lived on the same spot during the whole period of deposition, but have
disappeared and reappeared, perhaps many times, during the same
geological period. So that if such species
were to undergo a considerable amount of modification during any one
geological period, a section would not probably include all the fine
intermediate gradations which must on my theory have existed between
them, but abrupt, though perhaps very slight, changes of form.
It is all-important to remember that
naturalists have
no golden rule by which to distinguish species and
varieties; they grant some little variability to each species, but
when they meet with a somewhat greater amount of difference between
any two forms, they rank both as species, unless they are enabled to
connect them together by close intermediate gradations. And this from
the reasons just assigned we can seldom hope to effect in any one
geological section. Supposing B and C to be two species, and a third,
A, to be found in an underlying bed; even if A were strictly
intermediate between B and C, it would simply be ranked as a third and
distinct species, unless at the same time it could be most closely
connected with either one or both forms by intermediate varieties. Nor
should it be forgotten, as before explained, that A might be the
actual progenitor of B and C, and yet might not at all necessarily be
strictly intermediate between them in all points of structure. So that
we might obtain the parent-species and its several modified
descendants from the lower and upper beds of a formation, and unless
we obtained numerous transitional gradations, we should not recognise
their relationship, and should consequently be compelled to rank them
all as distinct species.
It is notorious on what excessively slight differences many
palaeontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking
many of the very fine species of D'Orbigny and others into the rank of
varieties; and on this view we do find the kind of evidence of change
which on my theory we ought to find. Moreover, if we look to rather
wider intervals, namely, to distinct but consecutive stages of the
same great formation, we find that the embedded fossils, though almost
universally ranked as specifically different,
yet are far more closely
allied to each other than are the species found in more widely
separated formations; but to this subject I shall have to return in
the following chapter.
One other consideration is worth notice: with animals and plants
that can propagate rapidly and are not highly locomotive, there is
reason to suspect, as we have formerly seen, that their varieties are
generally at first local; and that such local varieties do not spread
widely and supplant their parent-forms until they have been modified
and perfected in some considerable degree. According to this view, the
chance of discovering in a formation in any one country all the early
stages of transition between any two forms, is small, for the
successive changes are supposed to have been local or confined to some
one spot. Most marine animals have a wide range; and we have seen that
with plants it is those which have the widest range, that oftenest
present varieties; so that with shells and other marine animals, it is
probably those which have had the widest range, far exceeding the
limits of the known geological formations of Europe, which have
oftenest given rise, first to local varieties and ultimately to new
species; and this again would greatly lessen the chance of our being
able to trace the stages of transition in any one geological
formation.
It should not be forgotten, that at the present day, with perfect
specimens for examination, two forms can seldom be connected by
intermediate varieties and thus proved to be the same species, until
many specimens have been collected from many places; and in the case
of fossil species this could rarely be effected by palaeontologists.
We shall, perhaps, best perceive the improbability of our being
enabled to connect species by numerous, fine, intermediate, fossil
links, by asking
ourselves whether, for instance, geologists at some
future period will be able to prove, that our different
breeds of cattle, sheep, horses, and dogs have descended from a single
stock or from several aboriginal stocks; or, again, whether certain
sea-shells inhabiting the shores of North America, which are ranked by
some conchologists as distinct species from their European
representatives, and by other conchologists as only varieties, are
really varieties or are, as it is called, specifically distinct. This
could be effected only by the future geologist discovering in a fossil
state numerous intermediate gradations; and such success seems to me
improbable in the highest degree.
Geological research, though it has added numerous species to
existing and extinct genera, and has made the intervals between some
few groups less wide than they otherwise would have been, yet has done
scarcely anything in breaking down the distinction between species, by
connecting them together by numerous, fine, intermediate varieties;
and this not having been effected, is probably the gravest and most
obvious of all the many objections which may be urged against my
views. Hence it will be worth while to sum up the foregoing remarks,
under an imaginary illustration. The Malay Archipelago is of about the
size of Europe from the North Cape to the Mediterranean, and from
Britain to Russia; and therefore equals all the geological formations
which have been examined with any accuracy, excepting those of the
United States of America. I fully agree with Mr Godwin-Austen, that
the present condition of the Malay Archipelago, with its numerous
large islands separated by wide and shallow seas, probably represents
the former state of Europe, when most of our formations were
accumulating. The Malay Archipelago is one of the richest regions of
the
whole world in organic beings; yet if all the species were to be
collected which have ever lived there, how imperfectly would they
represent the natural history of the world!
But we have every reason to believe that the terrestrial
productions of the archipelago would be preserved in an excessively
imperfect manner in the formations which we suppose
to be there accumulating. I suspect that not many
of the strictly littoral animals, or of those which lived on naked
submarine rocks, would be embedded; and those embedded in gravel or
sand, would not endure to a distant epoch. Wherever
sediment did not accumulate on the bed of the sea, or where it did not
accumulate at a sufficient rate to protect organic bodies from decay,
no remains could be preserved.
In our archipelago, I believe that fossiliferous formations could
be formed of sufficient thickness to last to an age, as distant in
futurity as the secondary formations lie in the past, only during
periods of subsidence. These periods of subsidence would be separated
from each other by enormous intervals, during which the area would be
either stationary or rising; whilst rising, each fossiliferous
formation would be destroyed, almost as soon as accumulated, by the
incessant coast-action, as we now see on the shores of South America.
During the periods of subsidence there would probably be much
extinction of life; during the periods of elevation, there would be
much variation, but the geological record would then be least perfect.
It may be doubted whether the duration of any one great period of
subsidence over the whole or part of the archipelago, together with a
contemporaneous accumulation of sediment, would exceed the average duration of the same specific
forms; and these contingencies are
indispensable for the preservation
of all the transitional gradations between any two or more species. If
such gradations were not fully preserved, transitional varieties would
merely appear as so many distinct species. It is, also, probable that
each great period of subsidence would be interrupted by oscillations
of level, and that slight climatal changes would intervene during such
lengthy periods; and in these cases the inhabitants of the archipelago
would have to migrate, and no closely consecutive record of their
modifications could be preserved in any one formation.
Very many of the marine inhabitants of the archipelago now range
thousands of miles beyond its confines; and analogy leads me to
believe that it would be chiefly these far-ranging species which would
oftenest produce new varieties; and the varieties would at first
generally be local or confined to one place, but if possessed of any
decided advantage, or when further modified and improved, they would
slowly spread and supplant their parent-forms. When such varieties
returned to their ancient homes, as they would differ
from their former state, in a nearly uniform, though perhaps extremely
slight degree, they would, according to the principles followed by
many palaeontologists, be ranked as new and distinct species.
If then, there be some degree of truth in these remarks, we have no
right to expect to find in our geological formations, an infinite
number of those fine transitional forms, which on my theory assuredly
have connected all the past and present species of the same group into
one long and branching chain of life. We ought only to look for a few
links, some more closely, some more distantly related to each other;
and these links, let them be ever so close, if found in different
stages of the same formation, would, by most palaeontologists,
be
ranked as distinct species. But I do not pretend that I should ever
have suspected how poor a record of the mutations of life, the best
preserved geological section presented, had not the difficulty of our
not discovering innumerable transitional links between the species
which appeared at the commencement and close of each formation,
pressed so hardly on my theory.
On the sudden appearance of whole groups of
Allied Species.
The abrupt manner in which whole groups of
species suddenly appear in certain formations, has been urged by
several palaeontologists, for instance, by Agassiz, Pictet, and by
none more forcibly than by Professor Sedgwick, as a fatal objection to
the belief in the transmutation of species. If numerous species,
belonging to the same genera or families, have really started into
life all at once, the fact would be fatal to the theory of descent
with slow modification through natural selection. For the development
of a group of forms, all of which have descended from some one
progenitor, must have been an extremely slow process; and the
progenitors must have lived long ages before their modified
descendants. But we continually over-rate the perfection of the
geological record, and falsely infer, because certain genera or
families have not been found beneath a certain stage, that they did
not exist before that stage. We continually forget how large the world
is, compared with the area over which our geological formations have
been carefully examined; we forget that groups of species
may elsewhere have long existed and have slowly multiplied before they
invaded the ancient archipelagoes of Europe and of the United States.
We do not make due allowance for the enormous intervals of time, which
have
probably elapsed between our consecutive formations, --
longer perhaps in some cases than the time required for the
accumulation of each formation. These intervals will have given time
for the multiplication of species from some one or some few
parent-forms; and in the succeeding formation such species will appear
as if suddenly created.
I may here recall a remark formerly made, namely that it might
require a long succession of ages to adapt an organism to some new and
peculiar line of life, for instance to fly through the air; but that
when this had been effected, and a few species had thus acquired a
great advantage over other organisms, a comparatively short time would
be necessary to produce many divergent forms, which would be able to
spread rapidly and widely throughout the world.
I will now give a few examples to illustrate these remarks; and to
show how liable we are to error in supposing that whole groups of
species have suddenly been produced. I may recall the well-known fact
that in geological treatises, published not many years ago, the great
class of mammals was always spoken of as having abruptly come in at
the commencement of the tertiary series. And now one of the richest
known accumulations of fossil mammals belongs to the middle of the
secondary series; and one true mammal has been discovered in the new
red sandstone at nearly the commencement of this great series. Cuvier
used to urge that no monkey occurred in any tertiary stratum; but now
extinct species have been discovered in India, South America, and in
Europe even as far back as the eocene stage. The most striking case,
however, is that of the Whale family; as these animals have huge
bones, are marine, and range over the world, the fact of not a single
bone of a whale having been discovered in
any secondary formation,
seemed fully to justify the belief that this great and distinct order
had been suddenly produced in the interval between the latest
secondary and earliest tertiary formation. But now we may read in the
Supplement to Lyell's 'Manual,' published in 1858, clear
evidence of the existence of whales in the upper greensand, some time
before the close of the secondary period.
I may give another instance, which from having passed under my own
eyes has much struck me. In a memoir on Fossil Sessile Cirripedes, I
have stated that, from the number of existing and extinct tertiary
species; from the extraordinary abundance of the individuals of many
species all over the world, from the Arctic regions to
the equator, inhabiting various zones of depths
from the upper tidal limits to 50 fathoms; from the perfect manner in
which specimens are preserved in the oldest tertiary beds; from the
ease with which even a fragment of a valve can be recognised; from all
these circumstances, I inferred that had sessile cirripedes existed
during the secondary periods, they would certainly have been preserved
and discovered; and as not one species had been discovered in beds of
this age, I concluded that this great group had been suddenly
developed at the commencement of the tertiary series. This was a sore
trouble to me, adding as I thought one more instance of the abrupt
appearance of a great group of species. But my work had hardly been
published, when a skilful palaeontologist, M. Bosquet, sent me a
drawing of a perfect specimen of an unmistakeable sessile cirripede,
which he had himself extracted from the chalk of Belgium. And, as if
to make the case as striking as possible, this sessile cirripede was a
Chthamalus, a very common, large, and ubiquitous genus, of which not
one specimen has as yet been found even in any tertiary
stratum. Hence
we now positively know that sessile cirripedes existed during the
secondary period; and these cirripedes might have been the progenitors
of our many tertiary and existing species.
The case most frequently insisted on by palaeontologists of the
apparently sudden appearance of a whole group of species, is that of
the teleostean fishes, low down in the Chalk period. This group
includes the large majority of existing species. Lately, Professor
Pictet has carried their existence one sub-stage further back; and
some palaeontologists believe that certain much older fishes, of which
the affinities are as yet imperfectly known, are really teleostean.
Assuming, however, that the whole of them did appear, as
Agassiz believes, at the commencement of the chalk formation, the fact
would certainly be highly remarkable; but I cannot see that it would
be an insuperable difficulty on my theory, unless it could likewise be
shown that the species of this group appeared suddenly and
simultaneously throughout the world at this same period. It is almost
superfluous to remark that hardly any fossil-fish are known from south
of the equator; and by running through Pictet's palaeontology it will
be seen that very few species are known from several formations in
Europe. Some few families of fish now have a confined range; the
teleostean fish might formerly have had a similarly confined range,
and after having been largely developed in some one sea, might have
spread widely. Nor have we any right to suppose that the seas of the
world have always been so freely open from south to north as they are
at present. Even at this day, if the Malay Archipelago were converted
into land, the tropical parts of the Indian Ocean would form a large
and perfectly enclosed basin, in which any great group of marine
animals might be multiplied; and
here they would remain confined,
until some of the species became adapted to a cooler climate, and were
enabled to double the southern capes of Africa or Australia, and thus
reach other and distant seas.
From these and similar considerations, but chiefly from our
ignorance of the geology of other countries beyond the confines of
Europe and the United States; and from the revolution in our
palaeontological ideas on many points, which the discoveries of even
the last dozen years have effected, it seems to me to be about as rash
in us to dogmatize on the succession of organic beings throughout the
world, as it would be for a naturalist to land for five minutes on
some one barren point in Australia, and then to discuss the number and
range of its productions.
On the sudden appearance of groups of Allied
Species in the lowest known fossiliferous strata.
There is
another and allied difficulty, which is much graver. I allude to the
manner in which numbers of species of the same group, suddenly appear
in the lowest known fossiliferous rocks. Most of the arguments which
have convinced me that all the existing species of the same group have descended from one progenitor, apply with nearly equal
force to the earliest known species. For instance, I cannot doubt that
all the Silurian trilobites have descended from some one crustacean,
which must have lived long before the Silurian age, and which probably
differed greatly from any known animal. Some of the most ancient
Silurian animals, as the Nautilus, Lingula, c., do not differ
much from living species; and it cannot on my theory be supposed, that
these old species were the progenitors of all the species of the
orders to which they belong, for they do not present characters in any
degree intermediate between them.
If, moreover, they had been the
progenitors of these orders, they would
almost certainly have been long ago supplanted and exterminated by
their numerous and improved descendants.
Consequently, if my theory be true, it is indisputable that before
the lowest Silurian stratum was deposited, long periods elapsed, as
long as, or probably far longer than, the whole interval from the
Silurian age to the present day; and that during these vast, yet quite
unknown, periods of time, the world swarmed with living creatures.
To the question why we do not find records of these vast primordial
periods, I can give no satisfactory answer. Several of the most
eminent geologists, with Sir R. Murchison at their head, are
convinced that we see in the organic remains of the lowest Silurian
stratum the dawn of life on this planet. Other highly competent
judges, as Lyell and the late E. Forbes, dispute this conclusion. We
should not forget that only a small portion of the world is known with
accuracy. M. Barrande has lately added another and lower stage to the
Silurian system, abounding with new and peculiar species. Traces of
life have been detected in the Longmynd beds beneath Barrande's
so-called primordial zone. The presence of phosphatic nodules and
bituminous matter in some of the lowest azoic rocks, probably
indicates the former existence of life at these periods. But the
difficulty of understanding the absence of vast piles of fossiliferous
strata, which on my theory no doubt were somewhere accumulated before
the Silurian epoch, is very great. If these most ancient beds had been
wholly worn away by denudation, or obliterated by metamorphic action, we ought to find only small remnants of the
formations next succeeding them in age, and these
ought to be very generally in
a metamorphosed
condition. But the descriptions which we now possess of the Silurian
deposits over immense territories in Russia and in North America, do
not support the view, that the older a formation is, the more it has
suffered the extremity of denudation and metamorphism.
The case at present must remain inexplicable; and may be truly
urged as a valid argument against the views here entertained. To show
that it may hereafter receive some explanation, I will give the
following hypothesis. From the nature of the organic remains, which do
not appear to have inhabited profound depths, in the several
formations of Europe and of the United States; and from the amount of
sediment, miles in thickness, of which the
formations are composed, we may infer that from first to last large
islands or tracts of land, whence the sediment was derived, occurred
in the neighbourhood of the existing continents of Europe and North
America. But we do not know what was the state of things in the
intervals between the successive formations; whether Europe and the
United States during these intervals existed as dry land, or as a
submarine surface near land, on which sediment was not deposited, or
again as the bed of an open and unfathomable sea.
Looking to the existing oceans, which
are thrice as extensive as the land, we see them studded with many
islands; but not one oceanic island is as yet known to afford even a
remnant of any palaeozoic or secondary formation. Hence we may perhaps
infer, that during the palaeozoic and secondary periods, neither
continents nor continental islands existed where our oceans now
extend; for had they existed there, palaeozoic and secondary
formations would in all probability have been accumulated from
sediment derived from their wear and
tear; and would have been at
least partially upheaved by the oscillations of level, which we may
fairly conclude must have intervened during these enormously long
periods. If then we may infer anything from these facts, we may infer
that where our oceans now extend, oceans have extended from the
remotest period of which we have any record; and on the other hand,
that where continents now exist, large tracts of land
have existed, subjected no doubt to great oscillations of level, since
the earliest silurian period. The coloured map appended to my volume
on Coral Reefs, led me to conclude that the great oceans are still
mainly areas of subsidence, the great archipelagoes still areas of
oscillations of level, and the continents areas of elevation. But have
we any right to assume that things have thus remained from eternity?
Our continents seem to have been formed by a preponderance, during
many oscillations of level, of the force of elevation; but may not the
areas of preponderant movement have changed in the lapse of ages? At a
period immeasurably antecedent to the silurian epoch, continents may
have existed where oceans are now spread out; and clear and open
oceans may have existed where our continents now stand. Nor should we
be justified in assuming that if, for instance, the bed of the Pacific
Ocean were now converted into a continent, we should there find
formations older than the silurian strata, supposing such to have been
formerly deposited; for it might well happen that strata which had
subsided some miles nearer to the centre of the earth, and which had
been pressed on by an enormous weight of superincumbent water, might
have undergone far more metamorphic action than strata which have
always remained nearer to the surface. The immense areas in some parts
of the world, for instance in South America, of bare metamorphic
rocks, which
must have been heated under great pressure, have always
seemed to me to require some special explanation; and we may perhaps
believe that we see in these large areas, the many formations long
anterior to the silurian epoch in a completely metamorphosed
condition.
The several difficulties here discussed, namely our not finding in
the successive formations infinitely numerous transitional links
between the many species which now exist or have existed; the sudden
manner in which whole groups of species appear in our European
formations; the almost entire absence, as at present known, of
fossiliferous formations beneath the Silurian strata, are all
undoubtedly of the gravest nature. We see this in the plainest manner
by the fact that all the most eminent palaeontologists, namely Cuvier,
Owen, Agassiz, Barrande, Falconer, E. Forbes, c.,
and all our greatest geologists, as Lyell, Murchison, Sedgwick,
c., have unanimously, often vehemently, maintained the
immutability of species. But I have reason to believe that one great
authority, Sir Charles Lyell, from further reflexion entertains grave
doubts on this subject. I feel how rash it
is to differ from these great authorities, to whom, with others, we
owe all our knowledge. Those who think the
natural geological record in any degree perfect, and
who do not attach much weight to the facts and
arguments of other kinds even in this volume, will undoubtedly at once
reject my theory. For my part, following out Lyell's metaphor, I look
at the natural geological record, as a history of the world
imperfectly kept, and written in a changing dialect; of this history
we possess the last volume alone, relating only to two or three
countries. Of this volume, only here and there a short chapter has
been
preserved; and of each page, only here and there a few lines.
Each word of the slowly-changing language, in which the history is
supposed to be written, being more or less different in the
interrupted succession of chapters, may represent the apparently
abruptly changed forms of life, entombed in our consecutive, but
widely separated formations. On this view, the difficulties above
discussed are greatly diminished, or even disappear.
ON THE GEOLOGICAL SUCCESSION OF
ORGANIC BEINGS.
- On the slow and successive appearance of new species
- On their different rates of change
- Species once lost do not reappear
- Groups of species follow the same general rules in
their appearance and disappearance as do single species
- On Extinction
- On simultaneous changes in the forms of life
throughout the world
- On the affinities of extinct species
to each other and to living species
- On the state of development of ancient forms
- On the succession of the same types within the same areas
- Summary of preceding and present chapters
Let us now see whether the several facts and rules relating to the
geological succession of organic beings, better accord with the common
view of the immutability of species, or with that of their slow and
gradual modification, through descent and natural selection.
New species have appeared very slowly, one after another, both on
the land and in the waters. Lyell has shown that it is hardly possible
to resist the evidence on this head in the case of the several
tertiary stages; and every year tends to fill up the blanks between
them, and to make the percentage system of lost and new forms more
gradual. In some of the most recent beds, though undoubtedly of high
antiquity if measured by years, only one or two species are lost
forms, and only one or two are new forms, having here appeared for the
first time, either locally, or, as far as we know, on the face of the
earth. If we may trust the observations of Philippi in Sicily, the
successive changes in the marine inhabitants of that island have been
many and most gradual. The secondary formations are more broken; but,
as Bronn has remarked, neither the appearance
nor disappearance of
their many now extinct species has been simultaneous in each separate
formation.
Species of different genera and classes have not
changed at the same rate, or in the same degree. In the oldest
tertiary beds a few living shells may still be found in the midst of a
multitude of extinct forms. Falconer has given a striking instance of
a similar fact, in an existing crocodile associated with many strange
and lost mammals and reptiles in the sub-Himalayan deposits. The
Silurian Lingula differs but little from the living species of this
genus; whereas most of the other Silurian Molluscs and all the
Crustaceans have changed greatly. The productions of the land seem to
change at a quicker rate than those of the sea, of which a striking
instance has lately been observed in Switzerland. There is some reason
to believe that organisms, considered high in the scale of nature,
change more quickly than those that are low: though there are
exceptions to this rule. The amount of organic change, as Pictet has
remarked, does not strictly correspond with the succession of our
geological formations; so that between each two consecutive
formations, the forms of life have seldom changed in exactly the same
degree. Yet if we compare any but the most closely related formations,
all the species will be found to have undergone some change. When a
species has once disappeared from the face of the earth, we have
reason to believe that the same identical form never reappears. The
strongest apparent exception to this latter rule, is that of the
so-called `colonies' of M. Barrande, which intrude for a period in the
midst of an older formation, and then allow the pre-existing fauna to
reappear; but Lyell's explanation, namely, that it is a case of
temporary migration from a distinct geographical province, seems to me
satisfactory.
These several facts accord well with my theory. I believe in no
fixed law of development, causing all the inhabitants of a country to
change abruptly, or simultaneously, or to an equal degree. The process
of modification must be extremely slow. The variability of each
species is quite independent of that of all others. Whether such
variability be taken advantage of by natural selection, and whether
the variations be accumulated to a greater or lesser amount, thus
causing a greater or lesser amount of modification in the varying
species, depends on many complex contingencies, — on the
variability being of a beneficial nature, on the power of
intercrossing, on the rate of breeding, on the slowly changing
physical conditions of the country, and more especially on the nature
of the other inhabitants with which the varying species comes into
competition. Hence it is by no means surprising that one species
should retain the same identical form much longer than others; or, if
changing, that it should change less. We see the same fact in
geographical distribution; for instance, in the land-shells and
coleopterous insects of Madeira having come to differ considerably
from their nearest allies on the continent of Europe, whereas the
marine shells and birds have remained unaltered. We can perhaps
understand the apparently quicker rate of change in terrestrial and in
more highly organised productions compared with marine and lower
productions, by the more complex relations of the higher beings to
their organic and inorganic conditions of life, as explained in a
former chapter. When many of the inhabitants of a country have become
modified and improved, we can understand, on the principle of
competition, and on that of the many all-important relations of
organism to organism, that any form which does not become in some
degree modified and improved,
will be liable to be exterminated. Hence
we can see why all the species in the same region do at last, if we
look to wide enough intervals of time, become modified; for those
which do not change will become extinct.
In members of the same class the average amount of change, during
long and equal periods of time, may, perhaps, be nearly the same; but
as the accumulation of long-enduring fossiliferous formations depends
on great masses of sediment having been deposited on areas whilst
subsiding, our formations have been almost necessarily accumulated at
wide and irregularly intermittent intervals; consequently the amount
of organic change exhibited by the fossils embedded in consecutive
formations is not equal. Each formation, on this view, does not mark a
new and complete act of creation, but only an occasional scene, taken
almost at hazard, in a slowly changing drama.
We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and
inorganic, should recur. For though the offspring of one
species might be adapted (and no doubt this has occurred in
innumerable instances) to fill the exact place of another species in
the economy of nature, and thus supplant it; yet the two forms --
the old and the new — would not be identically the same; for
both would almost certainly inherit different characters from their
distinct progenitors. For instance, it is just possible, if our
fantail-pigeons were all destroyed, that fanciers, by striving during
long ages for the same object, might make a new breed hardly
distinguishable from our present fantail; but if the parent
rock-pigeon were also destroyed, and in nature we have every reason to
believe that the parent-form will generally be supplanted and
exterminated by its improved offspring, it is quite incredible that a
fantail, identical with the existing breed, could be raised from any
other species of pigeon, or even from the other well-established races
of the domestic pigeon, for the newly-formed fantail would be almost
sure to inherit from its new progenitor some slight characteristic
differences.
Groups of species, that is, genera and families, follow the same
general rules in their appearance and disappearance as do single
species, changing more or less quickly, and in a greater or lesser
degree. A group does not reappear after it has once disappeared; or
its existence, as long as it lasts, is continuous. I am aware that
there are some apparent exceptions to this rule, but the exceptions
are surprisingly few, so few, that E. Forbes, Pictet, and Woodward
(though all strongly opposed to such views as I maintain) admit its
truth; and the rule strictly accords with my theory. For as all the
species of the same group have descended from some one species, it is
clear that as long as any species of the group have appeared in the
long succession of ages, so long must its members have continuously
existed, in order to have generated either new and modified or the
same old and unmodified forms. Species of the genus Lingula, for
instance, must have continuously existed by an unbroken succession of
generations, from the lowest Silurian stratum to the present day.
We have seen in the last chapter that the species of a group
sometimes falsely appear to have come in abruptly; and I have
attempted to give an explanation of this fact, which if true would
have been fatal to my views. But such cases are certainly exceptional;
the general rule being a gradual increase in number, till
the group reaches its maximum, and then, sooner or later, it gradually
decreases. If the
number of the species of a genus, or the number of
the genera of a family, be represented by a vertical line of varying
thickness, crossing the successive geological formations in which the
species are found, the line will sometimes falsely appear to begin at
its lower end, not in a sharp point, but abruptly; it then gradually
thickens upwards, sometimes keeping for a space of equal thickness,
and ultimately thins out in the upper beds, marking the decrease and
final extinction of the species. This gradual increase in number of
the species of a group is strictly conformable with my theory; as the
species of the same genus, and the genera of the same family, can
increase only slowly and progressively; for the process of
modification and the production of a number of allied forms must be
slow and gradual, — one species giving rise first to two or
three varieties, these being slowly converted into species, which in
their turn produce by equally slow steps other species, and so on,
like the branching of a great tree from a single stem, till the group
becomes large.
On Extinction.
We have as yet spoken
only incidentally of the disappearance of species and of groups of
species. On the theory of natural selection the extinction of old
forms and the production of new and improved forms are intimately
connected together. The old notion of all the inhabitants of the earth
having been swept away at successive periods by catastrophes, is very
generally given up, even by those geologists, as Elie de Beaumont,
Murchison, Barrande, c., whose general views would naturally lead
them to this conclusion. On the contrary, we have every reason to
believe, from the study of the tertiary formations, that species and
groups of species gradually disappear, one after another, first from
one spot, then from another, and
finally from the world. Both single
species and whole groups of species last for very unequal periods;
some groups, as we have seen, having endured from the earliest known
dawn of life to the present day; some having disappeared before the
close of the palaeozoic period. No fixed law seems to determine the
length of time during which any single species or any single genus
endures. There is reason to believe that the complete
extinction of the species of a group is generally a slower process
than their production: if the appearance and disappearance of a group
of species be represented, as before, by a vertical line of varying
thickness, the line is found to taper more gradually at its upper end,
which marks the progress of extermination, than at its lower end,
which marks the first appearance and increase in numbers of the
species. In some cases, however, the extermination of whole groups of
beings, as of ammonites towards the close of the secondary period, has
been wonderfully sudden.
The whole subject of the extinction of species has been involved in
the most gratuitous mystery. Some authors have even supposed that as
the individual has a definite length of life, so have species a
definite duration. No one I think can have marvelled more at the
extinction of species, than I have done. When I found in La Plata the
tooth of a horse embedded with the remains of Mastodon, Megatherium,
Toxodon, and other extinct monsters, which all co-existed with still
living shells at a very late geological period, I was filled with
astonishment; for seeing that the horse, since its introduction by the
Spaniards into South America, has run wild over the whole country and
has increased in numbers at an unparalleled rate, I asked myself what
could so recently have exterminated the former horse under conditions
of life apparently so favourable. But
how utterly groundless was my
astonishment! Professor Owen soon perceived that the tooth, though so
like that of the existing horse, belonged to an extinct species. Had
this horse been still living, but in some degree rare, no naturalist
would have felt the least surprise at its rarity; for rarity is the
attribute of a vast number of species of all classes, in all
countries. If we ask ourselves why this or that species is rare, we
answer that something is unfavourable in its conditions of life; but
what that something is, we can hardly ever tell. On the supposition of
the fossil horse still existing as a rare species, we might have felt
certain from the analogy of all other mammals, even of the
slow-breeding elephant, and from the history of the naturalisation of
the domestic horse in South America, that under more favourable
conditions it would in a very few years have stocked the whole
continent. But we could not have told what the
unfavourable conditions were which checked its increase, whether some
one or several contingencies, and at what period of the horse's life,
and in what degree, they severally acted. If the conditions had gone
on, however slowly, becoming less and less favourable, we assuredly
should not have perceived the fact, yet the fossil horse would
certainly have become rarer and rarer, and finally extinct; --
its place being seized on by some more successful competitor.
It is most difficult always to remember that the increase of every
living being is constantly being checked by unperceived injurious
agencies; and that these same unperceived agencies are amply
sufficient to cause rarity, and finally extinction. We see in many
cases in the more recent tertiary formations, that rarity precedes
extinction; and we know that this has been the progress of events with
those animals which have
been exterminated, either locally or wholly,
through man's agency. I may repeat what I published in 1845, namely,
that to admit that species generally become rare before they become
extinct — to feel no surprise at the rarity of a species, and
yet to marvel greatly when it ceases to exist, is much the same as to
admit that sickness in the individual is the forerunner of death
-- to feel no surprise at sickness, but when the sick man dies,
to wonder and to suspect that he died by some unknown deed of
violence.
The theory of natural selection is grounded on the belief that each
new variety, and ultimately each new species, is produced and
maintained by having some advantage over those with which it comes
into competition; and the consequent extinction of less-favoured forms
almost inevitably follows. It is the same with our domestic
productions: when a new and slightly improved variety has been raised,
it at first supplants the less improved varieties in the same
neighbourhood; when much improved it is transported far and near, like
our short-horn cattle, and takes the place of other breeds in other
countries. Thus the appearance of new forms and the disappearance of
old forms, both natural and artificial, are bound together. In certain
flourishing groups, the number of new specific forms which have been
produced within a given time is probably greater than that of the old
forms which have been exterminated; but we know that the
number of species has not gone on indefinitely increasing, at least
during the later geological periods, so that looking to later times we
may believe that the production of new forms has caused the extinction
of about the same number of old forms.
The competition will generally be most severe, as formerly
explained and illustrated by examples, between the forms which are
most like each other in all respects.
Hence the improved and modified
descendants of a species will generally cause the extermination of the
parent-species; and if many new forms have been developed from any one
species, the nearest allies of that species, i.e.
the species of the
same genus, will be the most liable to extermination. Thus, as I
believe, a number of new species descended from one species, that is a
new genus, comes to supplant an old genus, belonging to the same
family. But it must often have happened that a new species belonging
to some one group will have seized on the place occupied by a species
belonging to a distinct group, and thus caused its extermination; and
if many allied forms be developed from the successful intruder, many
will have to yield their places; and it will generally be allied
forms, which will suffer from some inherited inferiority in common.
But whether it be species belonging to the same or to a distinct
class, which yield their places to other species which have been
modified and improved, a few of the sufferers may often long be
preserved, from being fitted to some peculiar line of life, or from
inhabiting some distant and isolated station, where they have escaped
severe competition. For instance, a single species of Trigonia, a
great genus of shells in the secondary formations, survives in the
Australian seas; and a few members of the great and almost extinct
group of Ganoid fishes still inhabit our fresh waters. Therefore the
utter extinction of a group is generally, as we have seen, a slower
process than its production.
With respect to the apparently sudden extermination of whole
families or orders, as of Trilobites at the close of the palaeozoic
period and of Ammonites at the close of the secondary period, we must
remember what has been already said on the probable wide intervals of
time
between our consecutive formations; and in these intervals there
may have been much slow extermination. Moreover, when by
sudden immigration or by unusually rapid development, many species of
a new group have taken possession of a new area, they will have
exterminated in a correspondingly rapid manner many of the old
inhabitants; and the forms which thus yield their places will commonly
be allied, for they will partake of some inferiority in common.
Thus, as it seems to me, the manner in which single species and
whole groups of species become extinct, accords well with the theory
of natural selection. We need not marvel at extinction; if we must
marvel, let it be at our presumption in imagining for a moment that we
understand the many complex contingencies, on which the existence of
each species depends. If we forget for an instant, that each species
tends to increase inordinately, and that some check is always in
action, yet seldom perceived by us, the whole economy of nature will
be utterly obscured. Whenever we can precisely say why this species is
more abundant in individuals than that; why this species and not
another can be naturalised in a given country; then, and not till
then, we may justly feel surprise why we cannot account for the
extinction of this particular species or group of species.
On the Forms of Life changing almost
simultaneously throughout the World.
Scarcely any
palaeontological discovery is more striking than the fact, that the
forms of life change almost simultaneously throughout the world. Thus
our European Chalk formation can be recognised in many distant parts
of the world, under the most different climates, where not a fragment
of the mineral chalk itself can be found; namely, in North
America, in
equatorial South America, in Tierra del Fuego, at the Cape of Good
Hope, and in the peninsula of India. For at these distant points, the
organic remains in certain beds present an unmistakeable degree of
resemblance to those of the Chalk. It is not that the same species are
met with; for in some cases not one species is identically the same,
but they belong to the same families, genera, and sections of genera,
and sometimes are similarly characterised in such trifling points as
mere superficial sculpture. Moreover other forms, which are not found
in the Chalk of Europe, but which occur in the formations either above
or below, are similarly absent at these distant points of
the world. In the several successive palaeozoic formations of Russia,
Western Europe and North America, a similar parallelism in the forms
of life has been observed by several authors: so it is, according to
Lyell, with the several European and North American tertiary deposits.
Even if the few fossil species which are common to the Old and New
Worlds be kept wholly out of view, the general parallelism in the
successive forms of life, in the stages of the widely separated
palaeozoic and tertiary periods, would still be manifest, and the
several formations could be easily correlated.
These observations, however, relate to the marine inhabitants of
distant parts of the world: we have not sufficient data to judge
whether the productions of the land and of fresh water change at
distant points in the same parallel manner. We may doubt whether they
have thus changed: if the Megatherium, Mylodon, Macrauchenia, and
Toxodon had been brought to Europe from La Plata, without any
information in regard to their geological position, no one would have
suspected that they had coexisted with still living sea-shells; but as
these anomalous monsters coexisted with the Mastodon
and Horse, it
might at least have been inferred that they had lived during one of
the latter tertiary stages.
When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same thousandth or hundred-thousandth year,
or even that it has a very strict geological sense; for if all the
marine animals which live at the present day in Europe, and all those
that lived in Europe during the pleistocene period (an enormously
remote period as measured by years, including the whole glacial
epoch), were to be compared with those now living in South America or
in Australia, the most skilful naturalist would hardly be able to say
whether the existing or the pleistocene inhabitants of Europe
resembled most closely those of the southern hemisphere. So, again,
several highly competent observers believe that the existing
productions of the United States are more closely related to those
which lived in Europe during certain later tertiary stages, than to
those which now live here; and if this be so, it is evident that
fossiliferous beds deposited at the present day on the
shores of North America would hereafter be liable to be classed with
somewhat older European beds. Nevertheless, looking to a remotely
future epoch, there can, I think, be little doubt that all the more
modern marine formations, namely,
the upper pliocene, the pleistocene
and strictly modern beds, of Europe, North and South America, and
Australia, from containing fossil remains in some degree allied, and
from not including those forms which are only found in the older
underlying deposits, would be correctly ranked as simultaneous in a
geological sense.
The fact of the forms of life changing simultaneously, in the above
large sense, at distant parts of the world, has greatly struck those
admirable observers, MM.
de Verneuil and d'Archiac. After referring
to the parallelism of the palaeozoic forms of life in various parts of
Europe, they add, `If struck by this strange sequence, we turn our
attention to North America, and there discover a series of analogous
phenomena, it will appear certain that all these modifications of
species, their extinction, and the introduction of new ones, cannot be
owing to mere changes in marine currents or other causes more or less
local and temporary, but depend on general laws which govern the whole
animal kingdom.' M. Barrande has made forcible remarks to precisely
the same effect. It is, indeed, quite futile to look to changes of
currents, climate, or other physical conditions, as the cause of these
great mutations in the forms of life throughout the world, under the
most different climates. We must, as Barrande has remarked, look to
some special law. We shall see this more clearly when we treat of the
present distribution of organic beings, and find how slight is the
relation between the physical conditions of various countries, and the
nature of their inhabitants.
This great fact of the parallel succession of the forms of life
throughout the world, is explicable on the theory of natural
selection. New species are formed by new varieties arising, which have
some advantage over older forms; and those forms, which are already
dominant, or have some advantage over the other forms in their own
country, would naturally oftenest give rise to new varieties or
incipient species; for these latter must be victorious in a still
higher degree in order to be preserved and to survive.
We have distinct evidence on this head, in the plants which are
dominant, that is, which are commonest in their own homes, and are
most widely diffused, having produced the greatest number of new
varieties. It is also natural that the dominant,
varying, and
far-spreading species, which already have invaded to a certain extent
the territories of other species, should be those which would have the
best chance of spreading still further, and of giving rise in new
countries to new varieties and species. The process of diffusion may
often be very slow, being dependent on climatal and geographical
changes, or on strange accidents, but in the long run the dominant
forms will generally succeed in spreading. The diffusion would, it is
probable, be slower with the terrestrial inhabitants of distinct
continents than with the marine inhabitants of the continuous sea. We
might therefore expect to find, as we apparently do find, a less
strict degree of parallel succession in the productions of the land
than of the sea.
Dominant species spreading from any region might encounter still
more dominant species, and then their triumphant course, or even their
existence, would cease. We know not at all precisely what are all the
conditions most favourable for the multiplication of new and dominant
species; but we can, I think, clearly see that a number of
individuals, from giving a better chance of the appearance of
favourable variations, and that severe competition with many already
existing forms, would be highly favourable, as would be the power of
spreading into new territories. A certain amount of isolation,
recurring at long intervals of time, would probably be also
favourable, as before explained. One quarter of the world may have
been most favourable for the production of new and dominant species on
the land, and another for those in the waters of the sea. If two great
regions had been for a long period favourably circumstanced in an
equal degree, whenever their inhabitants met, the battle would be
prolonged and severe; and some from one birthplace and some from the
other might be victorious. But in the course of time, the
forms
dominant in the highest degree, wherever produced, would tend
everywhere to prevail. As they prevailed, they would cause the
extinction of other and inferior forms; and as these
inferior forms would be allied in groups by inheritance, whole groups
would tend slowly to disappear; though here and there a single member
might long be enabled to survive.
Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the
world, accords well with the principle of new species having been
formed by dominant species spreading widely and varying; the new
species thus produced being themselves dominant owing to inheritance,
and to having already had some advantage over their parents or over
other species; these again spreading, varying, and producing new
species. The forms which are beaten and which yield their places to
the new and victorious forms, will generally be allied in groups, from
inheriting some inferiority in common; and therefore as new and
improved groups spread throughout the world, old groups will disappear
from the world; and the succession of forms in both ways will
everywhere tend to correspond.
There is one other remark connected with this subject worth making.
I have given my reasons for believing that all our greater
fossiliferous formations were deposited during periods of subsidence;
and that blank intervals of vast duration occurred during the periods
when the bed of the sea was either stationary or rising, and likewise
when sediment was not thrown down quickly enough to embed and preserve
organic remains. During these long and blank intervals I suppose that
the inhabitants of each region underwent a considerable amount of
modification and extinction, and that there was much migration from
other parts of the world. As we have reason to believe that large
areas are affected by the same movement, it is probable that strictly
contemporaneous formations have often been accumulated over very wide
spaces in the same quarter of the world; but we are far from having
any right to conclude that this has invariably been the case, and that
large areas have invariably been affected by the same movements. When
two formations have been deposited in two regions during nearly, but
not exactly the same period, we should find in both, from the causes
explained in the foregoing paragraphs, the same general succession in
the forms of life; but the species would not exactly
correspond; for there will have been a little more time in the one
region than in the other for modification, extinction, and
immigration.
I suspect that cases of this nature have occurred in Europe. Mr.
Prestwich, in his admirable Memoirs on the eocene deposits of England
and France, is able to draw a close general parallelism between the
successive stages in the two countries; but when he compares certain
stages in England with those in France, although he finds in both a
curious accordance in the numbers of the species belonging to the same
genera, yet the species themselves differ in a manner very difficult
to account for, considering the proximity of the two areas, --
unless, indeed, it be assumed that an isthmus separated two seas
inhabited by distinct, but contemporaneous, faunas. Lyell has made
similar observations on some of the later tertiary formations.
Barrande, also, shows that there is a striking general parallelism in
the successive Silurian deposits of Bohemia and Scandinavia;
nevertheless he finds a surprising amount of difference in the
species. If the several formations in these regions have not been
deposited during the same exact
periods, — a formation in one
region often corresponding with a blank interval in the other, --
and if in both regions the species have gone on slowly changing during
the accumulation of the several formations and during the long
intervals of time between them; in this case, the several formations
in the two regions could be arranged in the same order, in accordance
with the general succession of the form of life, and the order would
falsely appear to be strictly parallel; nevertheless the species would
not all be the same in the apparently corresponding stages in the two
regions.
On the Affinities of extinct Species to each
other, and to living forms.
Let us now look to the mutual
affinities of extinct and living species. They all fall into one grand
natural system; and this fact is at once explained on the principle of
descent. The more ancient any form is, the more, as a general rule, it
differs from living forms. But, as Buckland long ago remarked, all
fossils can be classed either in still existing groups, or between
them. That the extinct forms of life help to fill up the wide
intervals between existing genera, families, and orders,
cannot be disputed. For if we confine our attention either to the
living or to the extinct alone, the series is far less perfect than if
we combine both into one general system. With respect to the
Vertebrata, whole pages could be filled with striking illustrations
from our great palaeontologist, Owen, showing how extinct animals fall
in between existing groups. Cuvier ranked the Ruminants and
Pachyderms, as the two most distinct orders of mammals; but Owen has
discovered so many fossil links, that he has had to alter the whole
classification of these two orders; and has placed certain pachyderms
in the same sub-order with ruminants: for example, he dissolves by
fine gradations the apparently
wide difference between the pig and the
camel. In regard to the Invertebrata, Barrande, and a higher authority
could not be named, asserts that he is every day taught that
palaeozoic animals, though belonging to the same orders, families, or
genera with those living at the present day, were not at this early
epoch limited in such distinct groups as they now are.
Some writers have objected to any extinct species or group of
species being considered as intermediate between living species or
groups. If by this term it is meant that an extinct form is directly
intermediate in all its characters between two living forms, the
objection is probably valid. But I apprehend that in a perfectly
natural classification many fossil species would have to stand between
living species, and some extinct genera between living genera, even
between genera belonging to distinct families. The most common case,
especially with respect to very distinct groups, such as fish and
reptiles, seems to be, that supposing them to be distinguished at the
present day from each other by a dozen characters, the ancient members
of the same two groups would be distinguished by a somewhat lesser
number of characters, so that the two groups, though formerly quite
distinct, at that period made some small approach to each other.
It is a common belief that the more ancient a form is, by so much
the more it tends to connect by some of its characters groups now
widely separated from each other. This remark no doubt must be
restricted to those groups which have undergone much change in the
course of geological ages; and it would be difficult to
prove the truth of the proposition, for every now and then even a
living animal, as the Lepidosiren, is discovered having affinities
directed towards very distinct groups. Yet if we compare the older
Reptiles and
Batrachians, the older Fish, the older Cephalopods, and
the eocene Mammals, with the more recent members of the same classes,
we must admit that there is some truth in the remark.
Let us see how far these several facts and inferences accord with
the theory of descent with modification. As the subject is somewhat
complex, I must request the reader to turn to the diagram in the
fourth chapter. We may suppose that the numbered letters represent
genera, and the dotted lines diverging from them the species in each
genus. The diagram is much too simple, too few genera and too few
species being given, but this is unimportant for us. The horizontal
lines may represent successive geological formations, and all the
forms beneath the uppermost line may be considered as extinct. The
three existing genera, a14, q14,
p14, will form a small
family; b14 and f14 a closely allied family or
sub-family; and o14,
e14, m14, a third family. These
three families, together with the many extinct genera on the several
lines of descent diverging from the parent-form A, will form an order;
for all will have inherited something in common from their ancient and
common progenitor. On the principle of the continued tendency to
divergence of character, which was formerly illustrated by this
diagram, the more recent any form is, the more it will generally
differ from its ancient progenitor. Hence we can understand the rule
that the most ancient fossils differ most from existing forms. We must
not, however, assume that divergence of character is a necessary
contingency; it depends solely on the descendants from a species being
thus enabled to seize on many and different places in the economy of
nature. Therefore it is quite possible, as we have seen in the case
of some Silurian forms, that a species might go on being slightly
modified in relation to its slightly altered conditions of life, and
yet retain throughout a vast period the same general characteristics.
This is represented in the diagram by the letter F14.
All the many forms, extinct and recent, descended from A, make, as before remarked, one order; and this order, from the
continued effects of extinction and divergence of character, has
become divided into several sub-families and families, some of which
are supposed to have perished at different periods, and some to have
endured to the present day.
By looking at the diagram we can see that if many of the extinct
forms, supposed to be embedded in the successive formations, were
discovered at several points low down in the series, the three
existing families on the uppermost line would be rendered less
distinct from each other. If, for instance, the genera a1, a5, a10, m3, m6, m9 were disinterred, these
three families would be so closely linked together that they probably
would have to be united into one great family, in nearly the same
manner as has occurred with ruminants and pachyderms. Yet he who
objected to call the extinct genera, which thus linked the living
genera of three families together, intermediate in character, would be
justified, as they are intermediate, not directly, but only by a long
and circuitous course through many widely different forms. If many
extinct forms were to be discovered above one of the middle horizontal
lines or geological formations — for instance, above No. VI.
-- but none from beneath this line, then only the two families on
the left hand (namely, a14, c., and
b14, c.) would have to be united into
one family; and the two other families (namely,
a14 to f14
now including five genera, and o14 to
m14) would yet remain distinct. These two families,
however, would be less distinct from each other than they were before
the
discovery of the fossils. If, for instance, we suppose the
existing genera of the two families to differ from each other by a
dozen characters, in this case the genera, at the early period marked
VI., would differ by a lesser number of characters; for at this early
stage of descent they have not diverged in character from the common
progenitor of the order, nearly so much as they subsequently diverged.
Thus it comes that ancient and extinct genera are often in some slight
degree intermediate in character between their modified descendants,
or between their collateral relations.
In nature the case will be far more complicated than is represented
in the diagram; for the groups will have been more
numerous, they will have endured for extremely unequal lengths of
time, and will have been modified in various degrees. As we possess
only the last volume of the geological record, and that in a very
broken condition, we have no right to expect, except in very rare
cases, to fill up wide intervals in the natural system, and thus unite
distinct families or orders. All that we have a right to expect, is
that those groups, which have within known geological periods
undergone much modification, should in the older formations make some
slight approach to each other; so that the older members should differ
less from each other in some of their characters than do the existing
members of the same groups; and this by the concurrent evidence of our
best palaeontologists seems frequently to be the case.
Thus, on the theory of descent with modification, the main facts
with respect to the mutual affinities of the extinct forms of life to
each other and to living forms, seem to me explained in a satisfactory
manner. And they are wholly inexplicable on any other view.
On this same theory, it is evident that the fauna of any great
period in the earth's history will be intermediate
in general
character between that which preceded and that which succeeded it.
Thus, the species which lived at the sixth great stage of descent in
the diagram are the modified offspring of those which lived at the
fifth stage, and are the parents of those which became still more
modified at the seventh stage; hence they could hardly fail to be
nearly intermediate in character between the forms of life above and
below. We must, however, allow for the entire extinction of some
preceding forms, and for the coming in of quite new forms by
immigration, and for a large amount of modification, during the long
and blank intervals between the successive formations. Subject to
these allowances, the fauna of each geological period undoubtedly is
intermediate in character, between the preceding and succeeding
faunas. I need give only one instance, namely, the manner in which the
fossils of the Devonian system, when this system was first discovered,
were at once recognised by palaeontologists as intermediate in
character between those of the overlying carboniferous, and underlying
Silurian system. But each fauna is not necessarily
exactly intermediate, as unequal intervals of time have elapsed
between consecutive formations.
It is no real objection to the truth of the statement, that the
fauna of each period as a whole is nearly intermediate in character
between the preceding and succeeding faunas, that certain genera offer
exceptions to the rule. For instance, mastodons and elephants, when
arranged by Dr Falconer in two series, first according to their mutual
affinities and then according to their periods of existence, do not
accord in arrangement. The species extreme in character are not the
oldest, or the most recent; nor are those which are intermediate in
character, intermediate in age. But
supposing for an instant, in this
and other such cases, that the record of the first appearance and
disappearance of the species was perfect, we have no reason to believe
that forms successively produced necessarily endure for corresponding
lengths of time: a very ancient form might occasionally last much
longer than a form elsewhere subsequently produced, especially in the
case of terrestrial productions inhabiting separated districts. To
compare small things with great: if the principal living and extinct
races of the domestic pigeon were arranged as well as they could be in
serial affinity, this arrangement would not closely accord with the
order in time of their production, and still less with the order of
their disappearance; for the parent rock-pigeon now lives; and many
varieties between the rock-pigeon and the carrier have become extinct;
and carriers which are extreme in the important character of length of
beak originated earlier than short-beaked tumblers, which are at the
opposite end of the series in this same respect.
Closely connected with the statement, that the organic remains from
an intermediate formation are in some degree intermediate in
character, is the fact, insisted on by all palaeontologists, that
fossils from two consecutive formations are far more closely related
to each other, than are the fossils from two remote formations. Pictet
gives as a well-known instance, the general resemblance of the organic
remains from the several stages of the chalk formation, though the
species are distinct in each stage. This fact alone, from its
generality, seems to have shaken Professor Pictet in his firm belief
in the immutability of species. He who is acquainted
with the distribution of existing species over the globe, will not
attempt to account for the close resemblance of the distinct species
in closely consecutive
formations, by the physical conditions of the
ancient areas having remained nearly the same. Let it be remembered
that the forms of life, at least those inhabiting the sea, have
changed almost simultaneously throughout the world, and therefore
under the most different climates and conditions. Consider the
prodigious vicissitudes of climate during the pleistocene period,
which includes the whole glacial period, and note how little the
specific forms of the inhabitants of the sea have been affected.
On the theory of descent, the full meaning of the fact of fossil
remains from closely consecutive formations, though ranked as distinct
species, being closely related, is obvious. As the accumulation of
each formation has often been interrupted, and as long blank intervals
have intervened between successive formations, we ought not to expect
to find, as I attempted to show in the last chapter, in any one or two
formations all the intermediate varieties between the species which
appeared at the commencement and close of these periods; but we ought
to find after intervals, very long as measured by years, but only
moderately long as measured geologically, closely allied forms, or, as
they have been called by some authors, representative species; and
these we assuredly do find. We find, in short, such evidence of the
slow and scarcely sensible mutation of specific forms, as we have a
just right to expect to find.
On the state of Development of Ancient Forms.
There has been much discussion whether recent forms are more
highly developed than ancient. I will not here enter on this subject,
for naturalists have not as yet defined to each other's satisfaction
what is meant by high and low forms. But in one particular sense the
more recent forms must, on my theory, be higher than the more ancient;
for each new species is formed by having had some advantage in the
struggle for life over other and preceding forms. If under a nearly
similar climate, the eocene inhabitants of one quarter of the world
were put into competition with the existing inhabitants of the same or
some other quarter, the eocene fauna or flora would
certainly be beaten and exterminated; as would a secondary fauna by an
eocene, and a palaeozoic fauna by a secondary fauna. I do not doubt
that this process of improvement has affected in a marked and sensible
manner the organisation of the more recent and victorious forms of
life, in comparison with the ancient and beaten forms; but I can see
no way of testing this sort of progress. Crustaceans, for instance,
not the highest in their own class, may have beaten the highest
molluscs. From the extraordinary manner in which European productions
have recently spread over New Zealand, and have seized on places which
must have been previously occupied, we may believe, if all the animals
and plants of Great Britain were set free in New Zealand, that in the
course of time a multitude of British forms would become thoroughly
naturalized there, and would exterminate many of the natives. On the
other hand, from what we see now occurring in New Zealand, and from
hardly a single inhabitant of the southern hemisphere having become
wild in any part of Europe, we may doubt, if all the productions of
New Zealand were set free in Great Britain, whether any considerable
number would be enabled to seize on places now occupied by our native
plants and animals. Under this point of view, the productions of
Great Britain, may be said to be higher than those of New Zealand. Yet
the most skilful naturalist from an examination of the
species of the
two countries could not have foreseen this result.
Agassiz insists that ancient animals resemble to a certain extent
the embryos of recent animals of the same classes; or that the
geological succession of extinct forms is in some degree parallel to
the embryological development of recent forms. I must follow Pictet
and Huxley in thinking that the truth of this doctrine is very far
from proved. Yet I fully expect to see it hereafter confirmed, at
least in regard to subordinate groups, which have branched off from
each other within comparatively recent times. For this doctrine of
Agassiz accords well with the theory of natural selection. In a future
chapter I shall attempt to show that the adult differs from its
embryo, owing to variations supervening at a not early age, and being
inherited at a corresponding age. This process, whilst it leaves the
embryo almost unaltered, continually adds, in the course
of successive generations, more and more difference to the adult.
Thus the embryo comes to be left as a sort of picture, preserved by
nature, of the ancient and less modified condition of each animal.
This view may be true, and yet it may never be capable of full proof.
Seeing, for instance, that the oldest known mammals, reptiles, and
fish strictly belong to their own proper classes, though some of these
old forms are in a slight degree less distinct from each other than
are the typical members of the same groups at the present day, it
would be vain to look for animals having the common embryological
character of the Vertebrata, until beds far beneath the lowest
Silurian strata are discovered — a discovery of which the chance
is very small.
On the Succession of the same Types within the
same
areas, during the later tertiary periods.
Mr Clift many
years ago showed that the fossil mammals from the Australian caves
were closely allied to the living marsupials of that continent. In
South America, a similar relationship is manifest, even to an
uneducated eye, in the gigantic pieces of armour like those of the
armadillo, found in several parts of La Plata; and Professor Owen has
shown in the most striking manner that most of the fossil mammals,
buried there in such numbers, are related to South American types.
This relationship is even more clearly seen in the wonderful
collection of fossil bones made by MM. Lund and Clausen in the caves
of Brazil. I was so much impressed with these facts that I strongly
insisted, in 1839 and 1845, on this `law of the succession of types,'
-- on `this wonderful relationship in the same continent between
the dead and the living.' Professor Owen has subsequently extended the
same generalisation to the mammals of the Old World. We see the same
law in this author's restorations of the extinct and gigantic birds of
New Zealand. We see it also in the birds of the caves of Brazil. Mr
Woodward has shown that the same law holds good with sea-shells, but
from the wide distribution of most genera of molluscs, it is not well
displayed by them. Other cases could be added, as the relation between
the extinct and living land-shells of Madeira; and
between the extinct and living brackish-water shells of the
Aralo-Caspian Sea.
Now what does this remarkable law of the succession of the same
types within the same areas mean? He would be a bold man, who after
comparing the present climate of Australia and of parts of South
America under the same latitude, would attempt to account, on the one
hand, by dissimilar physical conditions for the dissimilarity of the
inhabitants of these two continents,
and, on the other hand, by
similarity of conditions, for the uniformity of the same types in each
during the later tertiary periods. Nor can it be pretended that it is
an immutable law that marsupials should have been chiefly or solely
produced in Australia; or that Edentata and other American types
should have been solely produced in South America. For we know that
Europe in ancient times was peopled by numerous marsupials; and I have
shown in the publications above alluded to, that in America the law of
distribution of terrestrial mammals was formerly different from what
it now is. North America formerly partook strongly of the present
character of the southern half of the continent; and the southern half
was formerly more closely allied, than it is at present, to the
northern half. In a similar manner we know from Falconer and Cautley's
discoveries, that northern India was formerly more closely related in
its mammals to Africa than it is at the present time. Analogous facts
could be given in relation to the distribution of marine animals.
On the theory of descent with modification, the great law of the
long enduring, but not immutable, succession of the same types within
the same areas, is at once explained; for the inhabitants of each
quarter of the world will obviously tend to leave in that quarter,
during the next succeeding period of time, closely allied though in
some degree modified descendants. If the inhabitants of one continent
formerly differed greatly from those of another continent, so will
their modified descendants still differ in nearly the same manner and
degree. But after very long intervals of time and after great
geographical changes, permitting much inter-migration, the feebler
will yield to the more dominant forms, and there will be nothing
immutable in the laws of past and present distribution.
It may be asked in ridicule, whether I suppose that
the megatherium and other allied huge monsters have left behind them
in South America the sloth, armadillo, and anteater, as their
degenerate descendants. This cannot for an instant be admitted. These
huge animals have become wholly extinct, and have left no progeny. But
in the caves of Brazil, there are many extinct species which are
closely allied in size and in other characters to the species still
living in South America; and some of these fossils may be the actual
progenitors of living species. It must not be forgotten that, on my
theory, all the species of the same genus have descended from some one
species; so that if six genera, each having eight species, be found in
one geological formation, and in the next succeeding formation there
be six other allied or representative genera with the same number of
species, then we may conclude that only one species of each of the six
older genera has left modified descendants, constituting the six new
genera. The other seven species of the old genera have all died out
and have left no progeny. Or, which would probably be a far commoner
case, two or three species of two or three alone of the six older
genera will have been the parents of the six new genera; the other old
species and the other whole genera having become utterly extinct. In
failing orders, with the genera and species decreasing in numbers, as
apparently is the case of the Edentata of South America, still fewer
genera and species will have left modified blood-descendants.
Summary of the preceding and
present Chapters.
I have attempted to show that the geological record is
extremely imperfect; that only a small portion of the globe has been
geologically explored with care; that
only certain classes of organic
beings have been largely preserved in a fossil state; that the number
both of specimens and of species, preserved in our museums, is
absolutely as nothing compared with the incalculable number of
generations which must have passed away even during a single
formation; that, owing to subsidence being necessary for the
accumulation of fossiliferous deposits thick enough to resist future
degradation, enormous intervals of time have elapsed
between the successive formations; that there has probably been more
extinction during the periods of subsidence, and more variation during
the periods of elevation, and during the latter the record will have
been least perfectly kept; that each single formation has not been
continuously deposited; that the duration of each formation is,
perhaps, short compared with the average duration of specific forms;
that migration has played an important part in the first appearance of
new forms in any one area and formation; that widely ranging species
are those which have varied most, and have oftenest given rise to new
species; and that varieties have at first often been local. All these
causes taken conjointly, must have tended to make the geological
record extremely imperfect, and will to a large extent explain why we
do not find interminable varieties, connecting together all the
extinct and existing forms of life by the finest graduated steps.
He who rejects these views on the nature of the geological record,
will rightly reject my whole theory. For he may ask in vain where are
the numberless transitional links which must formerly have connected
the closely allied or representative species, found in the several
stages of the same great formation. He may disbelieve in the enormous
intervals of time which have elapsed between our consecutive
formations; he
may overlook how important a part migration must have
played, when the formations of any one great region alone, as that of
Europe, are considered; he may urge the apparent, but often falsely
apparent, sudden coming in of whole groups of species. He may ask
where are the remains of those infinitely numerous organisms which
must have existed long before the first bed of the Silurian system was
deposited: I can answer this latter question only hypothetically, by
saying that as far as we can see, where our oceans now extend they
have for an enormous period extended, and where our oscillating
continents now stand they have stood ever since the Silurian epoch;
but that long before that period, the world may have presented a
wholly different aspect; and that the older continents, formed of
formations older than any known to us, may now all be in a
metamorphosed condition, or may lie buried under the ocean.
Passing from these difficulties, all the other great
leading facts in palaeontology seem to me simply to follow on the
theory of descent with modification through natural selection. We can
thus understand how it is that new species come in slowly and
successively; how species of different classes do not necessarily
change together, or at the same rate, or in the same degree; yet in
the long run that all undergo modification to some extent. The
extinction of old forms is the almost inevitable consequence of the
production of new forms. We can understand why when a species has once
disappeared it never reappears. Groups of species increase in numbers
slowly, and endure for unequal periods of time; for the process of
modification is necessarily slow, and depends on many complex
contingencies. The dominant species of the larger dominant groups
tend to leave many modified
descendants, and thus new sub-groups and
groups are formed. As these are formed, the species of the less
vigorous groups, from their inferiority inherited from a common
progenitor, tend to become extinct together, and to leave no modified
offspring on the face of the earth. But the utter extinction of a
whole group of species may often be a very slow process, from the
survival of a few descendants, lingering in protected and isolated
situations. When a group has once wholly disappeared, it does not
reappear; for the link of generation has been broken.
We can understand how the spreading of the dominant forms of life,
which are those that oftenest vary, will in the long run tend to
people the world with allied, but modified, descendants; and these
will generally succeed in taking the places of those groups of species
which are their inferiors in the struggle for existence. Hence, after
long intervals of time, the productions of the world will appear to
have changed simultaneously.
We can understand how it is that all the forms of life, ancient and
recent, make together one grand system; for all are connected by
generation. We can understand, from the continued tendency to
divergence of character, why the more ancient a form is, the more it
generally differs from those now living. Why ancient and extinct forms
often tend to fill up gaps between existing forms, sometimes blending
two groups previously classed as distinct into one; but
more commonly only bringing them a little closer together. The more
ancient a form is, the more often, apparently, it displays characters
in some degree intermediate between groups now distinct; for the more
ancient a form is, the more nearly it will be related to, and
consequently resemble, the common progenitor of groups, since become
widely divergent. Extinct forms are seldom directly intermediate
between existing forms; but are intermediate only by a long and
circuitous course through many extinct and very different forms. We
can clearly see why the organic remains of closely consecutive
formations are more closely allied to each other, than are those of
remote formations; for the forms are more closely linked together by
generation: we can clearly see why the remains of an intermediate
formation are intermediate in character.
The inhabitants of each successive period in the world's history
have beaten their predecessors in the race for life, and are, in so
far, higher in the scale of nature; and this may account for that
vague yet ill-defined sentiment, felt by many palaeontologists, that
organisation on the whole has progressed. If it should hereafter be
proved that ancient animals resemble to a certain extent the embryos
of more recent animals of the same class, the fact will be
intelligible. The succession of the same types of structure within the
same areas during the later geological periods ceases to be
mysterious, and is simply explained by inheritance.
If then the geological record be as imperfect as I believe it to
be, and it may at least be asserted that the record cannot be proved
to be much more perfect, the main objections to the theory of natural
selection are greatly diminished or disappear. On the other hand, all
the chief laws of palaeontology plainly proclaim, as it seems to me,
that species have been produced by ordinary generation: old forms
having been supplanted by new and improved forms of life, produced by
the laws of variation still acting round us, and preserved by Natural
Selection.
GEOGRAPHICAL DISTRIBUTION
- Present distribution cannot be accounted for by differences in
physical conditions
- Importance of barriers
- Affinity of the productions of the same continent
- Centres of creation
- Means of dispersal, by changes of climate and of the level of
the land, and by occasional means
- Dispersal during the Glacial period co-extensive with the world
In considering the
distribution of organic beings over the face of the globe, the first
great fact which strikes us is, that neither the similarity nor the
dissimilarity of the inhabitants of various regions can be accounted
for by their climatal and other physical conditions. Of late, almost
every author who has studied the subject has come to this conclusion.
The case of America alone would almost suffice to prove its truth: for
if we exclude the northern parts where the circumpolar land is almost
continuous, all authors agree that one of the most fundamental
divisions in geographical distribution is that between the New and Old
Worlds; yet if we travel over the vast American continent, from the
central parts of the United States to its extreme southern point, we
meet with the most diversified conditions; the most humid districts,
arid deserts, lofty mountains, grassy plains, forests, marshes, lakes,
and great rivers, under almost every temperature. There is hardly a
climate or condition in the Old World which cannot be paralleled in
the New — at least as closely as the same species generally
require; for it is a most rare case to find a group of organisms
confined to any small spot, having conditions peculiar in only a
slight
degree; for instance, small areas in the Old World could be
pointed out hotter than any in the New World, yet these are not
inhabited by a peculiar fauna or flora. Notwithstanding this
parallelism in the conditions of the Old and New Worlds, how widely
different are their living productions!
In the southern hemisphere, if we compare large tracts
of land in Australia, South Africa, and western South America, between
latitudes 25 and 35, we shall find parts extremely similar in
all their conditions, yet it would not be possible to point out three
faunas and floras more utterly dissimilar. Or again we may compare the
productions of South America south of lat. 35 with those north of
25, which consequently inhabit a considerably different climate, and
they will be found incomparably more closely related to each other,
than they are to the productions of Australia or Africa under nearly
the same climate. Analogous facts could be given with respect to the
inhabitants of the sea.
A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions
of various regions. We see this in the great difference of nearly all
the terrestrial productions of the New and Old Worlds, excepting in
the northern parts, where the land almost joins, and where, under a
slightly different climate, there might have been free migration for
the northern temperate forms, as there now is for the strictly arctic
productions. We see the same fact in the great difference between the
inhabitants of Australia, Africa, and South America under the same
latitude: for these countries are almost as much isolated from each
other as is possible. On each continent, also, we see the same fact;
for on the opposite sides of
lofty and continuous mountain-ranges, and
of great deserts, and sometimes even of large rivers, we find
different productions; though as mountain chains, deserts, c.,
are not as impassable, or likely to have endured so long as the oceans
separating continents, the differences are very inferior in degree to
those characteristic of distinct continents.
Turning to the sea, we find the same law. No two marine faunas are
more distinct, with hardly a fish, shell, or crab in common, than
those of the eastern and western shores of South and Central America;
yet these great faunas are separated only by the narrow, but
impassable, isthmus of panama. Westward of the shores of America, a
wide space of open ocean extends, with not an island as a
halting-place for emigrants; here we have a barrier of another kind,
and as soon as this is passed we meet in the eastern islands of the
Pacific, with another and totally distinct fauna. So that here three
marine faunas range far northward and southward, in parallel lines not
far from each other, under corresponding climates; but from being
separated from each other by impassable barriers, either of land or
open sea, they are wholly distinct. On the other hand, proceeding
still further westward from the eastern islands of the tropical parts
of the Pacific, we encounter no impassable barriers, and we have
innumerable islands as halting-places, until after travelling over a
hemisphere we come to the shores of Africa; and over this vast space
we meet with no well-defined and distinct marine faunas. Although
hardly one shell, crab or fish is common to the above-named three
approximate faunas of Eastern and Western America and the eastern
Pacific islands, yet many fish range from the Pacific into the Indian
Ocean, and many shells are common to the eastern islands of the
Pacific
and the eastern shores of Africa, on almost exactly opposite
meridians of longitude.
A third great fact, partly included in the foregoing statements, is
the affinity of the productions of the same continent or sea, though
the species themselves are distinct at different points and stations.
It is a law of the widest generality, and every continent offers
innumerable instances. Nevertheless the naturalist in travelling, for
instance, from north to south never fails to be struck by the manner
in which successive groups of beings, specifically distinct, yet
clearly related, replace each other. He hears from closely allied, yet
distinct kinds of birds, notes nearly similar, and sees their nests
similarly constructed, but not quite alike, with eggs coloured in
nearly the same manner. The plains near the Straits of Magellan are
inhabited by one species of Rhea (American ostrich), and northward the
plains of La Plata by another species of the same genus; and not by a
true ostrich or emeu, like those found in Africa and Australia under
the same latitude. On these same plains of La Plata, we see the agouti
and bizcacha, animals having nearly the same habits as our hares and
rabbits and belonging to the same order of Rodents, but they plainly
display an American type of structure. We ascend the
lofty peaks of the Cordillera and we find an alpine species of
bizcacha; we look to the waters, and we do not find the beaver or
musk-rat, but the coypu and capybara, rodents of the American type.
Innumerable other instances could be given. If we look to the islands
off the American shore, however much they may differ in geological
structure, the inhabitants, though they may be all peculiar species,
are essentially American. We may look back to past ages, as shown in
the last chapter, and we find American types then prevalent on
the
American continent and in the American seas. We see in these facts
some deep organic bond, prevailing throughout space and time, over the
same areas of land and water, and independent of their physical
conditions. The naturalist must feel little curiosity, who is not led
to inquire what this bond is.
This bond, on my theory, is simply inheritance, that cause which
alone, as far as we positively know, produces organisms quite like,
or, as we see in the case of varieties nearly like each other. The
dissimilarity of the inhabitants of different regions may be
attributed to modification through natural selection, and in a quite
subordinate degree to the direct influence of different physical
conditions. The degree of dissimilarity will depend on the migration
of the more dominant forms of life from one region into another having
been effected with more or less ease, at periods more or less remote;
-- on the nature and number of the former immigrants; — and
on their action and reaction, in their mutual struggles for life;
-- the relation of organism to organism being, as I have already
often remarked, the most important of all relations. Thus the high
importance of barriers comes into play by checking migration; as does
time for the slow process of modification through natural selection.
Widely-ranging species, abounding in individuals, which have already
triumphed over many competitors in their own widely-extended homes
will have the best chance of seizing on new places, when they spread
into new countries. In their new homes they will be exposed to new
conditions, and will frequently undergo further modification and
improvement; and thus they will become still further victorious, and
will produce groups of modified descendants. On this principle of
inheritance with modification, we can understand how it
is that sections of genera, whole genera,
and even families are
confined to the same areas, as is so commonly and notoriously the
case.
I believe, as was remarked in the last chapter, in no law of
necessary development. As the variability of each species is an
independent property, and will be taken advantage of by natural
selection, only so far as it profits the individual in its complex
struggle for life, so the degree of modification in different species
will be no uniform quantity. If, for instance, a number of species,
which stand in direct competition with each other, migrate in a body
into a new and afterwards isolated country, they will be little liable
to modification; for neither migration nor isolation in themselves can
do anything. These principles come into play only by bringing
organisms into new relations with each other, and in a lesser degree
with the surrounding physical conditions. As we have seen in the last
chapter that some forms have retained nearly the same character from
an enormously remote geological period, so certain species have
migrated over vast spaces, and have not become greatly modified.
On these views, it is obvious, that the several species of the same
genus, though inhabiting the most distant quarters of the world, must
originally have proceeded from the same source, as they have descended
from the same progenitor. In the case of those species, which have
undergone during whole geological periods but little modification,
there is not much difficulty in believing that they may have migrated
from the same region; for during the vast geographical and climatal
changes which will have supervened since ancient times, almost any
amount of migration is possible. But in many other cases, in which we
have reason to believe that the species of a genus have been produced
within comparatively recent times, there is great difficulty on this
head. It
is also obvious that the individuals of the same species,
though now inhabiting distant and isolated regions, must have
proceeded from one spot, where their parents were first produced: for,
as explained in the last chapter, it is incredible that individuals
identically the same should ever have been produced through natural
selection from parents specifically distinct.
We are thus brought to the question which has been
largely discussed by naturalists, namely, whether species have been
created at one or more points of the earth's surface. Undoubtedly
there are very many cases of extreme difficulty, in understanding how
the same species could possibly have migrated from some one point to
the several distant and isolated points, where now found. Nevertheless
the simplicity of the view that each species was first produced within
a single region captivates the mind. He who rejects it, rejects the
vera causa of ordinary generation with
subsequent migration, and calls in the agency of a miracle. It is
universally admitted, that in most cases the area inhabited by a
species is continuous; and when a plant or animal inhabits two points
so distant from each other, or with an interval of such a nature, that
the space could not be easily passed over by migration, the fact is
given as something remarkable and exceptional. The capacity of
migrating across the sea is more distinctly limited in terrestrial
mammals, than perhaps in any other organic beings; and, accordingly,
we find no inexplicable cases of the same mammal inhabiting distant
points of the world. No geologist will feel any difficulty in such
cases as Great Britain having been formerly united to Europe, and
consequently possessing the same quadrupeds. But if the same species
can be produced at two separate points, why do we not find a single
mammal common to Europe and Australia or South America? The conditions
of life are
nearly the same, so that a multitude of European animals
and plants have become naturalised in America and Australia; and some
of the aboriginal plants are identically the same at these distant
points of the northern and southern hemispheres? The answer, as I
believe, is, that mammals have not been able to migrate, whereas some
plants, from their varied means of dispersal, have migrated across the
vast and broken interspace. The great and striking influence which
barriers of every kind have had on distribution, is intelligible only
on the view that the great majority of species have been produced on
one side alone, and have not been able to migrate to the other side.
Some few families, many sub-families, very many genera, and a still
greater number of sections of genera are confined to a single region;
and it has been observed by several naturalists, that the
most natural genera, or those genera in which the species are most
closely related to each other, are generally local, or confined to one
area. What a strange anomaly it would be, if, when coming one step
lower in the series, to the individuals of the same species, a
directly opposite rule prevailed; and species were not local, but had
been produced in two or more distinct areas!
Hence it seems to me, as it has to many other naturalists, that the
view of each species having been produced in one area alone, and
having subsequently migrated from that area as far as its powers of
migration and subsistence under past and present conditions permitted,
is the most probable. Undoubtedly many cases occur, in which we
cannot explain how the same species could have passed from one point
to the other. But the geographical and climatal changes, which have
certainly occurred within recent geological times, must have
interrupted or rendered discontinuous the formerly continuous range of
many species. So that we are reduced to consider whether the
exceptions to
continuity of range are so numerous and of so grave a
nature, that we ought to give up the belief, rendered probable by
general considerations, that each species has been produced within one
area, and has migrated thence as far as it could. It would be
hopelessly tedious to discuss all the exceptional cases of the same
species, now living at distant and separated points; nor do I for a
moment pretend that any explanation could be offered of many such
cases. But after some preliminary remarks, I will discuss a few of
the most striking classes of facts; namely, the existence of the same
species on the summits of distant mountain-ranges, and at distant
points in the arctic and antarctic regions; and secondly (in the
following chapter), the wide distribution of freshwater productions;
and thirdly, the occurrence of the same terrestrial species on islands
and on the mainland, though separated by hundreds of miles of open
sea. If the existence of the same species at distant and isolated
points of the earth's surface, can in many instances be explained on
the view of each species having migrated from a single birthplace;
then, considering our ignorance with respect to former climatal and
geographical changes and various occasional means of transport, the belief that this has been the universal law, seems to me
incomparably the safest.
In discussing this subject, we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
distinct species of a genus, which on my theory have all descended
from a common progenitor, can have migrated (undergoing modification
during some part of their migration) from the area inhabited by their
progenitor. If it can be shown to be almost invariably the case, that
a region, of which most of its inhabitants are closely related to, or
belong to the same genera with the species of a second region,
has
probably received at some former period immigrants from this other
region, my theory will be strengthened; for we can clearly understand,
on the principle of modification, why the inhabitants of a region
should be related to those of another region, whence it has been
stocked. A volcanic island, for instance, upheaved and formed at the
distance of a few hundreds of miles from a continent, would probably
receive from it in the course of time a few colonists, and their
descendants, though modified, would still be plainly related by
inheritance to the inhabitants of the continent. Cases of this nature
are common, and are, as we shall hereafter more fully see,
inexplicable on the theory of independent creation. This view of the
relation of species in one region to those in another, does not differ
much (by substituting the word variety for species) from that lately
advanced in an ingenious paper by Mr Wallace, in which he concludes,
that `every species has come into existence coincident both in space
and time with a pre-existing closely allied species.' And I now know
from correspondence, that this coincidence he attributes to generation
with modification.
The previous remarks on `single and multiple centres of creation'
do not directly bear on another allied question, — namely
whether all the individuals of the same species have descended from a
single pair, or single hermaphrodite, or whether, as some authors
suppose, from many individuals simultaneously created. With those
organic beings which never intercross (if such exist), the species, on
my theory, must have descended from a succession of improved
varieties, which will never have blended with other
individuals or varieties, but will have supplanted each other; so
that, at each successive stage of modification and improvement, all
the individuals of each variety will have descended from
a single
parent. But in the majority of cases, namely, with all organisms which
habitually unite for each birth, or which often intercross, I believe
that during the slow process of modification the individuals of the
species will have been kept nearly uniform by intercrossing; so that
many individuals will have gone on simultaneously changing, and the
whole amount of modification will not have been due, at each stage, to
descent from a single parent. To illustrate what I mean: our English
racehorses differ slightly from the horses of every other breed; but
they do not owe their difference and superiority to descent from any
single pair, but to continued care in selecting and training many
individuals during many generations.
Before discussing the three classes of facts, which I have selected
as presenting the greatest amount of difficulty on the theory of
`single centres of creation,' I must say a few words on the means of
dispersal.
Means of Dispersal.
Sir C. Lyell and
other authors have ably treated this subject. I can give here only the
briefest abstract of the more important facts. Change of climate must
have had a powerful influence on migration: a region when its climate
was different may have been a high road for migration, but now be
impassable; I shall, however, presently have to discuss this branch of
the subject in some detail. Changes of level in the land must also
have been highly influential: a narrow isthmus now separates two
marine faunas; submerge it, or let it formerly have been submerged,
and the two faunas will now blend or may formerly have blended: where
the sea now extends, land may at a former period have connected
islands or possibly even continents together, and thus have allowed
terrestrial productions to pass from one to the other.
No geologist
will dispute that great mutations of level have occurred within the
period of existing organisms. Edward Forbes insisted that all the
islands in the Atlantic must recently have been connected with Europe
or Africa, and Europe likewise with America. Other
authors have thus hypothetically bridged over every ocean, and have
united almost every island to some mainland. If indeed the arguments
used by Forbes are to be trusted, it must be admitted that scarcely a
single island exists which has not recently been united to some
continent. This view cuts the Gordian knot of the dispersal of the
same species to the most distant points, and removes many a
difficulty: but to the best of any judgement we are not authorised in
admitting such enormous geographical changes within the period of
existing species. It seems to me that we have abundant evidence of
great oscillations of level in our continents; but not of such vast
changes in their position and extension, as to have united them within
the recent period to each other and to the several intervening oceanic
islands. I freely admit the former existence of many islands, now
buried beneath the sea, which may have served as halting places for
plants and for many animals during their migration. In the
coral-producing oceans such sunken islands are now marked, as I
believe, by rings of coral or atolls standing over them. Whenever it
is fully admitted, as I believe it will some day be, that each species
has proceeded from a single birthplace, and when in the course of time
we know something definite about the means of distribution, we shall
be enabled to speculate with security on the former extension of the
land. But I do not believe that it will ever be proved that within the
recent period continents which are now quite separate, have been
continuously, or almost continuously, united
with each other, and with
the many existing oceanic islands. Several facts in distribution,
-- such as the great difference in the marine faunas on the
opposite sides of almost every continent, — the close relation
of the tertiary inhabitants of several lands and even seas to their
present inhabitants, — a certain degree of relation (as we shall
hereafter see) between the distribution of mammals and the depth of
the sea, — these and other such facts seem to me opposed to the
admission of such prodigious geographical revolutions within the
recent period, as are necessitated in the view advanced by Forbes and
admitted by his many followers. The nature and relative proportions of
the inhabitants of oceanic islands likewise seem to me opposed to
the belief of their former continuity with continents.
Nor does their almost universally volcanic composition favour the
admission that they are the wrecks of sunken continents; — if
they had originally existed as mountain-ranges on the land, some at
least of the islands would have been formed, like other
mountain-summits, of granite, metamorphic schists, old fossiliferous
or other such rocks, instead of consisting of mere piles of volcanic
matter.
I must now say a few words on what are called accidental means, but
which more properly might be called occasional means of distribution.
I shall here confine myself to plants. In botanical works, this or
that plant is stated to be ill adapted for wide dissemination; but for
transport across the sea, the greater or less facilities may be said
to be almost wholly unknown. Until I tried, with Mr Berkeley's aid, a
few experiments, it was not even known how far seeds could resist the
injurious action of sea-water. To my surprise I found that out of 87
kinds, 64 germinated after an immersion of 28 days, and a few survived
an immersion of 137 days.
For convenience sake I chiefly tried small
seeds, without the capsule or fruit; and as all of these sank in a few
days, they could not be floated across wide spaces of the sea, whether
or not they were injured by the salt-water. Afterwards I tried some
larger fruits, capsules, c., and some of these floated for a long
time. It is well known what a difference there is in the buoyancy of
green and seasoned timber; and it occurred to me that floods might
wash down plants or branches, and that these might be dried on the
banks, and then by a fresh rise in the stream be washed into the sea.
Hence I was led to dry stems and branches of 94 plants with ripe
fruit, and to place them on sea water. The majority sank quickly, but
some which whilst green floated for a very short time, when dried
floated much longer; for instance, ripe hazel-nuts sank immediately,
but when dried, they floated for 90 days and afterwards when planted
they germinated; an asparagus plant with ripe berries floated for 23
days, when dried it floated for 85 days, and the seeds afterwards
germinated: the ripe seeds of Helosciadium sank in two days, when
dried they floated for above 90 days, and afterwards germinated.
Altogether out of the 94 dried plants, 18 floated for above 28 days,
and some of the 18 floated for a very much longer period.
So that as 64/87 seeds germinated after an immersion of 28 days; and
as 18/94 plants with ripe fruit (but not all the same species as in
the foregoing experiment) floated, after being dried, for above 28
days, as far as we may infer anything from these scanty facts, we may
conclude that the seeds of 14/100 plants of any country might be
floated by sea-currents during 28 days, and would retain their power
of germination. In Johnston's physical Atlas, the average rate of the
several Atlantic currents is 33 miles per diem (some currents running
at the rate of 60 miles
per diem); on this average, the seeds of
14/100 plants belonging to one country might be floated across 924
miles of sea to another country; and when stranded, if blown to a
favourable spot by an inland gale, they would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but
in a much better manner, for he placed the seeds in a box in the
actual sea, so that they were alternately wet and exposed to the air
like really floating plants. He tried 98 seeds, mostly different from
mine; but he chose many large fruits and likewise seeds from plants
which live near the sea; and this would have favoured the average
length of their flotation and of their resistance to the injurious
action of the salt-water. On the other hand he did not previously dry
the plants or branches with the fruit; and this, as we have seen,
would have caused some of them to have floated much longer. The result
was that 18/98 of his seeds floated for 42 days, and were then capable
of germination. But I do not doubt that plants exposed to the waves
would float for a less time than those protected from violent movement
as in our experiments. Therefore it would perhaps be safer to assume
that the seeds of about 10/100 plants of a flora, after having been
dried, could be floated across a space of sea 900 miles in width, and
would then germinate. The fact of the larger fruits often floating
longer than the small, is interesting; as plants with large seeds or
fruit could hardly be transported by any other means; and Alph. de
Candolle has shown that such plants generally have restricted ranges.
But seeds may be occasionally transported in another manner. Drift
timber is thrown up on most islands, even on those in the midst of the
widest oceans; and the natives of the coral-islands in
the Pacific, procure
stones for their tools, solely from the roots of
drifted trees, these stones being a valuable royal tax. I find on
examination, that when irregularly shaped stones are embedded in the
roots of trees, small parcels of earth are very frequently enclosed
in their interstices and behind them, — so perfectly that not a
particle could be washed away in the longest transport: out of one
small portion of earth thus completely
enclosed by wood in an oak about 50 years old, three dicotyledonous
plants germinated: I am certain of the accuracy of this observation.
Again, I can show that the carcasses of birds, when floating on the
sea, sometimes escape being immediately devoured; and seeds of many
kinds in the crops of floating birds long retain their vitality: peas
and vetches, for instance, are killed by even a few days' immersion in
sea-water; but some taken out of the crop of a pigeon, which had
floated on artificial salt-water for 30 days, to my surprise nearly
all germinated.
Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how
frequently birds of many kinds are blown by gales to vast distances
across the ocean. We may I think safely assume that under such
circumstances their rate of flight would often be 35 miles an hour;
and some authors have given a far higher estimate. I have never seen
an instance of nutritious seeds passing through the intestines of a
bird; but hard seeds of fruit will pass uninjured through even the
digestive organs of a turkey. In the course of two months, I picked up
in my garden 12 kinds of seeds, out of the excrement of small birds,
and these seemed perfect, and some of them, which I tried, germinated.
But the following fact is more important: the crops of birds do not
secrete gastric juice, and do not in the
least injure, as I know by
trial, the germination of seeds; now after a bird has found and
devoured a large supply of food, it is positively asserted that all
the grains do not pass into the gizzard for 12 or even 18 hours. A
bird in this interval might easily be blown to the distance of 500
miles, and hawks are known to look out for tired birds, and the
contents of their torn crops might thus readily get scattered. Mr
Brent informs me that a friend of his had to give up flying
carrier-pigeons from France to England, as the hawks on the English coast destroyed so many on their arrival. Some hawks
and owls bolt their prey whole, and after an interval of from twelve
to twenty hours, disgorge pellets, which, as I know from experiments
made in the Zoological Gardens, include seeds capable of germination.
Some seeds of the oat, wheat, millet, canary, hemp, clover, and beet
germinated after having been from twelve to twenty-one hours in the
stomachs of different birds of prey; and two seeds of beet grew after
having been thus retained for two days and fourteen hours. Freshwater
fish, I find, eat seeds of many land and water plants: fish are
frequently devoured by birds, and thus the seeds might be transported
from place to place. I forced many kinds of seeds into the stomachs of
dead fish, and then gave their bodies to fishing-eagles, storks, and
pelicans; these birds after an interval of many hours, either rejected
the seeds in pellets or passed them in their excrement; and several of
these seeds retained their power of germination. Certain seeds,
however, were always killed by this process.
Although the beaks and feet of birds are generally quite clean, I
can show that earth sometimes adheres to them: in one instance I
removed twenty-two grains of dry argillaceous earth from one foot of a
partridge, and in this earth there was a pebble quite as large as
the
seed of a vetch. Thus seeds might occasionally be transported to great
distances; for many facts could be given showing that soil almost
everywhere is charged with seeds. Reflect for a moment on the millions
of quails which annually cross the Mediterranean; and can we doubt
that the earth adhering to their feet would sometimes include a few
minute seeds? But I shall presently have to recur to this subject.
As icebergs are known to be sometimes loaded with earth and
stones, and have even carried brushwood, bones, and the nest of a
land-bird, I can hardly doubt that they must occasionally have
transported seeds from one part to another of the arctic and antarctic
regions, as suggested by Lyell; and during the Glacial period from one
part of the now temperate regions to another. In the Azores, from the
large number of the species of plants common to Europe, in comparison
with the plants of other oceanic islands nearer to the mainland, and
(as remarked by Mr H. C. Watson) from the somewhat northern character
of the flora in comparison with the latitude, I suspected
that these islands had been partly stocked by ice-borne seeds, during
the Glacial epoch. At my request Sir C. Lyell wrote to M. Hartung to
inquire whether he had observed erratic boulders on these islands, and
he answered that he had found large fragments of granite and other
rocks, which do not occur in the archipelago. Hence we may safely
infer that icebergs formerly landed their rocky burthens on the shores
of these mid-ocean islands, and it is at least possible that they may
have brought thither the seeds of northern plants.
Considering that the several above means of transport, and that
several other means, which without doubt remain to be discovered, have
been in action year after year, for centuries and tens of thousands of
years, it would I think be a marvellous fact if many plants had not
thus become widely transported. These means of transport are sometimes
called accidental, but this is not strictly correct: the currents of
the sea are not accidental, nor is the direction of prevalent gales of
wind. It should be observed that scarcely any means of transport would
carry seeds for very great distances; for seeds do not retain their
vitality when exposed for a great length of time to the action of
seawater; nor could they be long carried in the crops or intestines of
birds. These means, however, would suffice for occasional transport
across tracts of sea some hundred miles in breadth, or from island to
island, or from a continent to a neighbouring island, but not from one
distant continent to another. The floras of distant continents would
not by such means become mingled in any great degree; but would remain
as distinct as we now see them to be. The currents, from their course,
would never bring seeds from North America to Britain, though they
might and do bring seeds from the West Indies to our western shores,
where, if not killed by so long an immersion in salt-water, they could
not endure our climate. Almost every year, one or two land-birds are
blown across the whole Atlantic Ocean, from North America to the
western shores of Ireland and England; but seeds could be transported
by these wanderers only by one means, namely, in dirt sticking to
their feet, which is in itself a rare accident. Even in this case, how
small would the chance be of a seed falling on favourable
soil, and coming to maturity! But it would be a great error to argue
that because a well-stocked island, like Great Britain, has not, as
far as is known (and it would be very difficult to prove this),
received within the last few centuries, through occasional means
of
transport, immigrants from Europe or any other continent, that a
poorly-stocked island, though standing more remote from the mainland,
would not receive colonists by similar means. I do not doubt that out
of twenty seeds or animals transported to an island, even if far less
well-stocked than Britain, scarcely more than one would be so well
fitted to its new home, as to become naturalised. But this, as it
seems to me, is no valid argument against what would be effected by
occasional means of transport, during the long lapse of geological
time, whilst an island was being upheaved and formed, and before it
had become fully stocked with inhabitants. On almost bare land, with
few or no destructive insects or birds living there, nearly every
seed, which chanced to arrive, would be sure to germinate and survive.
Dispersal during the Glacial period.
The identity of many plants and animals, on mountain-summits,
separated from each other by hundreds of miles of lowlands, where the
Alpine species could not possibly exist, is one of the most striking
cases known of the same species living at distant points, without the
apparent possibility of their having migrated from one to the other.
It is indeed a remarkable fact to see so many of the same plants
living on the snowy regions of the Alps or Pyrenees, and in the
extreme northern parts of Europe; but it is far more remarkable, that
the plants on the White Mountains, in the United States of America,
are all the same with those of Labrador, and nearly all the same, as
we hear from Asa Gray, with those on the loftiest mountains of Europe.
Even as long ago as 1747, such facts led Gmelin to conclude that the
same species must have been independently created at several distinct
points; and we might have remained
in this same belief, had not
Agassiz and others called vivid attention to the Glacial period,
which, as we shall immediately see, affords a simple explanation of
these facts. We have evidence of almost every conceivable kind,
organic and inorganic, that within a very recent
geological period, central Europe and North America suffered under an
Arctic climate. The ruins of a house burnt by fire do not tell their
tale more plainly, than do the mountains of Scotland and Wales, with
their scored flanks, polished surfaces, and perched boulders, of the
icy streams with which their valleys were lately filled. So greatly
has the climate of Europe changed, that in Northern Italy, gigantic
moraines, left by old glaciers, are now clothed by the vine and maize.
Throughout a large part of the United States, erratic boulders, and
rocks scored by drifted icebergs and coast-ice, plainly reveal a
former cold period.
The former influence of the glacial climate on the distribution of
the inhabitants of Europe, as explained with remarkable clearness by
Edward Forbes, is substantially as follows. But we shall follow the
changes more readily, by supposing a new glacial period to come slowly
on, and then pass away, as formerly occurred. As the cold came on, and
as each more southern zone became fitted for arctic beings and
ill-fitted for their former more temperate inhabitants, the latter
would be supplanted and arctic productions would take their places.
The inhabitants of the more temperate regions would at the same time
travel southward, unless they were stopped by barriers, in which case
they would perish. The mountains would become covered with snow and
ice, and their former Alpine inhabitants would descend to the plains.
By the time that the cold had reached its maximum, we should have a
uniform arctic fauna and flora, covering the central parts of Europe,
as far
south as the Alps and Pyrenees, and even stretching into Spain.
The now temperate regions of the United States would likewise be
covered by arctic plants and animals, and these would be nearly the
same with those of Europe; for the present circumpolar inhabitants,
which we suppose to have everywhere travelled southward, are
remarkably uniform round the world. We may suppose that the Glacial
period came on a little earlier or later in North America than in
Europe, so will the southern migration there have been a little
earlier or later; but this will make no difference in the final
result.
As the warmth returned, the arctic forms would retreat northward,
closely followed up in their retreat by the productions of the more temperate regions. And as the snow melted from the
bases of the mountains, the arctic forms would seize on the cleared
and thawed ground, always ascending higher and higher, as the warmth
increased, whilst their brethren were pursuing their northern journey.
Hence, when the warmth had fully returned, the same arctic species,
which had lately lived in a body together on the lowlands of the Old
and New Worlds, would be left isolated on distant mountain-summits
(having been exterminated on all lesser heights) and in the arctic
regions of both hemispheres.
Thus we can understand the identity of many plants at points so
immensely remote as on the mountains of the United States and of
Europe. We can thus also understand the fact that the Alpine plants of
each mountain-range are more especially related to the arctic forms
living due north or nearly due north of them: for the migration as the
cold came on, and the re-migration on the returning warmth, will
generally have been due south and north. The Alpine plants, for
example, of Scotland, as remarked by Mr H. C. Watson,
and those of the
Pyrenees, as remarked by Ramond, are more especially allied to the
plants of northern Scandinavia; those of the United States to
Labrador; those of the mountains of Siberia to the arctic regions of
that country. These views, grounded as they are on the perfectly
well-ascertained occurrence of a former Glacial period, seem to me to
explain in so satisfactory a manner the present distribution of the
Alpine and Arctic productions of Europe and America, that when in
other regions we find the same species on distant mountain-summits, we
may almost conclude without other evidence, that a colder climate
permitted their former migration across the low intervening tracts,
since become too warm for their existence.
If the climate, since the Glacial period, has ever been in any
degree warmer than at present (as some geologists in the United States
believe to have been the case, chiefly from the distribution of the
fossil Gnathodon), then the arctic and temperate productions will at a
very late period have marched a little further north, and subsequently
have retreated to their present homes; but I have met with no
satisfactory evidence with respect to this intercalated
slightly warmer period, since the Glacial period.
The arctic forms, during their long southern migration and
re-migration northward, will have been exposed to nearly the same
climate, and, as is especially to be noticed, they will have kept in a
body together; consequently their mutual relations will not have been
much disturbed, and, in accordance with the principles inculcated in
this volume, they will not have been liable to much modification. But
with our Alpine productions, left isolated from the moment of the
returning warmth, first at the bases and ultimately on the summits of
the mountains, the case will have been somewhat different;
for it is
not likely that all the same arctic species will have been left on
mountain ranges distant from each other, and have survived there ever
since; they will, also, in all probability have become mingled with
ancient Alpine species, which must have existed on the mountains
before the commencement of the Glacial epoch, and which during its
coldest period will have been temporarily driven down to the plains;
they will, also, have been exposed to somewhat different climatal
influences. Their mutual relations will thus have been in some degree
disturbed; consequently they will have been liable to modification;
and this we find has been the case; for if we compare the present
Alpine plants and animals of the several great European
mountain-ranges, though very many of the species are identically the
same, some present varieties, some are ranked as doubtful forms, and
some few are distinct yet closely allied or representative species.
In illustrating what, as I believe, actually took place during the
Glacial period, I assumed that at its commencement the arctic
productions were as uniform round the polar regions as they are at the
present day. But the foregoing remarks on distribution apply not only
to strictly arctic forms, but also to many sub-arctic and to some few
northern temperate forms, for some of these are the same on the lower
mountains and on the plains of North America and Europe; and it may be
reasonably asked how I account for the necessary degree of uniformity
of the sub-arctic and northern temperate forms round the world, at the
commencement of the Glacial period. At the present day, the sub-arctic
and northern temperate productions of the Old and New
Worlds are separated from each other by the Atlantic Ocean and by the
extreme northern part of the Pacific. During the Glacial period, when
the inhabitants
of the Old and New Worlds lived further southwards
than at present, they must have been still more completely separated
by wider spaces of ocean. I believe the above difficulty may be
surmounted by looking to still earlier changes of climate of an
opposite nature. We have good reason to believe that during the newer
Pliocene period, before the Glacial epoch, and whilst the majority of
the inhabitants of the world were specifically the same as now, the
climate was warmer than at the present day. Hence we may suppose that
the organisms now living under the climate of latitude 60, during
the Pliocene period lived further north under the Polar Circle, in
latitude 66-67; and that the strictly arctic productions then
lived on the broken land still nearer to the pole. Now if we look at a
globe, we shall see that under the Polar Circle there is almost
continuous land from western Europe, through Siberia, to eastern
America. And to this continuity of the circumpolar land, and to the
consequent freedom for intermigration under a more favourable climate,
I attribute the necessary amount of uniformity in the sub-arctic and
northern temperate productions of the Old and New Worlds, at a period
anterior to the Glacial epoch.
Believing, from reasons before alluded to, that our continents have
long remained in nearly the same relative position, though subjected
to large, but partial oscillations of level, I am strongly inclined to
extend the above view, and to infer that during some earlier and still
warmer period, such as the older Pliocene period, a large number of
the same plants and animals inhabited the almost continuous
circumpolar land; and that these plants and animals, both in the Old
and New Worlds, began slowly to migrate southwards as the climate
became less warm, long before the commencement
of the Glacial period.
We now see, as I believe, their descendants, mostly in a modified
condition, in the central parts of Europe and the United States. On
this view we can understand the relationship, with very little
identity, between the productions of North America and Europe, --
a relationship which is most remarkable, considering the distance of the two areas, and their separation by the Atlantic Ocean.
We can further understand the singular fact remarked on by several
observers, that the productions of Europe and America during the later
tertiary stages were more closely related to each other than they are
at the present time; for during these warmer periods the northern
parts of the Old and New Worlds will have been almost continuously
united by land, serving as a bridge, since rendered impassable by
cold, for the inter-migration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as soon
as the species in common, which inhabited the New and Old Worlds,
migrated south of the Polar Circle, they must have been completely cut
off from each other. This separation, as far as the more temperate
productions are concerned, took place long ages ago. And as the plants
and animals migrated southward, they will have become mingled in the
one great region with the native American productions, and have had to
compete with them; and in the other great region, with those of the
Old World. Consequently we have here everything favourable for much
modification, — for far more modification than with the Alpine
productions, left isolated, within a much more recent period, on the
several mountain-ranges and on the arctic lands of the two Worlds.
Hence it has come, that when we compare the now living productions of
the temperate regions of the New and Old Worlds, we find very few
identical
species (though Asa Gray has lately shown that more plants
are identical than was formerly supposed), but we find in every great
class many forms, which some naturalists rank as geographical races,
and others as distinct species; and a host of closely allied or
representative forms which are ranked by all naturalists as
specifically distinct.
As on the land, so in the waters of the sea, a slow southern
migration of a marine fauna, which during the Pliocene or even a
somewhat earlier period, was nearly uniform along the continuous
shores of the Polar Circle, will account, on the theory of
modification, for many closely allied forms now living in areas
completely sundered. Thus, I think, we can understand the presence of
many existing and tertiary representative forms on the eastern and
western shores of temperate North America; and the still
more striking case of many closely allied crustaceans (as described in
Dana's admirable work), of some fish and other marine animals, in the
Mediterranean and in the seas of Japan, — areas now separated by
a continent and by nearly a hemisphere of equatorial ocean.
These cases of relationship, without identity, of the inhabitants
of seas now disjoined, and likewise of the past and present
inhabitants of the temperate lands of North America and Europe, are
inexplicable on the theory of creation. We cannot say that they have
been created alike, in correspondence with the nearly similar physical
conditions of the areas; for if we compare, for instance, certain
parts of South America with the southern continents of the Old World,
we see countries closely corresponding in all their physical
conditions, but with their inhabitants utterly dissimilar.
But we must return to our more immediate subject, the Glacial
period. I am convinced that Forbes's view
may be largely extended. In
Europe we have the plainest evidence of the cold period, from the
western shores of Britain to the Oural range, and southward to the
Pyrenees. We may infer, from the frozen mammals and nature of the
mountain vegetation, that Siberia was similarly affected. Along the
Himalaya, at points 900 miles apart, glaciers have left the marks of
their former low descent; and in Sikkim, Dr Hooker saw maize growing
on gigantic ancient moraines. South of the equator, we have some
direct evidence of former glacial action in New Zealand; and the same
plants, found on widely separated mountains in this island, tell the
same story. If one account which has been published can be trusted, we
have direct evidence of glacial action in the southeastern corner of
Australia.
Looking to America; in the northern half, ice-borne fragments of
rock have been observed on the eastern side as far south as lat.
36-37, and on the shores of the Pacific, where the climate is now
so different, as far south as lat. 46; erratic boulders have, also,
been noticed on the Rocky Mountains. In the Cordillera of Equatorial
South America, glaciers once extended far below their present level.
In central Chile I was astonished at the structure of a vast mound of
detritus, about 800 feet in height, crossing a valley of
the Andes; and this I now feel convinced was a gigantic moraine, left
far below any existing glacier. Further south on both sides of the
continent, from lat. 41 to the southernmost extremity, we have the
clearest evidence of former glacial action, in huge boulders
transported far from their parent source.
We do not know that the Glacial epoch was strictly simultaneous at
these several far distant points on opposite sides of the world. But
we have good evidence in almost every case, that the epoch was
included within
the latest geological period. We have, also, excellent
evidence, that it endured for an enormous time, as measured by
years, at each point. The cold may have come on, or have ceased,
earlier at one point of the globe than at another, but seeing that it
endured for long at each, and that it was contemporaneous in a
geological sense, it seems to me probable that it was, during a part
at least of the period, actually simultaneous throughout the world.
Without some distinct evidence to the contrary, we may at least admit
as probable that the glacial action was simultaneous on the eastern
and western sides of North America, in the Cordillera under the
equator and under the warmer temperate zones, and on both sides of the
southern extremity of the continent. If this be admitted, it is
difficult to avoid believing that the temperature of the whole world
was at this period simultaneously cooler. But it would suffice for my
purpose, if the temperature was at the same time lower along certain
broad belts of longitude.
On this view of the whole world, or at least of broad longitudinal
belts, having been simultaneously colder from pole to pole, much light
can be thrown on the present distribution of identical and allied
species. In America, Dr Hooker has shown that between forty and fifty
of the flowering plants of Tierra del Fuego, forming no inconsiderable
part of its scanty flora, are common to Europe, enormously remote as
these two points are; and there are many closely allied species. On
the lofty mountains of equatorial America a host of peculiar species
belonging to European genera occur. On the highest mountains of
Brazil, some few European genera were found by Gardner, which do not
exist in the wide intervening hot countries. So on the Silla of
Caraccas the illustrious Humboldt long ago found species belonging
to genera characteristic of the Cordillera. On the mountains
of Abyssinia, several European forms and some few representatives of
the peculiar flora of the Cape of Good Hope occur. At the Cape of Good
Hope a very few European species, believed not to have been introduced
by man, and on the mountains, some few representative European forms
are found, which have not been discovered in the intertropical parts
of Africa. On the Himalaya, and on the isolated mountain-ranges of the
peninsula of India, on the heights of Ceylon, and on the volcanic
cones of Java, many plants occur, either identically the same or
representing each other, and at the same time representing plants of
Europe, not found in the intervening hot lowlands. A list of the
genera collected on the loftier peaks of Java raises a picture of a
collection made on a hill in Europe! Still more striking is the fact
that southern Australian forms are clearly represented by plants
growing on the summits of the mountains of Borneo. Some of these
Australian forms, as I hear from Dr. Hooker, extend along the heights
of the peninsula of Malacca, and are thinly scattered, on the one hand
over India and on the other as far as Japan.
On the southern mountains of Australia, Dr. F. Mller has discovered
several European species; other species, not introduced by man, occur
on the lowlands; and a long list can be given, as I am informed by Dr.
Hooker, of European genera, found in Australia, but not in the
intermediate torrid regions. In the admirable `Introduction to the
Flora of New Zealand,' by Dr. Hooker, analogous and striking facts are
given in regard to the plants of that large island. Hence we see that
throughout the world, the plants growing on the more lofty mountains,
and on the temperate lowlands of the northern and southern
hemispheres, are sometimes
identically the same; but they are much
oftener specifically distinct, though related to each other in a most
remarkable manner.
This brief abstract applies to plants alone: some strictly
analogous facts could be given on the distribution of terrestrial
animals. In marine productions, similar cases occur; as an example, I
may quote a remark by the highest authority, Prof. Dana, that `it is
certainly a wonderful fact that New Zealand should have a
closer resemblance in its crustacea to Great Britain, its antipode,
than to any other part of the world.' Sir J. Richardson, also, speaks
of the reappearance on the shores of New Zealand, Tasmania, c.,
of northern forms of fish. Dr Hooker informs me that twenty-five
species of Algae are common to New Zealand and to Europe, but have not
been found in the intermediate tropical seas.
It should be observed that the northern species and forms found in
the southern parts of the southern hemisphere, and on the
mountain-ranges of the intertropical regions, are not arctic, but
belong to the northern temperate zones. As Mr. H. C. Watson has
recently remarked, `In receding from polar towards equatorial
latitudes, the Alpine or mountain floras really become less and less
arctic.' Many of the forms living on the mountains of the warmer
regions of the earth and in the southern hemisphere are of doubtful
value, being ranked by some naturalists as specifically distinct, by
others as varieties; but some are certainly identical, and many,
though closely related to northern forms, must be ranked as distinct
species.
Now let us see what light can be thrown on the foregoing facts, on
the belief, supported as it is by a large body of geological evidence,
that the whole world, or a large part of it, was during the Glacial
period simultaneously
much colder than at present. The Glacial period,
as measured by years, must have been very long; and when we remember
over what vast spaces some naturalised plants and animals have spread
within a few centuries, this period will have been ample for any
amount of migration. As the cold came slowly on, all the tropical
plants and other productions will have retreated from both sides
towards the equator, followed in the rear by the temperate
productions, and these by the arctic; but with the latter we are not
now concerned. The tropical plants probably suffered much extinction;
how much no one can say; perhaps formerly the tropics supported as
many species as we see at the present day crowded together at the Cape
of Good Hope, and in parts of temperate Australia. As we know that
many tropical plants and animals can withstand a considerable amount
of cold, many might have escaped extermination during a moderate fall
of temperature, more especially by escaping into the
warmest spots. But the great fact to bear in mind is, that all
tropical productions will have suffered to a certain extent. On the
other hand, the temperate productions, after migrating nearer to the
equator, though they will have been placed under somewhat new
conditions, will have suffered less. And it is certain that many
temperate plants, if protected from the inroads of competitors, can
withstand a much warmer climate than their own. Hence, it seems to me
possible, bearing in mind that the tropical productions were in a
suffering state and could not have presented a firm front against
intruders, that a certain number of the more vigorous and dominant
temperate forms might have penetrated the native ranks and have
reached or even crossed the equator. The invasion would, of course,
have been greatly favoured by high land, and perhaps
by a dry climate;
for Dr. Falconer informs me that it is the damp with the heat of the
tropics which is so destructive to perennial plants from a temperate
climate. On the other hand, the most humid and hottest districts will
have afforded an asylum to the tropical natives. The mountain-ranges
north-west of the Himalaya, and the long line of the Cordillera, seem
to have afforded two great lines of invasion: and it is a striking
fact, lately communicated to me by Dr. Hooker, that all the flowering
plants, about forty-six in number, common to Tierra del Fuego and to
Europe still exist in North America, which must have lain on the line
of march. But I do not doubt that some temperate productions entered
and crossed even the lowlands of the
tropics at the period when the
cold was most intense, — when arctic forms had migrated some
twenty-five degrees of latitude from their native country and covered
the land at the foot of the Pyrenees. At this period of extreme cold,
I believe that the climate under the equator at the level of the sea
was about the same with that now felt there at the height of six or
seven thousand feet. During this the coldest period, I suppose that
large spaces of the tropical lowlands were clothed with a mingled
tropical and temperate vegetation, like that now growing with strange
luxuriance at the base of the Himalaya, as graphically described by
Hooker.
Thus, as I believe, a considerable number of plants, a few terrestrial animals, and some marine productions, migrated
during the Glacial period from the northern and southern temperate
zones into the intertropical regions, and some even crossed the
equator. As the warmth returned, these temperate forms would naturally
ascend the higher mountains, being exterminated on the lowlands; those
which had not reached the equator, would re-migrate northward or
southward towards their former
homes; but the forms, chiefly northern,
which had crossed the equator, would travel still further from their
homes into the more temperate latitudes of the opposite hemisphere.
Although we have reason to believe from geological evidence that the
whole body of arctic shells underwent scarcely any modification during
their long southern migration and re-migration northward, the case may
have been wholly different with those intruding forms which settled
themselves on the intertropical mountains, and in the southern
hemisphere. These being surrounded by strangers will have had to
compete with many new forms of life; and it is probable that selected
modifications in their structure, habits, and constitutions will have
profited them. Thus many of these wanderers, though still plainly
related by inheritance to their brethren of the northern or southern
hemispheres, now exist in their new homes as well-marked varieties or
as distinct species.
It is a remarkable fact, strongly insisted on by Hooker in regard
to America, and by Alph. de Candolle in regard to Australia, that many
more identical plants and allied forms have apparently migrated from
the north to the south, than in a reversed direction. We see, however,
a few southern vegetable forms on the mountains of Borneo and
Abyssinia. I suspect that this preponderant migration from north to
south is due to the greater extent of land in the north, and to the
northern forms having existed in their own homes in greater numbers,
and having consequently been advanced through natural selection and
competition to a higher stage of perfection or dominating power, than
the southern forms. And thus, when they became commingled during the
Glacial period, the northern forms were enabled to beat the less
powerful southern forms. Just in the same manner as we see at the
present day,
that very many European productions cover
the ground in La Plata, and in a lesser degree in Australia, and have
to a certain extent beaten the natives; whereas extremely few southern
forms have become naturalised in any part of Europe, though hides,
wool, and other objects likely to carry seeds have been largely
imported into Europe during the last two or three centuries from La
Plata, and during the last thirty or forty years from Australia.
Something of the same kind must have occurred on the intertropical
mountains: no doubt before the Glacial period they were stocked with
endemic Alpine forms; but these have almost everywhere largely yielded
to the more dominant forms, generated in the larger areas and more
efficient workshops of the north. In many islands the native
productions are nearly equalled or even outnumbered by the
naturalised; and if the natives have not been actually exterminated,
their numbers have been greatly reduced, and this is the first stage
towards extinction. A mountain is an island on the land; and the
intertropical mountains before the Glacial period must have been
completely isolated; and I believe that the productions of these
islands on the land yielded to those produced within the larger areas
of the north, just in the same way as the productions of real islands
have everywhere lately yielded to continental forms, naturalised by
man's agency.
I am far from supposing that all difficulties are removed on the
view here given in regard to the range and affinities of the allied
species which live in the northern and southern temperate zones and on
the mountains of the intertropical regions. Very many difficulties
remain to be solved. I do not pretend to indicate the exact lines and
means of migration, or the reason
why certain species and not others
have migrated; why certain species have been modified and have given
rise to new groups of forms, and others have remained unaltered. We
cannot hope to explain such facts, until we can say why one species
and not another becomes naturalised by man's agency in a foreign land;
why one ranges twice or thrice as far, and is twice or thrice as
common, as another species within their own homes.
I have said that many difficulties remain to be solved: some of the
most remarkable are stated with admirable clearness by Dr. Hooker in
his botanical works on the antarctic regions. These
cannot be here discussed. I will only say that as far as regards the
occurrence of identical species at points so enormously remote as
Kerguelen Land, New Zealand, and Fuegia, I believe that towards the
close of the Glacial period, icebergs, as suggested by Lyell, have
been largely concerned in their dispersal. But the existence of
several quite distinct species, belonging to genera exclusively
confined to the south, at these and other distant points of the
southern hemisphere, is, on my theory of descent with modification, a
far more remarkable case of difficulty. For some of these species are
so distinct, that we cannot suppose that there has been time since the
commencement of the Glacial period for their migration, and for their
subsequent modification to the necessary degree. The facts seem to me
to indicate that peculiar and very distinct species have migrated in
radiating lines from some common centre; and I am inclined to look in
the southern, as in the northern hemisphere, to a former and warmer
period, before the commencement of the Glacial period, when the
antarctic lands, now covered with ice, supported a highly peculiar and
isolated flora. I suspect that before this flora was exterminated by
the Glacial epoch, a few forms were
widely dispersed to various points
of the southern hemisphere by occasional means of transport, and by
the aid, as halting-places, of existing and now sunken islands, and
perhaps at the commencement of the Glacial period, by icebergs. By
these means, as I believe, the southern shores of America, Australia,
New Zealand have become slightly tinted by the same peculiar forms of
vegetable life.
Sir C. Lyell in a striking passage has speculated, in language
almost identical with mine, on the effects of great alterations of
climate on geographical distribution. I believe that the world has
recently felt one of his great cycles of change; and that on this
view, combined with modification through natural selection, a
multitude of facts in the present distribution both of the same and of
allied forms of life can be explained. The living waters may be said
to have flowed during one short period from the north and from the
south, and to have crossed at the equator; but to have flowed with
greater force from the north so as to have freely inundated the south.
As the tide leaves its drift in horizontal lines, though
rising higher on the shores where the tide rises highest, so have the
living waters left their living drift on our mountain-summits, in a
line gently rising from the arctic lowlands to a great height under
the equator. The various beings thus left stranded may be compared
with savage races of man, driven up and surviving in the
mountain-fastnesses of almost every land, which serve as a record,
full of interest to us, of the former inhabitants of the surrounding
lowlands.
GEOGRAPHICAL DISTRIBUTION —
continued
- Distribution of fresh-water productions
- On the inhabitants of oceanic islands
- Absence of Batrachians and of terrestrial Mammals
- On the relations of the inhabitants of islands to
those of the nearest mainland
- On colonisation from the
nearest source with subsequent modification
- Summary of the last and present chapters
As lakes and river-systems are separated
from each other by barriers of land, it might have been thought that
fresh-water productions would not have ranged widely within the same
country, and as the sea is apparently a still more impassable barrier,
that they never would have extended to distant countries. But the case
is exactly the reverse. Not only have many fresh-water species,
belonging to quite different classes, an enormous range, but allied
species prevail in a remarkable manner throughout the world. I well
remember, when first collecting in the fresh waters of Brazil, feeling
much surprise at the similarity of the fresh-water insects, shells,
c., and at the dissimilarity of the surrounding terrestrial
beings, compared with those of Britain.
But this power in fresh-water productions of ranging widely, though
so unexpected, can, I think, in most cases be explained by their
having become fitted, in a manner highly useful to them, for short and
frequent migrations from pond to pond, or from stream to stream; and
liability to wide dispersal would follow from this capacity as an
almost necessary consequence. We can here consider only a few cases.
In regard to
fish, I believe that the same species never occur in the
fresh waters of distant continents. But on the same continent the
species often range widely and almost capriciously; for two
river-systems will have some fish in common and some different. A few
facts seem to favour the possibility of their occasional transport by
accidental means; like that of the live fish not rarely
dropped by whirlwinds in India, and the vitality of their ova when
removed from the water. But I am inclined to attribute the dispersal
of fresh-water fish mainly to slight changes within the recent period
in the level of the land, having caused rivers to flow into each
other. Instances, also, could be given of this having occurred during
floods, without any change of level. We have evidence in the loess of
the Rhine of considerable changes of level in the land within a very
recent geological period, and when the surface was peopled by existing
land and fresh-water shells. The wide difference of the fish on
opposite sides of continuous mountain-ranges, which from an early
period must have parted river-systems and completely prevented their
inosculation, seems to lead to this same conclusion. With respect to
allied fresh-water fish occurring at very distant points of the world,
no doubt there are many cases which cannot at present be explained:
but some fresh-water fish belong to very ancient forms, and in such
cases there will have been ample time for great geographical changes,
and consequently time and means for much migration. In the second
place, salt-water fish can with care be slowly accustomed to live in
fresh water; and, according to Valenciennes, there is hardly a single
group of fishes confined exclusively to fresh water, so that we may
imagine that a marine member of a fresh-water group might travel far
along the shores of the sea, and subsequently
become modified and
adapted to the fresh waters of a distant land.
Some species of fresh-water shells have a very wide range, and
allied species, which, on my theory, are descended from a common
parent and must have proceeded from a single source, prevail
throughout the world. Their distribution at first perplexed me much,
as their ova are not likely to be transported by birds, and they are
immediately killed by sea water, as are the adults. I could not even
understand how some naturalised species have rapidly spread throughout
the same country. But two facts, which I have observed — and no
doubt many others remain to be observed — throw some light on
this subject. When a duck suddenly emerges from a pond covered with
duck-weed, I have twice seen these little plants adhering to its back;
and it has happened to me, in removing a little duck-weed
from one aquarium to another, that I have quite unintentionally
stocked the one with fresh-water shells from the other. But another
agency is perhaps more effectual: I suspended a duck's feet, which
might represent those of a bird sleeping in a natural pond, in an
aquarium, where many ova of fresh-water shells were hatching; and I
found that numbers of the extremely minute and just hatched shells
crawled on the feet, and clung to them so firmly that when taken out
of the water they could not be jarred off, though at a somewhat more
advanced age they would voluntarily drop off. These just hatched
molluscs, though aquatic in their nature, survived on the duck's feet,
in damp air, from twelve to twenty hours; and in this length of time a
duck or heron might fly at least six or seven hundred miles, and would
be sure to alight on a pool or rivulet, if blown across sea to an
oceanic island or to any other distant point. Sir Charles Lyell also
informs me that a Dyticus has been caught with an Ancylus (a
fresh-water shell like a limpet) firmly adhering to it; and a
water-beetle of the same family, a Colymbetes, once flew on board the
`Beagle,' when forty-five miles distant
from the nearest land: how much farther it might have flown with a
favouring gale no one can tell.
With respect to plants, it has long been known what enormous ranges
many fresh-water and even marsh-species have, both over continents and
to the most remote oceanic islands. This is strikingly shown, as
remarked by Alph. de Candolle, in large groups of terrestrial plants,
which have only a very few aquatic members; for these latter seem
immediately to acquire, as if in consequence, a very wide range. I
think favourable means of dispersal explain this fact. I have before
mentioned that earth occasionally, though rarely, adheres in some
quantity to the feet and beaks of birds. Wading birds, which frequent
the muddy edges of ponds, if suddenly flushed, would be the most
likely to have muddy feet. Birds of this order I can show are the
greatest wanderers, and are occasionally found on the most remote and
barren islands in the open ocean; they would not be likely to alight
on the surface of the sea, so that the dirt would not be washed off
their feet; when making land, they would be sure to fly
to their natural fresh-water haunts. I do not believe that botanists
are aware how charged the mud of ponds is with seeds: I have tried
several little experiments, but will here give only the most striking
case: I took in February three table-spoonfuls of mud from three
different points, beneath water, on the edge of a little pond; this
mud when dry weighed only 6 3/4 ounces; I kept it covered up in my
study for six months, pulling up and counting each plant as it grew;
the plants were
of many kinds, and were altogether 537 in number; and
yet the viscid mud was all contained in a breakfast cup! Considering
these facts, I think it would be an inexplicable circumstance if
water-birds did not transport the seeds of fresh-water plants to vast
distances, and if consequently the range of these plants was not very
great. The same agency may have come into play with the eggs of some
of the smaller fresh-water animals.
Other and unknown agencies probably have also played a part. I have
stated that fresh-water fish eat some kinds of seeds, though they
reject many other kinds after having swallowed them; even small fish
swallow seeds of moderate size, as of the yellow water-lily and
Potamogeton. Herons and other birds, century after century, have gone
on daily devouring fish; they then take flight and go to other waters,
or are blown across the sea; and we have seen that seeds retain their
power of germination, when rejected in pellets or in excrement, many
hours afterwards. When I saw the great size of the seeds of that fine
water-lily, the Nelumbium, and remembered Alph. de Candolle's remarks
on this plant, I thought that its distribution must remain quite
inexplicable; but Audubon states that he found the seeds of the great
southern water-lily (probably, according to Dr Hooker, the Nelumbium
luteum) in a heron's stomach; although I do not know the fact, yet
analogy makes me believe that a heron flying to another pond and
getting a hearty meal of fish, would probably reject from its stomach
a pellet containing the seeds of the Nelumbium undigested; or the
seeds might be dropped by the bird whilst feeding its young, in the
same way as fish are known sometimes to be dropped.
In considering these several means of distribution,
it should be
remembered that when a pond or stream is first formed, for instance, on a rising islet, it will be unoccupied; and a
single seed or egg will have a good chance of succeeding. Although
there will always be a struggle for life between the individuals of
the species, however few, already occupying any pond, yet as the
number of kinds is small, compared with those on the land, the
competition will probably be less severe between aquatic than between
terrestrial species; consequently an intruder from the waters of a
foreign country, would have a better chance of seizing on a place,
than in the case of terrestrial colonists. We should, also, remember
that some, perhaps many, fresh-water productions are low in the scale
of nature, and that we have reason to believe that such low beings
change or become modified less quickly than the high; and this will
give longer time than the average for the migration of the same
aquatic species. We should not forget the probability of many species
having formerly ranged as continuously as fresh-water productions ever
can range, over immense areas, and having subsequently become extinct
in intermediate regions. But the wide distribution of fresh-water
plants and of the lower animals, whether retaining the same identical
form or in some degree modified, I believe mainly depends on the wide
dispersal of their seeds and eggs by animals, more especially by
fresh-water birds, which have large powers of flight, and naturally
travel from one to another and often distant piece of water. Nature,
like a careful gardener, thus takes her seeds from a bed of a
particular nature, and drops them in another equally well fitted for
them.
On the Inhabitants of Oceanic
Islands.
We now come to the last of the three classes of facts, which I
have
selected as presenting the greatest amount of difficulty, on the view
that all the individuals both of the same and of allied species have
descended from a single parent; and therefore have all proceeded from
a common birthplace, notwithstanding that in the course of time they
have come to inhabit distant points of the globe. I have already
stated that I cannot honestly admit Forbes's view on continental
extensions, which, if legitimately followed out, would lead to the
belief that within the recent period all existing islands have been
nearly or quite joined to some continent. This view
would remove many difficulties, but it would not, I think, explain all
the facts in regard to insular productions. In the following remarks I
shall not confine myself to the mere question of dispersal; but shall
consider some other facts, which bear on the truth of the two theories
of independent creation and of descent with modification.
The species of all kinds which inhabit oceanic islands are few in
number compared with those on equal continental areas: Alph. de
Candolle admits this for plants, and Wollaston for insects. If we look
to the large size and varied stations of New Zealand, extending over
780 miles of latitude, and compare its flowering plants, only 750 in
number, with those on an equal area at the Cape of Good Hope or in
Australia, we must, I think, admit that something quite independently
of any difference in physical conditions has caused so great a
difference in number. Even the uniform county of Cambridge has 847
plants, and the little island of Anglesea 764, but a few ferns and a
few introduced plants are included in these numbers, and the
comparison in some other respects is not quite fair. We have evidence
that the barren island of Ascension aboriginally possessed under
half-a-dozen flowering
plants; yet many have become naturalised on it,
as they have on New Zealand and on every other oceanic island which
can be named. In St. Helena there is reason to believe that the
naturalised plants and animals have nearly or quite exterminated many
native productions. He who admits the doctrine of the creation of
each separate species, will have to admit, that a sufficient number of
the best adapted plants and animals have not been created on oceanic
islands; for man has unintentionally stocked them from various sources
far more fully and perfectly than has nature.
Although in oceanic islands the number of kinds of inhabitants is
scanty, the proportion of endemic species (i.e. those found nowhere else in the world) is often extremely
large. If we compare, for instance, the number of the endemic
land-shells in Madeira, or of the endemic birds in the Galapagos
Archipelago, with the number found on any continent, and then compare
the area of the islands with that of the continent, we shall see that
this is true. This fact might have been expected on my
theory for, as already explained, species occasionally arriving after
long intervals in a new and isolated district, and having to compete
with new associates, will be eminently liable to modification, and
will often produce groups of modified descendants. But it by no means
follows, that, because in an island nearly all the species of one
class are peculiar, those of another class, or of another section of
the same class, are peculiar; and this difference seems to depend on
the species which do not become modified having immigrated with
facility and in a body, so that their mutual relations have not been
much disturbed. Thus in the Galapagos Islands nearly every land-bird,
but only two out of the eleven marine birds, are peculiar; and it is
obvious that
marine birds could arrive at these islands more easily
than land-birds. Bermuda, on the other hand, which lies at about the
same distance from North America as the Galapagos Islands do from
South America, and which has a very peculiar soil, does not possess
one endemic land bird; and we know from Mr. J. M. Jones's admirable
account of Bermuda, that very many North American birds, during their
great annual migrations, visit either periodically or occasionally
this island. Madeira does not possess one peculiar bird, and many
European and African birds are almost every year blown there, as I am
informed by Mr. E. V. Harcourt. So that these two islands of Bermuda
and Madeira have been stocked by birds, which for long ages have
struggled together in their former homes, and have become mutually
adapted to each other; and when settled in their new homes, each kind
will have been kept by the others to their proper places and habits,
and will consequently have been little liable to modification.
Madeira, again, is inhabited by a wonderful number of peculiar
land-shells, whereas not one species of sea-shell is confined to its
shores: now, though we do not know how seashells are dispersed, yet we
can see that their eggs or larvae, perhaps attached to seaweed or
floating timber, or to the feet of wading-birds, might be transported
far more easily than land-shells, across three or four hundred miles of
open sea. The different orders of insects in Madeira apparently
present analogous facts.
Oceanic islands are sometimes deficient in certain classes, and their places are apparently occupied by the other
inhabitants; in the Galapagos Islands reptiles, and in New Zealand
gigantic wingless birds, take the place of mammals. In the plants of
the Galapagos Islands, Dr. Hooker has shown that the proportional
numbers of the different orders are very different from
what they are
elsewhere. Such cases are generally accounted for by the physical
conditions of the islands; but this explanation seems to me not a
little doubtful. Facility of immigration, I believe, has been at least
as important as the nature of the conditions.
Many remarkable little facts could be given with respect to the
inhabitants of remote islands. For instance, in certain islands not
tenanted by mammals, some of the endemic plants have beautifully
hooked seeds; yet few relations are more striking than the adaptation
of hooked seeds for transportal by the wool and fur of quadrupeds.
This case presents no difficulty on my view, for a hooked seed might
be transported to an island by some other means; and the plant then
becoming slightly modified, but still retaining its hooked seeds,
would form an endemic species, having as useless an appendage as any
rudimentary organ, — for instance, as the shrivelled wings under
the soldered elytra of many insular beetles. Again, islands often
possess trees or bushes belonging to orders which elsewhere include
only herbaceous species; now trees, as Alph. de Candolle has shown,
generally have, whatever the cause may be, confined ranges. Hence
trees would be little likely to reach distant oceanic islands; and an
herbaceous plant, though it would have no chance of successfully
competing in stature with a fully developed tree, when established on
an island and having to compete with herbaceous plants alone, might
readily gain an advantage by growing taller and taller and overtopping
the other plants. If so, natural selection would often tend to add to
the stature of herbaceous plants when growing on an island, to
whatever order they belonged, and thus convert them first into bushes
and ultimately into trees.
With respect to the absence of whole orders on
oceanic islands,
Bory St. Vincent long ago remarked that Batrachians (frogs, toads,
newts) have never been found on any of the many islands with which the
great oceans are studded. I have taken pains to verify this assertion,
and I have found it strictly true. I have, however, been
assured that a frog exists on the mountains of the great island of New
Zealand; but I suspect that this exception (if the information be
correct) may be explained through glacial agency. This general absence
of frogs, toads, and newts on so many oceanic islands cannot be
accounted for by their physical conditions; indeed it seems that
islands are peculiarly well fitted for these animals; for frogs have
been introduced into Madeira, the Azores, and Mauritius, and have
multiplied so as to become a nuisance. But as these animals and their
spawn are known to be immediately killed by sea-water, on my view we
can see that there would be great difficulty in their transportal
across the sea, and therefore why they do not exist on any oceanic
island. But why, on the theory of creation, they should not have been
created there, it would be very difficult to explain.
Mammals offer another and similar case. I have carefully searched
the oldest voyages, but have not finished my search; as yet I have not
found a single instance, free from doubt, of a terrestrial mammal
(excluding domesticated animals kept by the natives) inhabiting an
island situated above 300 miles from a continent or great continental
island; and many islands situated at a much less distance are equally
barren. The Falkland Islands, which are inhabited by a wolf-like fox,
come nearest to an exception; but this group cannot be considered as
oceanic, as it lies on a bank connected with the mainland; moreover,
icebergs formerly brought boulders to its western shores, and they may
have formerly transported foxes, as so frequently now happens in the
arctic regions. Yet it cannot be said that small islands will not
support small mammals, for they occur in many parts of the world on
very small islands, if close to a continent; and hardly an island can
be named on which our smaller quadrupeds have not become naturalised
and greatly multiplied. It cannot be said, on the ordinary view of
creation, that there has not been time for the creation of mammals;
many volcanic islands are sufficiently ancient, as shown by the
stupendous degradation which they have suffered and by their tertiary
strata: there has also been time for the production of endemic species
belonging to other classes; and on continents it is thought that
mammals appear and disappear at a quicker rate than other and lower
animals. Though terrestrial mammals do not occur on
oceanic islands, arial mammals do occur on almost every island. New
Zealand possesses two bats found nowhere else in the world: Norfolk
Island, the Viti Archipelago, the Bonin Islands, the Caroline and
Marianne Archipelagoes, and Mauritius, all possess their peculiar
bats. Why, it may be asked, has the supposed creative force produced
bats and no other mammals on remote islands? On my view this question
can easily be answered; for no terrestrial mammal can be transported
across a wide space of sea, but bats can fly across. Bats have been
seen wandering by day far over the Atlantic Ocean; and two North
American species either regularly or occasionally visit Bermuda, at
the distance of 600 miles from the mainland. I hear from Mr. Tomes, who
has specially studied this family, that many of the same species have
enormous ranges, and are found on continents and on far distant
islands. Hence we have only to suppose that such wandering species
have been modified
through natural selection in their new homes in
relation to their new position, and we can understand the presence of
endemic bats on islands, with the absence of all terrestrial mammals.
Besides the absence of terrestrial mammals in relation to the
remoteness of islands from continents, there is also a relation, to a
certain extent independent of distance, between the depth of the sea
separating an island from the neighbouring mainland, and the presence
in both of the same mammiferous species or of allied species in a more
or less modified condition. Mr. Windsor Earl has made some striking
observations on this head in regard to the great Malay Archipelago,
which is traversed near Celebes by a space of deep ocean; and this
space separates two widely distinct mammalian faunas. On either side
the islands are situated on moderately deep submarine banks, and they
are inhabited by closely allied or identical quadrupeds. No doubt some
few anomalies occur in this great archipelago, and there is much
difficulty in forming a judgment in some cases owing to the probable
naturalisation of certain mammals through man's agency; but we shall
soon have much light thrown on the natural history of this archipelago
by the admirable zeal and researches of Mr Wallace. I have not as yet
had time to follow up this subject in all other quarters
of the world; but as far as I have gone, the relation generally holds
good. We see Britain separated by a shallow channel from Europe, and
the mammals are the same on both sides; we meet with analogous facts
on many islands separated by similar channels from Australia. The West
Indian Islands stand on a deeply submerged bank, nearly 1000 fathoms
in depth, and here we find American forms, but the species and even
the genera are distinct. As the amount of modification in all cases
depends to
a certain degree on the lapse of time, and as during
changes of level it is obvious that islands separated by shallow
channels are more likely to have been continuously united within a
recent period to the mainland than islands separated by deeper
channels, we can understand the frequent relation between the depth of
the sea and the degree of affinity of the mammalian inhabitants of
islands with those of a neighbouring continent, — an explicable
relation on the view of independent acts of creation.
All the foregoing remarks on the inhabitants of oceanic islands,
-- namely, the scarcity of kinds — the richness in endemic
forms in particular classes or sections of classes, — the
absence of whole groups, as of batrachians, and of terrestrial mammals
notwithstanding the presence of arial bats, — the
singular proportions of certain orders of plants, — herbaceous
forms having been developed into trees, c., — seem to me to
accord better with the view of occasional means of transport having
been largely efficient in the long course of time, than with the view
of all our oceanic islands having been formerly connected by
continuous land with the nearest continent; for on this latter view
the migration would probably have been more complete; and if
modification be admitted, all the forms of life would have been more
equally modified, in accordance with the paramount importance of the
relation of organism to organism.
I do not deny that there are many and grave difficulties in
understanding how several of the inhabitants of the more remote
islands, whether still retaining the same specific form or modified
since their arrival, could have reached their present homes. But the
probability of many islands having existed as halting-places, of which
not a wreck now remains, must not be overlooked.
I will
here give a single instance of one of the cases of difficulty. Almost
all oceanic islands, even the most isolated and smallest, are
inhabited by land-shells, generally by endemic species, but sometimes
by species found elsewhere. Dr. Aug. A. Gould has given several
interesting cases in regard to the land-shells of the islands of the
Pacific. Now it is notorious that land-shells are very easily killed
by salt; their eggs, at least such as I have tried, sink in sea-water
and are killed by it. Yet there must be, on my view, some unknown, but
highly efficient means for their transportal. Would the just-hatched
young occasionally crawl on and adhere to the feet of birds roosting
on the ground, and thus get transported? It occurred to me that
land-shells, when hybernating and having a membranous diaphragm over
the mouth of the shell, might be floated in chinks of drifted timber
across moderately wide arms of the sea. And I found that several
species did in this state withstand uninjured an immersion in
sea-water during seven days: one of these shells was the Helix
pomatia, and after it had again hybernated I put it in sea-water for
twenty days, and it perfectly recovered. As this species has a thick
calcareous operculum, I removed it, and when it had formed a new
membranous one, I immersed it for fourteen days in sea-water, and it
recovered and crawled away: but more experiments are wanted on this
head.
The most striking and important fact for us in regard to the
inhabitants of islands, is their affinity to those of the nearest
mainland, without being actually the same species. Numerous instances
could be given of this fact. I will give only one, that of the
Galapagos Archipelago, situated under the equator, between 500 and 600
miles from the shores of South America. Here
almost every product of
the land and water bears the unmistakeable stamp of the American
continent. There are twenty-six land birds, and twenty-five of those
are ranked by Mr Gould as distinct species, supposed to have been
created here; yet the close affinity of most of these birds to
American species in every character, in their habits, gestures, and
tones of voice, was manifest. So it is with the other animals, and
with nearly all the plants, as shown by Dr. Hooker in his admirable
memoir on the Flora of this archipelago. The naturalist, looking at
the inhabitants of these volcanic islands in the Pacific,
distant several hundred miles from the continent, yet feels that he is
standing on American land. Why should this be so? why should the
species which are supposed to have been created in the Galapagos
Archipelago, and nowhere else, bear so plain a stamp of affinity to
those created in America? There is nothing in the conditions of life,
in the geological nature of the islands, in their height or climate,
or in the proportions in which the several classes are associated
together, which resembles closely the conditions of the South American
coast: in fact there is a considerable dissimilarity in all these
respects. On the other hand, there is a considerable degree of
resemblance in the volcanic nature of the soil, in climate, height,
and size of the islands, between the Galapagos and Cape de Verde
Archipelagos: but what an entire and absolute difference in their
inhabitants! The inhabitants of the Cape de Verde Islands are related
to those of Africa, like those of the Galapagos to America. I believe
this grand fact can receive no sort of explanation on the ordinary
view of independent creation; whereas on the view here maintained, it
is obvious that the Galapagos Islands would be likely to receive
colonists, whether by occasional means of transport or
by formerly
continuous land, from America; and the Cape de Verde Islands from
Africa; and that such colonists would be liable to modifications;
-- the principle of inheritance still betraying their original
birthplace.
Many analogous facts could be given: indeed it is an almost
universal rule that the endemic productions of islands are related to
those of the nearest continent, or of other near islands. The
exceptions are few, and most of them can be explained. Thus the plants
of Kerguelen Land, though standing nearer to Africa than to America,
are related, and that very closely, as we know from Dr. Hooker's
account, to those of America: but on the view that this island has
been mainly stocked by seeds brought with earth and stones on
icebergs, drifted by the prevailing currents, this anomaly disappears.
New Zealand in its endemic plants is much more closely related to
Australia, the nearest mainland, than to any other region: and this is
what might have been expected; but it is also plainly related to South
America, which, although the next nearest continent, is
so enormously remote, that the fact becomes an anomaly. But this
difficulty almost disappears on the view that both New Zealand, South
America, and other southern lands were long ago partially stocked from
a nearly intermediate though distant point, namely from the antarctic
islands, when they were clothed with vegetation, before the
commencement of the Glacial period. The affinity, which, though
feeble, I am assured by Dr. Hooker is real, between the flora of the
south-western corner of Australia and of the Cape of Good Hope, is a
far more remarkable case, and is at present inexplicable: but this
affinity is confined to the plants, and will, I do not doubt, be some
day explained.
The law which causes the inhabitants of an archipelago,
though
specifically distinct, to be closely allied to those of the nearest
continent, we sometimes see displayed on a small scale, yet in a most
interesting manner, within the limits of the same archipelago. Thus
the several islands of the Galapagos Archipelago are tenanted, as I
have elsewhere shown, in a quite marvellous manner, by very closely
related species; so that the inhabitants of each separate island,
though mostly distinct, are related in an incomparably closer degree
to each other than to the inhabitants of any other part of the world.
And this is just what might have been expected on my view, for the
islands are situated so near each other that they would almost
certainly receive immigrants from the same original source, or from
each other. But this dissimilarity between the endemic inhabitants of
the islands may be used as an argument against my views; for it may be
asked, how has it happened in the several islands situated within
sight of each other, having the same geological nature, the same
height, climate, c., that many of the immigrants should have been
differently modified, though only in a small degree. This long
appeared to me a great difficulty: but it arises in chief part from
the deeply-seated error of considering the physical conditions of a
country as the most important for its inhabitants; whereas it cannot,
I think, be disputed that the nature of the other inhabitants, with
which each has to compete, is at least as important, and generally a
far more important element of success. Now if we look to those
inhabitants of the Galapagos Archipelago which are found
in other parts of the world (having on one side for the moment the
endemic species, which cannot be here fairly included, as we are
considering how they have come to be modified since their arrival), we
find a considerable amount
of difference in the several islands. This
difference might indeed have been expected on the view of the islands
having been stocked by occasional means of transport — a seed,
for instance, of one plant having been brought to one island, and that
of another plant to another island. Hence when in former times an
immigrant settled on any one or more of the islands, or when it
subsequently spread from one island to another, it would undoubtedly
be exposed to different conditions of life in the different islands,
for it would have to compete with different sets of organisms: a
plant, for instance, would find the best-fitted ground more perfectly
occupied by distinct plants in one island than in another, and it
would be exposed to the attacks of somewhat different enemies. If then
it varied, natural selection would probably favour different varieties
in the different islands. Some species, however, might spread and yet
retain the same character throughout the group, just as we see on
continents some species' spreading widely and remaining the same.
The really surprising fact in this case of the Galapagos
Archipelago, and in a lesser degree in some analogous instances, is
that the new species formed in the separate islands have not quickly
spread to the other islands. But the islands, though in sight of each
other, are separated by deep arms of the sea, in most cases wider than
the British Channel, and there is no reason to suppose that they have
at any former period been continuously united. The currents of the sea
are rapid and sweep across the archipelago, and gales of wind are
extraordinarily rare; so that the islands are far more effectually
separated from each other than they appear to be on a map.
Nevertheless a good many species, both those found in other parts of
the world and those confined to the archipelago, are common to
the
several islands, and we may infer from certain facts that these have
probably spread from some one island to the others. But we often take,
I think, an erroneous view of the probability of closely allied species invading each other's territory, when put into free
intercommunication. Undoubtedly if one species has any advantage
whatever over another, it will in a very brief time wholly or in part
supplant it; but if both are equally well fitted for their own places
in nature, both probably will hold their own places and keep separate
for almost any length of time. Being familiar with the fact that many
species, naturalised through man's agency, have spread with
astonishing rapidity over new countries, we are apt to infer that most
species would thus spread; but we should remember that the forms which
become naturalised in new countries are not generally closely allied
to the aboriginal inhabitants, but are very distinct species,
belonging in a large proportion of cases, as shown by Alph. de
Candolle, to distinct genera. In the Galapagos Archipelago, many even
of the birds, though so well adapted for flying from island to island,
are distinct on each; thus there are three closely-allied species of
mocking-thrush, each confined to its own island. Now let us suppose
the mocking-thrush of Chatham Island to be blown to Charles Island,
which has its own mocking-thrush: why should it succeed in
establishing itself there? We may safely infer that Charles Island is
well stocked with its own species, for annually more eggs are laid
there than can possibly be reared; and we may infer that the
mocking-thrush peculiar to Charles Island is at least as well fitted
for its home as is the species peculiar to Chatham Island. Sir C.
Lyell and Mr. Wollaston have communicated to me a remarkable fact
bearing on this subject; namely, that Madeira and the adjoining islet
of
Porto Santo possess many distinct but representative land-shells,
some of which live in crevices of stone; and although large quantities
of stone are annually transported from Porto Santo to Madeira, yet
this latter island has not become colonised by the Porto Santo
species: nevertheless both islands have been colonised by some
European land-shells, which no doubt had some advantage over the
indigenous species. From these considerations I think we need not
greatly marvel at the endemic and representative species, which
inhabit the several islands of the Galapagos Archipelago, not having
universally spread from island to island. In many other instances, as
in the several districts of the same continent,
pre-occupation has probably played an important part in checking the
commingling of species under the same conditions of life. Thus, the
south-east and south-west corners of Australia have nearly the same
physical conditions, and are united by continuous land, yet they are
inhabited by a vast number of distinct mammals, birds, and plants.
The principle which determines the general character of the fauna
and flora of oceanic islands, namely, that the inhabitants, when not
identically the same, yet are plainly related to the inhabitants of
that region whence colonists could most readily have been derived,
-- the colonists having been subsequently modified and better
fitted to their new homes, — is of the widest application
throughout nature. We see this on every mountain, in every lake and
marsh. For Alpine species, excepting in so far as the same forms,
chiefly of plants, have spread widely throughout the world during the
recent Glacial epoch, are related to those of the surrounding
lowlands; — thus we have in South America, Alpine humming-birds,
Alpine rodents, Alpine plants, c., all of strictly American
forms, and it is obvious
that a mountain, as it became slowly
upheaved, would naturally be colonised from the surrounding lowlands.
So it is with the inhabitants of lakes and marshes, excepting in so
far as great facility of transport has given the same general forms to
the whole world. We see this same principle in the blind animals
inhabiting the caves of America and of Europe. Other analogous facts
could be given. And it will, I believe, be universally found to be
true, that wherever in two regions, let them be ever so distant, many
closely allied or representative species occur, there will likewise be
found some identical species, showing, in accordance with the
foregoing view, that at some former period there has been
intercommunication or migration between the two regions. And wherever
many closely-allied species occur, there will be found many forms
which some naturalists rank as distinct species, and some as
varieties; these doubtful forms showing us the steps in
the process of modification.
This relation between the power and extent of migration of a
species, either at the present time or at some former period under
different physical conditions, and the existence at remote points of the world of other species allied to it, is shown
in another and more general way. Mr. Gould remarked to me long ago,
that in those genera of birds which range over the world, many of the
species have very wide ranges. I can hardly doubt that this rule is
generally true, though it would be difficult to prove it. Amongst
mammals, we see it strikingly displayed in Bats, and in a lesser
degree in the Felidae and Canidae. We see it, if we compare the
distribution of butterflies and beetles. So it is with most
fresh-water productions, in which so many genera range over the world,
and many individual species have enormous ranges. It is not meant that
in world-ranging genera all the species have a wide range, or even
that they have on an average a wide range; but only that some of the
species range very widely; for the facility with which widely-ranging
species vary and give rise to new forms will largely determine their
average range. For instance, two varieties of the same species inhabit
America and Europe, and the species thus has an immense range; but, if
the variation had been a little greater, the two varieties would have
been ranked as distinct species, and the common range would have been
greatly reduced. Still less is it meant, that a species which
apparently has the capacity of crossing barriers and ranging widely,
as in the case of certain powerfully-winged birds, will necessarily
range widely; for we should never forget that to range widely implies
not only the power of crossing barriers, but the more important power
of being victorious in distant lands in the struggle for life with
foreign associates. But on the view of all the species of a genus
having descended from a single parent, though now distributed to the
most remote points of the world, we ought to find, and I believe as a
general rule we do find, that some at least of the species range very
widely; for it is necessary that the unmodified parent should range
widely, undergoing modification during its diffusion, and should place
itself under diverse conditions favourable for the conversion of its
offspring, firstly into new varieties and ultimately into new species.
In considering the wide distribution of certain genera, we should
bear in mind that some are extremely ancient, and must have branched
off from a common parent at a remote epoch; so that in
such cases there will have been ample time for great climatal and
geographical changes and for accidents of transport; and consequently
for the migration of some of the species into all
quarters of the
world, where they may have become slightly modified in relation to
their new conditions. There is, also, some reason to believe from
geological evidence that organisms low in the scale within each great
class, generally change at a slower rate than the higher forms; and
consequently the lower forms will have had a better chance of ranging
widely and of still retaining the same specific character. This fact,
together with the seeds and eggs of many low forms being very minute
and better fitted for distant transportation, probably accounts for a
law which has long been observed, and which has lately been admirably
discussed by Alph. de Candolle in regard to plants, namely, that the
lower any group of organisms is, the more widely it is apt to range.
The relations just discussed, — namely, low and
slowly-changing organisms ranging more widely than the high, --
some of the species of widely-ranging genera themselves ranging
widely, — such facts, as alpine, lacustrine, and marsh
productions being related (with the exceptions before specified) to
those on the surrounding low lands and dry lands, though these
stations are so different — the very close relation of the
distinct species which inhabit the islets of the same archipelago,
-- and especially the striking relation of the inhabitants of
each whole archipelago or island to those of the nearest mainland,
-- are, I think, utterly inexplicable on the ordinary view of the
independent creation of each species, but are explicable on the view
of colonisation from the nearest and readiest source, together with
the subsequent modification and better adaptation of the colonists to
their new homes.
Summary of last and present Chapters.
In these chapters I have endeavoured to show, that if we make due
allowance for our ignorance of the full effects of all
the changes of
climate and of the level of the land, which have certainly occurred
within the recent period, and of other similar changes which may have
occurred within the same period; if we remember how
profoundly ignorant we are with respect to the many and curious means
of occasional transport, — a subject which has hardly ever been
properly experimentised on; if we bear in mind how often a species may
have ranged continuously over a wide area, and then have become
extinct in the intermediate tracts, I think the difficulties in
believing that all the individuals of the same species, wherever
located, have descended from the same parents, are not insuperable.
And we are led to this conclusion, which has been arrived at by many
naturalists under the designation of single centres of creation, by
some general considerations, more especially from the importance of
barriers and from the analogical distribution of sub-genera, genera,
and families.
With respect to the distinct species of the same genus, which on my
theory must have spread from one parent-source; if we make the same
allowances as before for our ignorance, and remember that some forms
of life change most slowly, enormous periods of time being thus
granted for their migration, I do not think that the difficulties are
insuperable; though they often are in this case, and in that of the
individuals of the same species, extremely grave.
As exemplifying the effects of climatal changes on distribution, I
have attempted to show how important has been the influence of the
modern Glacial period, which I am fully convinced simultaneously
affected the whole world, or at least great meridional belts. As
showing how diversified are the means of occasional transport, I have
discussed at some little length the means of dispersal of fresh-water
productions.
If the difficulties be not insuperable in admitting that in the
long course of time the individuals of the same species, and likewise
of allied species, have proceeded from some one source; then I think
all the grand leading facts of geographical distribution are
explicable on the theory of migration (generally of the more dominant
forms of life), together with subsequent modification and the
multiplication of new forms. We can thus understand the high
importance of barriers, whether of land or water, which separate our
several zoological and botanical provinces. We can thus understand
the localisation of sub-genera, genera, and families; and how it is
that under different latitudes, for instance in South
America, the inhabitants of the plains and mountains, of the forests,
marshes, and deserts, are in so mysterious a manner linked together by
affinity, and are likewise linked to the extinct beings which formerly
inhabited the same continent. Bearing in mind that the mutual
relations of organism to organism are of the highest importance, we
can see why two areas having nearly the same physical conditions
should often be inhabited by very different forms of life; for
according to the length of time which has elapsed since new
inhabitants entered one region; according to the nature of the
communication which allowed certain forms and not others to enter,
either in greater or lesser numbers; according or not, as those which
entered happened to come in more or less direct competition with each
other and with the aborigines; and according as the immigrants were
capable of varying more or less rapidly, there would ensue in
different regions, independently of their physical conditions,
infinitely diversified conditions of life, — there would be an
almost endless amount of organic action and reaction, — and we
should find, as we do find, some groups of beings greatly, and some
only slightly modified, — some developed
in great force, some
existing in scanty numbers — in the different great geographical
provinces of the world.
On these same principles, we can understand, as I have endeavoured
to show, why oceanic islands should have few inhabitants, but of these
a great number should be endemic or peculiar; and why, in relation to
the means of migration, one group of beings, even within the same
class, should have all its species endemic, and another group should
have all its species common to other quarters of the world. We can see
why whole groups of organisms, as batrachians and terrestrial mammals,
should be absent from oceanic islands, whilst the most isolated
islands possess their own peculiar species of arial mammals or bats.
We can see why there should be some relation between the presence of
mammals, in a more or less modified condition, and the depth of the
sea between an island and the mainland. We can clearly see why all the
inhabitants of an archipelago, though specifically distinct on the
several islets, should be closely related to each other, and likewise
be related, but less closely, to those of the nearest
continent or other source whence immigrants were probably derived. We
can see why in two areas, however distant from each other, there
should be a correlation, in the presence of identical species, of
varieties, of doubtful species, and of distinct but representative
species.
As the late Edward Forbes often insisted, there is a striking
parallelism in the laws of life throughout time and space: the laws
governing the succession of forms in past times being nearly the same
with those governing at the present time the differences in different
areas. We see this in many facts. The endurance of each species and
group of species is continuous in time; for the exceptions to the rule
are so few, that they may
fairly be attributed to our not having as
yet discovered in an intermediate deposit the forms which are therein
absent, but which occur above and below: so in space, it certainly is
the general rule that the area inhabited by a single species, or by a
group of species, is continuous; and the exceptions, which are not
rare, may, as I have attempted to show, be accounted for by migration
at some former period under different conditions or by occasional
means of transport, and by the species having become extinct in the
intermediate tracts. Both in time and space, species and groups of
species have their points of maximum development. Groups of species,
belonging either to a certain period of time, or to a certain area,
are often characterised by trifling characters in common, as of
sculpture or colour. In looking to the long succession of ages, as in
now looking to distant provinces throughout the world, we find that
some organisms differ little, whilst others belonging to a different
class, or to a different order, or even only to a different family of
the same order, differ greatly. In both time and space the lower
members of each class generally change less than the higher; but there
are in both cases marked exceptions to the rule. On my theory these
several relations throughout time and space are intelligible; for
whether we look to the forms of life which have changed during
successive ages within the same quarter of the world, or to those
which have changed after having migrated into distant quarters, in
both cases the forms within each class have been connected by the same
bond of ordinary generation; and the more nearly any two forms are related in blood, the nearer they will generally stand to
each other in time and space; in both cases the laws of variation have
been the same, and modifications have been accumulated by the same
power of natural selection.
MUTUAL AFFINITIES OF ORGANIC
BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY ORGANS
- CLASSIFICATION, groups subordinate to groups
- Natural system
- Rules and difficulties in classification, explained
on
the theory of descent with modification
- Classification of varieties
- Descent always used in classification
- Analogical or adaptive characters
- Affinities, general, complex and radiating
- Extinction separates and defines groups
- MORPHOLOGY, between members of the same class, between parts
of the same individual
- EMBRYOLOGY, laws of, explained by variations not supervening
at an early age, and being inherited at a corresponding age
- RUDIMENTARY ORGANS; their origin explained
- Summary
From the first dawn of
life, all organic beings are found to resemble each other in
descending degrees, so that they can be classed in groups under
groups. This classification is evidently not arbitrary like the
grouping of the stars in constellations. The existence of groups would
have been of simple signification, if one group had been exclusively
fitted to inhabit the land, and another the water; one to feed on
flesh, another on vegetable matter, and so on; but the case is widely
different in nature; for it is notorious how commonly members of even
the same subgroup have different habits. In our second and fourth
chapters, on Variation and on Natural Selection, I have attempted to
show that it is the widely ranging, the much diffused and common, that
is the dominant species belonging to the larger genera, which vary
most. The varieties, or incipient species, thus produced ultimately
become converted, as I believe, into new and distinct species; and
these, on the principle of inheritance, tend to produce other new and
dominant
species. Consequently the groups which are now large, and
which generally include many dominant species, tend to go
on increasing indefinitely in size. I further attempted to show that
from the varying descendants of each species trying to occupy as many
and as different places as possible in the economy of nature, there is
a constant tendency in their characters to diverge. This conclusion
was supported by looking at the great diversity of the forms of life
which, in any small area, come into the closest competition, and by
looking to certain facts in naturalisation.
I attempted also to show that there is a constant tendency in the
forms which are increasing in number and diverging in character, to
supplant and exterminate the less divergent, the less improved, and
preceding forms. I request the reader to turn to the diagram
illustrating the action, as formerly explained, of these several
principles; and he will see that the inevitable result is that the
modified descendants proceeding from one progenitor become broken up
into groups subordinate to groups. In the diagram each letter on the
uppermost line may represent a genus including several species; and
all the genera on this line form together one class, for all have
descended from one ancient but unseen parent, and, consequently, have
inherited something in common. But the three genera on the left hand
have, on this same principle, much in common, and form a sub-family,
distinct from that including the next two genera on the right hand,
which diverged from a common parent at the fifth stage of descent.
These five genera have also much, though less, in common; and they
form a family distinct from that including the three genera still
further to the right hand, which diverged at a still earlier period.
And all these genera, descended from (A), form an order distinct from
the
genera descended from (I). So that we here have many species
descended from a single progenitor grouped into genera; and the genera
are included in, or subordinate to, sub-families, families, and
orders, all united into one class. Thus, the grand fact in natural
history of the subordination of group under group, which, from its
familiarity, does not always sufficiently strike us, is in my
judgement fully explained.
Naturalists try to arrange the species, genera, and families in
each class, on what is called the Natural System. But what is meant by this system? Some authors look at it merely as a
scheme for arranging together those living objects which are most
alike, and for separating those which are most unlike; or as an
artificial means for enunciating, as briefly as possible, general
propositions, — that is, by one sentence to give the characters
common, for instance, to all mammals, by another those common to all
carnivora, by another those common to the dog-genus, and then by
adding a single sentence, a full description is given of each kind of
dog. The ingenuity and utility of this system are indisputable. But
many naturalists think that something more is meant by the Natural
System; they believe that it reveals the plan of the Creator; but
unless it be specified whether order in time or space, or what else is
meant by the plan of the Creator, it seems to me that nothing is thus
added to our knowledge. Such expressions as that famous one of
Linnaeus, and which we often meet with in a more or less concealed
form, that the characters do not make the genus, but that the genus
gives the characters, seem to imply that something more is included in
our classification, than mere resemblance. I believe that something
more is included; and that propinquity of descent, — the only
known cause of the similarity of organic beings, — is the bond,
hidden as it is by various degrees of modification,
which is partially
revealed to us by our classifications.
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification
either gives some unknown plan of creation, or is simply a scheme for
enunciating general propositions and of placing together the forms
most like each other. It might have been thought (and was in ancient
times thought) that those parts of the structure which determined the
habits of life, and the general place of each being in the economy of
nature, would be of very high importance in classification. Nothing
can be more false. No one regards the external similarity of a mouse
to a shrew, of a dugong to a whale, of a whale to a fish, as of any
importance. These resemblances, though so intimately connected with
the whole life of the being, are ranked as merely `adaptive or
analogical characters;' but to the consideration of these resemblances
we shall have to recur. It may even be given as a general rule, that
the less any part of the organisation is concerned with
special habits, the more important it becomes for classification. As
an instance: Owen, in speaking of the dugong, says, `The generative
organs being those which are most remotely related to the habits and
food of an animal, I have always regarded as affording very clear
indications of its true affinities. We are least likely in the
modifications of these organs to mistake a merely adaptive for an
essential character.' So with plants, how remarkable it is that the
organs of vegetation, on which their whole life depends, are of little
signification, excepting in the first main divisions; whereas the
organs of reproduction, with their product the seed, are of paramount
importance!
We must not, therefore, in classifying, trust to resemblances in
parts of the organisation, however important
they may be for the
welfare of the being in relation to the outer world. Perhaps from this
cause it has partly arisen, that almost all naturalists lay the
greatest stress on resemblances in organs of high vital or
physiological importance. No doubt this view of the classificatory
importance of organs which are important is generally, but by no means
always, true. But their importance for classification, I believe,
depends on their greater constancy throughout large groups of species;
and this constancy depends on such organs having generally been
subjected to less change in the adaptation of the species to their
conditions of life. That the mere physiological importance of an organ
does not determine the classificatory value, is almost shown by the
one fact, that in allied groups, in which the same organ, as we have
every reason to suppose, has nearly the same physiological value, its
classificatory value is widely different. No naturalist can have
worked at any group without being struck with this fact; and it has
been most fully acknowledged in the writings of almost every author.
It will suffice to quote the highest authority, Robert Brown, who in
speaking of certain organs in the Proteaceae, says their generic
importance, `like that of all their parts, not only in this but, as I
apprehend, in every natural family, is very unequal, and in some cases
seems to be entirely lost.' Again in another work he says, the genera
of the Connaraceae `differ in having one or more ovaria, in the
existence or absence of albumen, in the imbricate or
valvular aestivation. Any one of these characters singly is frequently
of more than generic importance, though here even when all taken
together they appear insufficient to separate Cnestis from Connarus.'
To give an example amongst insects, in one great division of the
Hymenoptera, the antennae, as Westwood has remarked, are most constant
in structure;
in another division they differ much, and the
differences are of quite subordinate value in classification; yet no
one probably will say that the antennae in these two divisions of the
same order are of unequal physiological importance. Any number of
instances could be given of the varying importance for classification
of the same important organ within the same group of beings.
Again, no one will say that rudimentary or atrophied organs are of
high physiological or vital importance; yet, undoubtedly, organs in
this condition are often of high value in classification. No one will
dispute that the rudimentary teeth in the upper jaws of young
ruminants, and certain rudimentary bones of the leg, are highly
serviceable in exhibiting the close affinity between Ruminants and
Pachyderms. Robert Brown has strongly insisted on the fact that the
rudimentary florets are of the highest importance in the
classification of the Grasses.
Numerous instances could be given of characters derived from parts
which must be considered of very trifling physiological importance,
but which are universally admitted as highly serviceable in the
definition of whole groups. For instance, whether or not there is an
open passage from the nostrils to the mouth, the only character,
according to Owen, which absolutely distinguishes fishes and reptiles
-- the inflection of the angle of the jaws in Marsupials --
the manner in which the wings of insects are folded — mere
colour in certain Algae — mere pubescence on parts of the flower
in grasses — the nature of the dermal covering, as hair or
feathers, in the Vertebrata. If the Ornithorhynchus had been covered
with feathers instead of hair, this external and trifling character
would, I think, have been considered by naturalists as important an
aid in determining the degree of affinity of this strange creature to
birds and reptiles, as an approach in structure in any one internal
and important organ.
The importance, for classification, of trifling characters, mainly
depends on their being correlated with several other
characters of more or less importance. The value indeed of an
aggregate of characters is very evident in natural history. Hence, as
has often been remarked, a species may depart from its allies in
several characters, both of high physiological importance and of
almost universal prevalence, and yet leave us in no doubt where it
should be ranked. Hence, also, it has been found, that a
classification founded on any single character, however important that
may be, has always failed; for no part of the organisation is
universally constant. The importance of an aggregate of characters,
even when none are important, alone explains, I think, that saying of
Linnaeus, that the characters do not give the genus, but the genus
gives the characters; for this saying seems founded on an appreciation
of many trifling points of resemblance, too slight to be defined.
Certain plants, belonging to the Malpighiaceae, bear perfect and
degraded flowers; in the latter, as A. de Jussieu has remarked, `the
greater number of the characters proper to the species, to the genus,
to the family, to the class, disappear, and thus laugh at our
classification.' But when Aspicarpa produced in France, during
several years, only degraded flowers, departing so wonderfully in a
number of the most important points of structure from the proper type
of the order, yet M. Richard sagaciously saw, as Jussieu observes,
that this genus should still be retained amongst the Malpighiaceae.
This case seems to me well to illustrate the spirit with which our
classifications are sometimes necessarily founded.
Practically when naturalists are at work, they do
not trouble
themselves about the physiological value of the characters which they
use in defining a group, or in allocating any particular species. If
they find a character nearly uniform, and common to a great number of
forms, and not common to others, they use it as one of high value; if
common to some lesser number, they use it as of subordinate value.
This principle has been broadly confessed by some naturalists to be
the true one; and by none more clearly than by that excellent
botanist, Aug. St. Hilaire. If certain characters are always found
correlated with others, though no apparent bond of connexion can be
discovered between them, especial value is set on them. As in most
groups of animals, important organs, such as those for
propelling the blood, or for arating it, or those for propagating the
race, are found nearly uniform, they are considered as highly
serviceable in classification; but in some groups of animals all
these, the most important vital organs, are found to offer characters
of quite subordinate value.
We can see why characters derived from the embryo should be of
equal importance with those derived from the adult, for our
classifications of course include all ages of each species. But it is
by no means obvious, on the ordinary view, why the structure of the
embryo should be more important for this purpose than that of the
adult, which alone plays its full part in the economy of nature. Yet
it has been strongly urged by those great naturalists, Milne Edwards
and Agassiz, that embryonic characters are the most important of any
in the classification of animals; and this doctrine has very generally
been admitted as true. The same fact holds good with flowering plants,
of which the two main divisions have been founded on characters
derived from the embryo, — on the number and position of the
embryonic
leaves or cotyledons, and on the mode of development of the
plumule and radicle. In our discussion on embryology, we shall see why
such characters are so valuable, on the view of classification tacitly
including the idea of descent.
Our classifications are often plainly influenced by chains of
affinities. Nothing can be easier than to define a number of
characters common to all birds; but in the case of crustaceans, such
definition has hitherto been found impossible. There are crustaceans
at the opposite ends of the series, which have hardly a character in
common; yet the species at both ends, from being plainly allied to
others, and these to others, and so onwards, can be recognised as
unequivocally belonging to this, and to no other class of the
Articulata.
Geographical distribution has often been used, though perhaps not
quite logically, in classification, more especially in very large
groups of closely allied forms. Temminck insists on the utility or
even necessity of this practice in certain groups of birds; and it has
been followed by several entomologists and botanists.
Finally, with respect to the comparative value of the
various groups of species, such as orders, sub-orders, families,
sub-families, and genera, they seem to be, at least at present, almost
arbitrary. Several of the best botanists, such as Mr Bentham and
others, have strongly insisted on their arbitrary value. Instances
could be given amongst plants and insects, of a group of forms, first
ranked by practised naturalists as only a genus, and then raised to
the rank of a sub-family or family; and this has been done, not
because further research has detected important structural
differences, at first overlooked, but because numerous allied species,
with slightly different grades of difference, have been subsequently
discovered.
All the foregoing rules and aids and difficulties in classification
are explained, if I do not greatly deceive myself, on the view that
the natural system is founded on descent with modification; that the
characters which naturalists consider as showing true affinity between
any two or more species, are those which have been inherited from a
common parent, and, in so far, all true classification is
genealogical; that community of descent is the hidden bond which
naturalists have been unconsciously seeking, and not some unknown plan
of creation, or the enunciation of general propositions, and the mere
putting together and separating objects more or less alike.
But I must explain my meaning more fully. I believe that the
arrangement of the groups within each class, in due subordination and
relation to the other groups, must be strictly genealogical in order
to be natural; but that the amount of difference in the several
branches or groups, though allied in the same degree in blood to their
common progenitor, may differ greatly, being due to the different
degrees of modification which they have undergone; and this is
expressed by the forms being ranked under different genera, families,
sections, or orders. The reader will best understand what is meant, if
he will take the trouble of referring to the diagram in the fourth
chapter. We will suppose the letters A to L to represent allied
genera, which lived during the Silurian epoch, and these have
descended from a species which existed at an unknown anterior period.
Species of three of these genera (A, F, and I) have transmitted
modified descendants to the present day, represented by
the fifteen genera (a14 to z14)
on the uppermost horizontal line. Now all these modified descendants
from a single species, are represented as related in blood or descent
to the same
degree; they may metaphorically be called cousins to the
same millionth degree; yet they differ widely and in different degrees
from each other. The forms descended from A, now broken up into two or
three families, constitute a distinct order from those descended from
I, also broken up into two families. Nor can the existing species,
descended from A, be ranked in the same genus with the parent A; or
those from I, with the parent I. But the existing genus F14 may be supposed to have been but slightly modified;
and it will then rank with the parent-genus F; just as some few still
living organic beings belong to Silurian genera. So that the amount or
value of the differences between organic beings all related to each
other in the same degree in blood, has come to be widely different.
Nevertheless their genealogical arrangement
remains strictly true, not
only at the present time, but at each successive period of descent.
All the modified descendants from A will have inherited something in
common from their common parent, as will all the descendants from I;
so will it be with each subordinate branch of descendants, at each
successive period. If, however, we choose to suppose that any of the
descendants of A or of I have been so much modified as to have more or
less completely lost traces of their parentage, in this case, their
places in a natural classification will have been more or less
completely lost, — as sometimes seems to have occurred with
existing organisms. All the descendants of the genus F, along its
whole line of descent, are supposed to have been but little modified,
and they yet form a single genus. But this genus, though much
isolated, will still occupy its proper intermediate position; for F
originally was intermediate in character between A and I, and the
several genera descended from these two genera will
have inherited to
a certain extent their characters. This natural arrangement is shown,
as far as is possible on paper, in the diagram, but in much too simple
a manner. If a branching diagram had not been used, and only the names
of the groups had been written in a linear series, it would have been
still less possible to have given a natural arrangement;
and it is notoriously not possible to represent in a series, on a flat
surface, the affinities which we discover in nature amongst the beings
of the same group. Thus, on the view which I hold, the natural system
is genealogical in its arrangement, like a pedigree; but the degrees
of modification which the different groups have undergone, have to be
expressed by ranking them under different so-called genera,
sub-families, families, sections, orders, and classes.
It may be worth while to illustrate this view of classification, by
taking the case of languages. If we possessed a perfect pedigree of
mankind, a genealogical arrangement of the races of man would afford
the best classification of the various languages now spoken throughout
the world; and if all extinct languages, and all intermediate and
slowly changing dialects, had to be included, such an arrangement
would, I think, be the only possible one. Yet it might be that some
very ancient language had altered little, and had given rise to few
new languages, whilst others (owing to the spreading and subsequent
isolation and states of civilisation of the several races, descended
from a common race) had altered much, and had given rise to many new
languages and dialects. The various degrees of difference in the
languages from the same stock, would have to be expressed by groups
subordinate to groups; but the proper or even only possible
arrangement would still be genealogical; and this would be strictly
natural, as
it would connect together all languages, extinct and
modern, by the closest affinities, and would give the filiation and
origin of each tongue.
In confirmation of this view, let us glance at the classification
of varieties, which are believed or known to have descended from one
species. These are grouped under species, with sub-varieties under
varieties; and with our domestic productions, several other grades of
difference are requisite, as we have seen with pigeons. The origin of
the existence of groups subordinate to groups, is the same with
varieties as with species, namely, closeness of descent with various
degrees of modification. Nearly the same rules are followed in
classifying varieties, as with species. Authors have insisted on the
necessity of classing varieties on a natural instead of an artificial
system; we are cautioned, for instance, not to class two
varieties of the pine-apple together, merely because their fruit,
though the most important part, happens to be nearly identical; no one
puts the swedish and common turnips together, though the esculent and
thickened stems are so similar. Whatever part is found to be most
constant, is used in classing varieties: thus the great agriculturist
Marshall says the horns are very useful for this purpose with cattle,
because they are less variable than the shape or colour of the body,
c.; whereas with sheep the horns are much less serviceable,
because less constant. In classing varieties, I apprehend if we had a
real pedigree, a genealogical classification would be universally
preferred; and it has been attempted by some authors. For we might
feel sure, whether there had been more or less modification, the
principle of inheritance would keep the forms together which were
allied in the greatest number of points. In tumbler pigeons, though
some sub-varieties differ from the others
in the important character
of having a longer beak, yet all are kept together from having the
common habit of tumbling; but the short-faced breed has nearly or
quite lost this habit; nevertheless, without any reasoning or thinking
on the subject, these tumblers are kept in the same group, because
allied in blood and alike in some other respects. If it could be
proved that the Hottentot had descended from the Negro, I think he
would be classed under the Negro group, however much he might differ
in colour and other important characters from negroes.
With species in a state of nature, every naturalist has in fact
brought descent into his classification; for he includes in his lowest
grade, or that of a species, the two sexes; and how enormously these
sometimes differ in the most important characters, is known to every
naturalist: scarcely a single fact can be predicated in common of the
males and hermaphrodites of certain cirripedes, when adult, and yet no
one dreams of separating them. The naturalist includes as one species
the several larval stages of the same individual, however much they
may differ from each other and from the adult; as he likewise includes
the so-called alternate generations of Steenstrup, which can only in a
technical sense be considered as the same individual. He includes
monsters; he includes varieties, not solely because they
closely resemble the parent-form, but because they are descended from
it. He who believes that the cowslip is descended from the primrose,
or conversely, ranks them together as a single species, and gives a
single definition. As soon as three Orchidean forms (Monochanthus,
Myanthus, and Catasetum), which had previously been ranked as three
distinct genera, were known to be sometimes produced on the same
spike, they were immediately included as a single species.
But it may
be asked, what ought we to do, if it could be proved that one species
of kangaroo had been produced, by a long course of modification, from
a bear? Ought we to rank this one species with bears, and what should
we do with the other species? The supposition is of course
preposterous; and I might answer by the argumentum ad hominem, and ask
what should be done if a perfect kangaroo were seen to come out of the
womb of a bear? According to all analogy, it would be ranked with
bears; but then assuredly all the other species of the kangaroo family
would have to be classed under the bear genus. The whole case is
preposterous; for where there has been close descent in common, there
will certainly be close resemblance or affinity.
As descent has universally been used in classing together the
individuals of the same species, though the males and females and
larvae are sometimes extremely different; and as it has been used in
classing varieties which have undergone a certain, and sometimes a
considerable amount of modification, may not this same element of
descent have been unconsciously used in grouping species under genera,
and genera under higher groups, though in these cases the modification
has been greater in degree, and has taken a longer time to complete? I
believe it has thus been unconsciously used; and only thus can I
understand the several rules and guides which have been followed by
our best systematists. We have no written pedigrees; we have to make
out community of descent by resemblances of any kind. Therefore we
choose those characters which, as far as we can judge, are the least
likely to have been modified in relation to the conditions of life to
which each species has been recently exposed. Rudimentary structures
on this view are as good as, or even sometimes better than, other
parts of the organisation.
We care not how trifling a
character may be — let it be the mere inflection of the angle of
the jaw, the manner in which an insect's wing is folded, whether the
skin be covered by hair or feathers — if it prevail throughout
many and different species, especially those having very different
habits of life, it assumes high value; for we can account for its
presence in so many forms with such different habits, only by its
inheritance from a common parent. We may err in this respect in regard
to single points of structure, but when several characters, let them
be ever so trifling, occur together throughout a large group of beings
having different habits, we may feel almost sure, on the theory of
descent, that these characters have been inherited from a common
ancestor. And we know that such correlated or aggregated characters
have especial value in classification.
We can understand why a species or a group of species may depart,
in several of its most important characteristics, from its allies, and
yet be safely classed with them. This may be safely done, and is often
done, as long as a sufficient number of characters, let them be ever
so unimportant, betrays the hidden bond of community of descent. Let
two forms have not a single character in common, yet if these extreme
forms are connected together by a chain of intermediate groups, we may
at once infer their community of descent, and we put them all into the
same class. As we find organs of high physiological importance --
those which serve to preserve life under the most diverse conditions
of existence — are generally the most constant, we attach
especial value to them; but if these same organs, in another group or
section of a group, are found to differ much, we at once value them
less in our classification. We shall hereafter, I think, clearly see
why embryological characters are of such high classificatory
importance.
Geographical distribution may sometimes be brought
usefully into play in classing large and widely-distributed genera,
because all the species of the same genus, inhabiting any distinct and
isolated region, have in all probability descended from the same
parents.
We can understand, on these views, the very important distinction
between real affinities and analogical or adaptive resemblances.
Lamarck first called attention to this distinction, and
he has been ably followed by Macleay and others. The resemblance, in
the shape of the body and in the fin-like anterior limbs, between the
dugong, which is a pachydermatous animal, and the whale, and between
both these mammals and fishes, is analogical. Amongst insects there
are innumerable instances: thus Linnaeus, misled by external
appearances, actually classed an homopterous insect as a moth. We see
something of the same kind even in our domestic varieties, as in the
thickened stems of the common and swedish turnip. The resemblance of
the greyhound and racehorse is hardly more fanciful than the analogies
which have been drawn by some authors between very distinct animals.
On my view of characters being of real importance for classification,
only in so far as they reveal descent, we can clearly understand why
analogical or adaptive character, although of the utmost importance to
the welfare of the being, are almost valueless to the systematist. For
animals, belonging to two most distinct lines of descent, may readily
become adapted to similar conditions, and thus assume a close external
resemblance; but such resemblances will not reveal — will rather
tend to conceal their blood-relationship to their proper lines of
descent. We can also understand the apparent paradox, that the very
same characters are analogical when one class or order is compared
with another, but give true affinities when the members of
the same
class or order are compared one with another: thus the shape of the
body and fin-like limbs are only analogical when whales are compared
with fishes, being adaptations in both classes for swimming through
the water; but the shape of the body and fin-like limbs serve as
characters exhibiting true affinity between the several members of the
whale family; for these cetaceans agree in so many characters, great
and small, that we cannot doubt that they have inherited their general
shape of body and structure of limbs from a common ancestor. So it is
with fishes.
As members of distinct classes have often been adapted by
successive slight modifications to live under nearly similar
circumstances, — to inhabit for instance the three elements of
land, air, and water, — we can perhaps understand how it is that
a numerical parallelism has sometimes been observed between the
sub-groups in distinct classes. A naturalist, struck by a parallelism
of this nature in any one class, by arbitrarily raising
or sinking the value of the groups in other classes (and all our
experience shows that this valuation has hitherto been arbitrary),
could easily extend the parallelism over a wide range; and thus the
septenary, quinary, quaternary, and ternary classifications have
probably arisen.
As the modified descendants of dominant species, belonging to the
larger genera, tend to inherit the advantages, which made the groups
to which they belong large and their parents dominant, they are almost
sure to spread widely, and to seize on more and more places in the
economy of nature. The larger and more dominant groups thus tend to go
on increasing in size; and they consequently supplant many smaller and
feebler groups. Thus we can account for the fact that all organisms,
recent and extinct, are included under a few great
orders, under still
fewer classes, and all in one great natural system. As showing how few
the higher groups are in number, and how widely spread they are
throughout the world, the fact is striking, that the discovery of
Australia has not added a single insect belonging to a new order; and
that in the vegetable kingdom, as I learn from Dr. Hooker, it has added
only two or three orders of small size.
In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character
during the long-continued process of modification, how it is that the
more ancient forms of life often present characters in some slight
degree intermediate between existing groups. A few old and
intermediate parent-forms having occasionally transmitted to the
present day descendants but little modified, will give to us our
so-called osculant or aberrant groups. The more aberrant any form is,
the greater must be the number of connecting forms which on my theory
have been exterminated and utterly lost. And we have some evidence of
aberrant forms having suffered severely from extinction, for they are
generally represented by extremely few species; and such species as do
occur are generally very distinct from each other, which again implies
extinction. The genera Ornithorhynchus and Lepidosiren, for example,
would not have been less aberrant had each been represented by a dozen
species instead of by a single one; but such richness in
species, as I find after some investigation, does not commonly fall to
the lot of aberrant genera. We can, I think, account for this fact
only by looking at aberrant forms as failing groups conquered by more
successful competitors, with a few members preserved by some unusual
coincidence of favourable circumstances.
Mr. Waterhouse has remarked that, when a member
belonging to one
group of animals exhibits an affinity to a quite distinct group, this
affinity in most cases is general and not special: thus, according to
Mr. Waterhouse, of all Rodents, the bizcacha is most nearly related to
Marsupials; but in the points in which it approaches this order, its
relations are general, and not to any one marsupial species more than
to another. As the points of affinity of the bizcacha to Marsupials
are believed to be real and not merely adaptive, they are due on my
theory to inheritance in common. Therefore we must suppose either that
all Rodents, including the bizcacha, branched off from some very
ancient Marsupial, which will have had a character in some degree
intermediate with respect to all existing Marsupials; or that both
Rodents and Marsupials branched off from a common progenitor, and that
both groups have since undergone much modification in divergent
directions. On either view we may suppose that the bizcacha has
retained, by inheritance, more of the character of its ancient
progenitor than have other Rodents; and therefore it will not be
specially related to any one existing Marsupial, but indirectly to all
or nearly all Marsupials, from having partially retained the character
of their common progenitor, or of an early member of the group. On the
other hand, of all Marsupials, as Mr. Waterhouse has remarked, the
phascolomys resembles most nearly, not any one species, but the
general order of Rodents. In this case, however, it may be strongly
suspected that the resemblance is only analogical, owing to the
phascolomys having become adapted to habits like those of a Rodent.
The elder De Candolle has made nearly similar observations on the
general nature of the affinities of distinct orders of plants.
On the principle of the multiplication and gradual divergence in
character of the species descended from
a common parent,
together with their retention by inheritance of some characters in
common, we can understand the excessively complex and radiating
affinities by which all the members of the same family or higher group
are connected together. For the common parent of a whole family of
species, now broken up by extinction into distinct groups and
sub-groups, will have transmitted some of its characters, modified in
various ways and degrees, to all; and the several species will
consequently be related to each other by circuitous lines of affinity
of various lengths (as may be seen in the diagram so often referred
to), mounting up through many predecessors. As it is difficult to show
the blood-relationship between the numerous kindred of any ancient and
noble family, even by the aid of a genealogical tree, and almost
impossible to do this without this aid, we can understand the
extraordinary difficulty which naturalists have experienced in
describing, without the aid of a diagram, the various affinities which
they perceive between the many living and extinct members of the same
great natural class.
Extinction, as we have seen in the fourth chapter, has played an
important part in defining and widening the intervals between the
several groups in each class. We may thus account even for the
distinctness of whole classes from each other — for instance, of
birds from all other vertebrate animals — by the belief that
many ancient forms of life have been utterly lost, through which the
early progenitors of birds were formerly connected with the early
progenitors of the other vertebrate classes. There has been less
entire extinction of the forms of life which once connected fishes
with batrachians. There has been still less in some other classes, as
in that of the Crustacea, for here the most wonderfully diverse forms
are still tied
together by a long, but broken, chain of affinities.
Extinction has only separated groups: it has by no means made them;
for if every form which has ever lived on this earth were suddenly to
reappear, though it would be quite impossible to give definitions by
which each group could be distinguished from other groups, as all
would blend together by steps as fine as those between the finest
existing varieties, nevertheless a natural classification, or at least
a natural arrangement, would be possible. We shall see this by turning
to the diagram: the letters, A to L, may represent eleven
Silurian genera, some of which have produced large groups of modified
descendants. Every intermediate link between these eleven genera and
their primordial parent, and every intermediate link in each branch
and sub-branch of their descendants, may be supposed to be still
alive; and the links to be as fine as those between the finest
varieties. In this case it would be quite impossible to give any
definition by which the several members of the several groups could be
distinguished from their more immediate parents; or these parents from
their ancient and unknown progenitor. Yet the natural arrangement in
the diagram would still hold good; and, on the principle of
inheritance, all the forms descended from A, or from I, would have
something in common. In a tree we can specify this or that branch,
though at the actual fork the two unite and blend together. We could
not, as I have said, define the several groups; but we could pick out
types, or forms, representing most of the characters of each group,
whether large or small, and thus give a general idea of the value of
the differences between them. This is what we should be driven to, if
we were ever to succeed in collecting all the forms in any class which
have lived throughout all time and space. We shall certainly never
succeed in making
so perfect a collection: nevertheless, in certain
classes, we are tending in this direction; and Milne Edwards has
lately insisted, in an able paper, on the high importance of looking
to types, whether or not we can separate and define the groups to
which such types belong.
Finally, we have seen that natural selection, which results from
the struggle for existence, and which almost inevitably induces
extinction and divergence of character in the many descendants from
one dominant parent-species, explains that great and universal feature
in the affinities of all organic beings, namely, their subordination
in group under group. We use the element of descent in classing the
individuals of both sexes and of all ages, although having few
characters in common, under one species; we use descent in classing
acknowledged varieties, however different they may be from their
parent; and I believe this element of descent is the hidden bond of
connexion which naturalists have sought under the term of the Natural
System. On this idea of the natural system being, in so
far as it has been perfected, genealogical in its arrangement, with
the grades of difference between the descendants from a common parent,
expressed by the terms genera, families, orders, c., we can
understand the rules which we are compelled to follow in our
classification. We can understand why we value certain resemblances
far more than others; why we are permitted to use rudimentary and
useless organs, or others of trifling physiological importance; why,
in comparing one group with a distinct group, we summarily reject
analogical or adaptive characters, and yet use these same characters
within the limits of the same group. We can clearly see how it is that
all living and extinct forms can be grouped together in one great
system; and how the several members of each class are connected
together by the most complex and radiating
lines of affinities. We
shall never, probably, disentangle the inextricable web of affinities
between the members of any one class; but when we have a distinct
object in view, and do not look to some unknown plan of creation, we
may hope to make sure but slow progress.
Morphology.
We have seen that the
members of the same class, independently of their habits of life,
resemble each other in the general plan of their organisation. This
resemblance is often expressed by the term `unity of type;' or by
saying that the several parts and organs in the different species of
the class are homologous. The whole subject is included under the
general name of Morphology. This is the most interesting department
of natural history, and may be said to be its very soul. What can be
more curious than that the hand of a man, formed for grasping, that of
a mole for digging, the leg of the horse, the paddle of the porpoise,
and the wing of the bat, should all be constructed on the same
pattern, and should include the same bones, in the same relative
positions? Geoffroy St Hilaire has insisted strongly on the high
importance of relative connexion in homologous organs: the parts may
change to almost any extent in form and size, and yet they always
remain connected together in the same order. We never find, for
instance, the bones of the arm and forearm, or of the thigh and leg,
transposed. Hence the same names can be given to the
homologous bones in widely different animals. We see the same great
law in the construction of the mouths of insects: what can be more
different than the immensely long spiral proboscis of a sphinx-moth,
the curious folded one of a bee or bug, and the great jaws of a
beetle? — yet all these organs, serving for such different
purposes,
are formed by infinitely numerous modifications of an upper
lip, mandibles, and two pairs of maxillae. Analogous laws govern the
construction of the mouths and limbs of crustaceans. So it is with the
flowers of plants.
Nothing can be more hopeless than to attempt to explain this
similarity of pattern in members of the same class, by utility or by
the doctrine of final causes. The hopelessness of the attempt has been
expressly admitted by Owen in his most interesting work on the `Nature
of Limbs.' On the ordinary view of the independent creation of each
being, we can only say that so it is; — that it has so pleased
the Creator to construct each animal and plant.
The explanation is manifest on the theory of the natural selection
of successive slight modifications, — each modification being
profitable in some way to the modified form, but often affecting by
correlation of growth other parts of the organisation. In changes of
this nature, there will be little or no tendency to modify the
original pattern, or to transpose parts. The bones of a limb might be
shortened and widened to any extent, and become gradually enveloped in
thick membrane, so as to serve as a fin; or a webbed foot might have
all its bones, or certain bones, lengthened to any extent, and the
membrane connecting them increased to any extent, so as to serve as a
wing: yet in all this great amount of modification there will be no
tendency to alter the framework of bones or the relative connexion of
the several parts. If we suppose that the ancient progenitor, the
archetype as it may be called, of all mammals, had its limbs
constructed on the existing general pattern, for whatever purpose they
served, we can at once perceive the plain signification of the
homologous construction of the limbs throughout the whole class. So
with the mouths of insects, we have only to
suppose that their common
progenitor had an upper lip, mandibles, and two pair of
maxillae, these parts being perhaps very simple in form; and then
natural selection will account for the infinite diversity in structure
and function of the mouths of insects. Nevertheless, it is conceivable
that the general pattern of an organ might become so much obscured as
to be finally lost, by the atrophy and ultimately by the complete
abortion of certain parts, by the soldering together of other parts,
and by the doubling or multiplication of others, — variations
which we know to be within the limits of possibility. In the paddles
of the extinct gigantic sea-lizards, and in the mouths of certain
suctorial crustaceans, the general pattern seems to have been thus to
a certain extent obscured.
There is another and equally curious branch of the present subject;
namely, the comparison not of the same part in different members of a
class, but of the different parts or organs in the same individual.
Most physiologists believe that the bones of the skull are homologous
with — that is correspond in number and in relative connexion
with — the elemental parts of a certain number of vertebrae. The
anterior and posterior limbs in each member of the vertebrate and
articulate classes are plainly homologous. We see the same law in
comparing the wonderfully complex jaws and legs in crustaceans. It is
familiar to almost every one, that in a flower the relative position
of the sepals, petals, stamens, and pistils, as well as their intimate
structure, are intelligible in the view that they consist of
metamorphosed leaves, arranged in a spire. In monstrous plants, we
often get direct evidence of the possibility of one organ being
transformed into another; and we can actually see in embryonic
crustaceans and in many other animals, and in flowers, that organs
which when mature
become extremely different, are at an early stage of
growth exactly alike.
How inexplicable are these facts on the ordinary view of creation!
Why should the brain be enclosed in a box composed of such numerous
and such extraordinarily shaped pieces of bone? As Owen has remarked,
the benefit derived from the yielding of the separate pieces in the
act of parturition of mammals, will by no means explain the same
construction in the skulls of birds. Why should similar bones have
been created in the formation of the wing and leg of a
bat, used as they are for such totally different purposes? Why should
one crustacean, which has an extremely complex mouth formed of many
parts, consequently always have fewer legs; or conversely, those with
many legs have simpler mouths? Why should the sepals, petals, stamens,
and pistils in any individual flower, though fitted for such widely
different purposes, be all constructed on the same pattern ?
On the theory of natural selection, we can satisfactorily answer
these questions. In the vertebrata, we see a series of internal
vertebrae bearing certain processes and appendages; in the articulata,
we see the body divided into a series of segments, bearing external
appendages; and in flowering plants, we see a series of successive
spiral whorls of leaves. An indefinite repetition of the same part or
organ is the common characteristic (as Owen has observed) of all low
or little-modified forms; therefore we may readily believe that the
unknown progenitor of the vertebrata possessed many vertebrae; the
unknown progenitor of the articulata, many segments; and the unknown
progenitor of flowering plants, many spiral whorls of leaves. We have
formerly seen that parts many times repeated are eminently liable to
vary in number and structure; consequently it is quite probable that
natural selection, during a long-continued course of modification,
should have seized on a certain number of the primordially similar
elements, many times repeated, and have adapted them to the most
diverse purposes. And as the whole amount of modification will have
been effected by slight successive steps, we need not wonder at
discovering in such parts or organs, a certain degree of fundamental
resemblance, retained by the strong principle of inheritance.
In the great class of molluscs, though we can homologise the parts
of one species with those of another and distinct species, we can
indicate but few serial homologies; that is, we are seldom enabled to
say that one part or organ is homologous with another in the same
individual. And we can understand this fact; for in molluscs, even in
the lowest members of the class, we do not find nearly so much
indefinite repetition of any one part, as we find in the other great
classes of the animal and vegetable kingdoms.
Naturalists frequently speak of the skull as formed of
metamorphosed vertebrae: the jaws of crabs as metamorphosed legs; the
stamens and pistils of flowers as metamorphosed leaves; but it would
in these cases probably be more correct, as Professor Huxley has
remarked, to speak of both skull and vertebrae, both jaws and legs,
c., — as having been metamorphosed, not one from the other,
but from some common element. Naturalists, however, use such language
only in a metaphorical sense: they are far from meaning that during a
long course of descent, primordial organs of any kind --
vertebrae in the one case and legs in the other — have actually
been modified into skulls or jaws. Yet so strong is the appearance of
a modification of this nature having occurred, that naturalists can
hardly avoid employing language having this plain signification. On my
view
these terms may be used literally; and the wonderful fact of the
jaws, for instance, of a crab retaining numerous characters, which
they would probably have retained through inheritance, if they had
really been metamorphosed during a long course of descent from true
legs, or from some simple appendage, is explained.
Embryology.
It has already been
casually remarked that certain organs in the individual, which when
mature become widely different and serve for different purposes, are
in the embryo exactly alike. The embryos, also, of distinct animals
within the same class are often strikingly similar: a better proof of
this cannot be given, than a circumstance mentioned by Agassiz,
namely, that having forgotten to ticket the embryo of some vertebrate
animal, he cannot now tell whether it be that of a mammal, bird, or
reptile. The vermiform larvae of moths, flies, beetles, c.,
resemble each other much more closely than do the mature insects; but
in the case of larvae, the embryos are active, and have been adapted
for special lines of life. A trace of the law of embryonic
resemblance, sometimes lasts till a rather late age: thus birds of the
same genus, and of closely allied genera, often resemble each other in
their first and second plumage; as we see in the spotted feathers in
the thrush group. In the cat tribe, most of the species are striped or
spotted in lines; and stripes can be plainly distinguished in the
whelp of the lion. We occasionally though rarely see something of this
kind in plants: thus the embryonic leaves of the ulex or
furze, and the first leaves of the phyllodineous acaceas, are pinnate
or divided like the ordinary leaves of the leguminosae.
The points of structure, in which the embryos of widely different
animals of the same class resemble each other, often have no direct
relation to their conditions
of existence. We cannot, for instance,
suppose that in the embryos of the vertebrata the peculiar loop-like
course of the arteries near the branchial slits are related to similar
conditions, — in the young mammal which is nourished in the womb
of its mother, in the egg of the bird which is hatched in a nest, and
in the spawn of a frog under water. We have no more reason to believe
in such a relation, than we have to believe that the same bones in the
hand of a man, wing of a bat, and fin of a porpoise, are related to
similar conditions of life. No one will suppose that the stripes on
the whelp of a lion, or the spots on the young blackbird, are of any
use to these animals, or are related to the conditions to which they
are exposed.
The case, however, is different when an animal during any part of
its embryonic career is active, and has to provide for itself. The
period of activity may come on earlier or later in life; but whenever
it comes on, the adaptation of the larva to its conditions of life is
just as perfect and as beautiful as in the adult animal. From such
special adaptations, the similarity of the larvae or active embryos of
allied animals is sometimes much obscured; and cases could be given of
the larvae of two species, or of two groups of species, differing
quite as much, or even more, from each other than do their adult
parents. In most cases, however, the larvae, though active, still obey
more or less closely the law of common embryonic resemblance.
Cirripedes afford a good instance of this: even the illustrious Cuvier
did not perceive that a barnacle was, as it certainly is, a
crustacean; but a glance at the larva shows this to be the case in an
unmistakeable manner. So again the two main divisions of cirripedes,
the pedunculated and sessile, which differ widely in external
appearance, have larvae in all their several stages barely
distinguishable.
The embryo in the course of development generally rises in
organisation: I use this expression, though I am aware that it is hardly possible to define clearly what is meant by the
organisation being higher or lower. But no one probably will dispute
that the butterfly is higher than the caterpillar. In some cases,
however, the mature animal is generally considered as lower in the
scale than the larva, as with certain parasitic crustaceans. To refer
once again to cirripedes: the larvae in the first stage have three
pairs of legs, a very simple single eye, and a probosciformed mouth,
with which they feed largely, for they increase much in size. In the
second stage, answering to the chrysalis stage of butterflies, they
have six pairs of beautifully constructed natatory legs, a pair of
magnificent compound eyes, and extremely complex antennae; but they
have a closed and imperfect mouth, and cannot feed: their function at
this stage is, to search by their well-developed organs of sense, and
to reach by their active powers of swimming, a proper place on which
to become attached and to undergo their final metamorphosis. When this
is completed they are fixed for life: their legs are now converted
into prehensile organs; they again obtain a well-constructed mouth;
but they have no antennae, and their two eyes are now reconverted into
a minute, single, and very simple eye-spot. In this last and complete
state, cirripedes may be considered as either more highly or more
lowly organised than they were in the larval condition. But in some
genera the larvae become developed either into hermaphrodites having
the ordinary structure, or into what I have called complemental males:
and in the latter, the development has assuredly been retrograde; for
the male is a mere sack, which lives for a short time, and is
destitute of mouth, stomach, or other organ of importance, excepting
for reproduction.
We are so much accustomed to see differences in structure between
the embryo and the adult, and likewise a close similarity in the
embryos of widely different animals within the same class, that we
might be led to look at these facts as necessarily contingent in some
manner on growth. But there is no obvious reason why, for instance,
the wing of a bat, or the fin of a porpoise, should not have been
sketched out with all the parts in proper proportion, as soon as any
structure became visible in the embryo. And in some whole groups of
animals and in certain members of other groups, the
embryo does not at any period differ widely from the adult: thus Owen
has remarked in regard to cuttle-fish, `there is no metamorphosis; the
cephalopodic character is manifested long before the parts of the
embryo are completed;' and again in spiders, `there is nothing worthy
to be called a metamorphosis.' The larvae of insects, whether adapted
to the most diverse and active habits, or quite inactive, being fed by
their parents or placed in the midst of proper nutriment, yet nearly
all pass through a similar worm-like stage of development; but in some
few cases, as in that of Aphis, if we look to the admirable drawings
by Professor Huxley of the development of this insect, we see no trace
of the vermiform stage.
How, then, can we explain these several facts in embryology,
-- namely the very general, but not universal difference in
structure between the embryo and the adult; — of parts in the
same individual embryo, which ultimately become very unlike and serve
for diverse purposes, being at this early period of growth alike;
-- of embryos of different species within the same class,
generally, but not universally, resembling each other; — of the
structure of the embryo not being closely related to its conditions of
existence, except when the
embryo becomes at any period of life active
and has to provide for itself; — of the embryo apparently having
sometimes a higher organisation than the mature animal, into which it
is developed. I believe that all these facts can be explained, as
follows, on the view of descent with modification.
It is commonly assumed, perhaps from monstrosities often affecting
the embryo at a very early period, that slight variations necessarily
appear at an equally early period. But we have little evidence on
this head — indeed the evidence rather points the other way; for
it is notorious that breeders of cattle, horses, and various fancy
animals, cannot positively tell, until some time after the animal has
been born, what its merits or form will ultimately turn out. We see
this plainly in our own children; we cannot always tell whether the
child will be tall or short, or what its precise features will be. The
question is not, at what period of life any variation has been caused,
but at what period it is fully displayed. The cause may
have acted, and I believe generally has acted, even before the embryo
is formed; and the variation may be due to the male and female sexual
elements having been affected by the conditions to which either
parent, or their ancestors, have been exposed. Nevertheless an effect
thus caused at a very early period, even before the formation of the
embryo, may appear late in life; as when an hereditary disease, which
appears in old age alone, has been communicated to the offspring from
the reproductive element of one parent. Or again, as when the horns of
cross-bred cattle have been affected by the shape of the horns of
either parent. For the welfare of a very young animal, as long as it
remains in its mother's womb, or in the egg, or as long as it is
nourished and protected by its parent, it must be quite unimportant
whether most of its characters are fully
acquired a little earlier or
later in life. It would not signify, for instance, to a bird which
obtained its food best by having a long beak, whether or not it
assumed a beak of this particular length, as long as it was fed by its
parents. Hence, I conclude, that it is quite possible, that each of
the many successive modifications, by which each species has acquired
its present structure, may have supervened at a not very early period
of life; and some direct evidence from our domestic animals supports
this view. But in other cases it is quite possible that each
successive modification, or most of them, may have appeared at an
extremely early period.
I have stated in the first chapter, that there is some evidence to
render it probable, that at whatever age any variation first appears
in the parent, it tends to reappear at a corresponding age in the
offspring. Certain variations can only appear at corresponding ages,
for instance, peculiarities in the caterpillar, cocoon, or imago
states of the silk-moth; or, again, in the horns of almost full-grown
cattle. But further than this, variations which, for all that we can
see, might have appeared earlier or later in life, tend to appear at a
corresponding age in the offspring and parent. I am far from meaning
that this is invariably the case; and I could give a good many cases
of variations (taking the word in the largest sense) which have
supervened at an earlier age in the child than in the parent.
These two principles, if their truth be admitted, will, I believe,
explain all the above specified leading facts in embryology. But first
let us look at a few analogous cases in domestic varieties. Some
authors who have written on Dogs, maintain that the greyhound and
bulldog, though appearing so different, are really varieties most
closely allied, and have probably descended from
the same wild stock;
hence I was curious to see how far their puppies differed from each
other: I was told by breeders that they differed just as much as their
parents, and this, judging by the eye, seemed almost to be the case;
but on actually measuring the old dogs and their six-days old puppies,
I found that the puppies had not nearly acquired their full amount of
proportional difference. So, again, I was told that the foals of cart
and race-horses differed as much as the full-grown animals; and this
surprised me greatly, as I think it probable that the difference
between these two breeds has been wholly caused by selection under
domestication; but having had careful measurements made of the dam and
of a three-days old colt of a race and heavy cart-horse, I find that
the colts have by no means acquired their full amount of proportional
difference.
As the evidence appears to me conclusive, that the several domestic
breeds of pigeon have descended from one wild species, I compared
young pigeons of various breeds, within twelve hours after being
hatched; I carefully measured the proportions (but will not here give
details) of the beak, width of mouth, length of nostril and of eyelid,
size of feet and length of leg, in the wild stock, in pouters,
fantails, runts, barbs, dragons, carriers, and tumblers. Now some of
these birds, when mature, differ so extraordinarily in length and form
of beak, that they would, I cannot doubt, be ranked in distinct
genera, had they been natural productions. But when the nestling birds
of these several breeds were placed in a row, though most of them
could be distinguished from each other, yet their proportional
differences in the above specified several points were incomparably
less than in the full-grown birds. Some characteristic points of
difference — for instance, that of the width of mouth --
could hardly be detected in the
young. But there was one remarkable
exception to this rule, for the young of the short-faced tumbler differed from the young of the wild rock-pigeon and of the
other breeds, in all its proportions, almost exactly as much as in the
adult state.
The two principles above given seem to me to explain these facts in
regard to the later embryonic stages of our domestic varieties.
Fanciers select their horses, dogs, and pigeons, for breeding, when
they are nearly grown up: they are indifferent whether the desired
qualities and structures have been acquired earlier or later in life,
if the full-grown animal possesses them. And the cases just given,
more especially that of pigeons, seem to show that the characteristic
differences which give value to each breed, and which have been
accumulated by man's selection, have not generally first appeared at
an early period of life, and have been inherited by the offspring at a
corresponding not early period. But the case of the short-faced
tumbler, which when twelve hours old had acquired its proper
proportions, proves that this is not the universal rule; for here the
characteristic differences must either have appeared at an earlier
period than usual, or, if not so, the differences must have been
inherited, not at the corresponding, but at an earlier age.
Now let us apply these facts and the above two principles --
which latter, though not proved true, can be shown to be in some
degree probable — to species in a state of nature. Let us take
a genus of birds, descended on my theory from some one parent-species,
and of which the several new species have become modified through
natural selection in accordance with their diverse habits. Then, from
the many slight successive steps of variation having supervened at a
rather late age, and having been inherited at a corresponding
age, the
young of the new species of our supposed genus will manifestly tend to
resemble each other much more closely than do the adults, just as we
have seen in the case of pigeons. We may extend this view to whole
families or even classes. The fore-limbs, for instance, which served
as legs in the parent-species, may become, by a long course of
modification, adapted in one descendant to act as hands, in another as
paddles, in another as wings; and on the above two principles --
namely of each successive modification supervening at a rather late
age, and being inherited at a corresponding late age
-- the fore-limbs in the embryos of the several descendants of
the parent-species will still resemble each other closely, for they
will not have been modified. But in each individual new species, the
embryonic fore-limbs will differ greatly from the fore-limbs in the
mature animal; the limbs in the latter having undergone much
modification at a rather late period of life, and having thus been
converted into hands, or paddles, or wings. Whatever influence
long-continued exercise or use on the one hand, and disuse on the
other, may have in modifying an organ, such influence will mainly
affect the mature animal, which has come to its full powers of
activity and has to gain its own living; and the effects thus produced
will be inherited at a corresponding mature age. Whereas the young
will remain unmodified, or be modified in a lesser degree, by the
effects of use and disuse.
In certain cases the successive steps of variation might supervene,
from causes of which we are wholly ignorant, at a very early period of
life, or each step might be inherited at an earlier period than that
at which it first appeared. In either case (as with the short-faced
tumbler) the young or embryo would closely
resemble the mature
parent-form. We have seen that this is the rule of development in
certain whole groups of animals, as with cuttle-fish and spiders, and
with a few members of the great class of insects, as with Aphis. With
respect to the final cause of the young in these cases not undergoing
any metamorphosis, or closely resembling their parents from their
earliest age, we can see that this would result from the two following
contingencies; firstly, from the young, during a course of
modification carried on for many generations, having to provide for
their own wants at a very early stage of development, and secondly,
from their following exactly the same habits of life with their
parents; for in this case, it would be indispensable for the existence
of the species, that the child should be modified at a very early age
in the same manner with its parents, in accordance with their similar
habits. Some further explanation, however, of the embryo not
undergoing any metamorphosis is perhaps requisite. If, on the other
hand, it profited the young to follow habits of life in any degree
different from those of their parent, and consequently to be
constructed in a slightly different manner, then, on the
principle of inheritance at corresponding ages, the active young or
larvae might easily be rendered by natural selection different to any
conceivable extent from their parents. Such differences might, also,
become correlated with successive stages of development; so that the
larvae, in the first stage, might differ greatly from the larvae in
the second stage, as we have seen to be the case with cirripedes. The
adult might become fitted for sites or habits, in which organs of
locomotion or of the senses, c., would be useless; and in this
case the final metamorphosis would be said to be retrograde.
As all the organic beings, extinct and recent, which
have ever
lived on this earth have to be classed together, and as all have been
connected by the finest gradations, the best, or indeed, if our
collections were nearly perfect, the only possible arrangement, would
be genealogical. Descent being on my view the hidden bond of connexion
which naturalists have been seeking under the term of the natural
system. On this view we can understand how it is that, in the eyes of
most naturalists, the structure of the embryo is even more important
for classification than that of the adult. For the embryo is the
animal in its less modified state; and in so far it reveals the
structure of its progenitor. In two groups of animal, however much
they may at present differ from each other in structure and habits, if
they pass through the same or similar embryonic stages, we may feel
assured that they have both descended from the same or nearly similar
parents, and are therefore in that degree closely related. Thus,
community in embryonic structure reveals community of descent. It will
reveal this community of descent, however much the structure of the
adult may have been modified and obscured; we have seen, for instance,
that cirripedes can at once be recognised by their larvae as belonging
to the great class of crustaceans. As the embryonic state of each
species and group of species partially shows us the structure of their
less modified ancient progenitors, we can clearly see why ancient and
extinct forms of life should resemble the embryos of their
descendants, — our existing species. Agassiz believes this to be
a law of nature; but I am bound to confess that I only hope to see the
law hereafter proved true. It can be proved true in those cases alone
in which the ancient state, now supposed to be
represented in many embryos, has not been obliterated, either by the
successive variations in a long course of modification having
supervened
at a very early age, or by the variations having been
inherited at an earlier period than that at which they first appeared.
It should also be borne in mind, that the supposed law of resemblance
of ancient forms of life to the embryonic stages of recent forms, may
be true, but yet, owing to the geological record not extending far
enough back in time, may remain for a long period, or for ever,
incapable of demonstration.
Thus, as it seems to me, the leading facts in embryology, which are
second in importance to none in natural history, are explained on the
principle of slight modifications not appearing, in the many
descendants from some one ancient progenitor, at a very early period
in the life of each, though perhaps caused at the earliest, and being
inherited at a corresponding not early period. Embryology rises
greatly in interest, when we thus look at the embryo as a picture,
more or less obscured, of the common parent-form of each great class
of animals.
Rudimentary, atrophied, or aborted organs.
Organs or parts in this strange condition, bearing the stamp
of inutility, are extremely common throughout nature. For instance,
rudimentary mammae are very general in the males of mammals: I presume
that the `bastard-wing' in birds may be safely considered as a digit
in a rudimentary state: in very many snakes one lobe of the lungs is
rudimentary; in other snakes there are rudiments of the pelvis and
hind limbs. Some of the cases of rudimentary organs are extremely
curious; for instance, the presence of teeth in foetal whales, which
when grown up have not a tooth in their heads; and the presence of
teeth, which never cut through the gums, in the upper jaws of our
unborn calves. It has even been stated on good authority that
rudiments of teeth can be detected
in the beaks of certain embryonic
birds. Nothing can be plainer than that wings are formed for flight,
yet in how many insects do we see wings so reduced in size as to be
utterly incapable of flight, and not rarely lying under wing-cases,
firmly soldered together!
The meaning of rudimentary organs is often quite
unmistakeable: for instance there are beetles of the same genus (and
even of the same species) resembling each other most closely in all
respects, one of which will have full-sized wings, and another mere
rudiments of membrane; and here it is impossible to doubt, that the
rudiments represent wings. Rudimentary organs sometimes retain their
potentiality, and are merely not developed: this seems to be the case
with the mammae of male mammals, for many instances are on record of
these organs having become well developed in full-grown males, and
having secreted milk. So again there are normally four developed and
two rudimentary teats in the udders of the genus Bos, but in our
domestic cows the two sometimes become developed and give milk. In
individual plants of the same species the petals sometimes occur as
mere rudiments, and sometimes in a well-developed state. In plants
with separated sexes, the male flowers often have a rudiment of a
pistil; and Klreuter found that by crossing such male plants with an
hermaphrodite species, the rudiment of the pistil in the hybrid
offspring was much increased in size; and this shows that the rudiment
and the perfect pistil are essentially alike in nature.
An organ serving for two purposes, may become rudimentary or
utterly aborted for one, even the more important purpose;, and remain
perfectly efficient for the other. Thus in plants, the office of the
pistil is to allow the pollen-tubes to reach the ovules protected in
the ovarium at its base. The pistil consists of a stigma
supported on
the style; but in some Compositae, the male florets, which of course
cannot be fecundated, have a pistil, which is in a rudimentary state,
for it is not crowned with a stigma; but the style remains well
developed, and is clothed with hairs as in other compositae, for the
purpose of brushing the pollen out of the surrounding anthers. Again,
an organ may become rudimentary for its proper purpose, and be used
for a distinct object: in certain fish the swim-bladder seems to be
rudimentary for its proper function of giving buoyancy, but has become
converted into a nascent breathing organ or lung. Other similar
instances could be given.
Rudimentary organs in the individuals of the same species are very liable to vary in degree of development and in other
respects. Moreover, in closely allied species, the degree to which the
same organ has been rendered rudimentary occasionally differs much.
This latter fact is well exemplified in the state of the wings of the
female moths in certain groups. Rudimentary organs may be utterly
aborted; and this implies, that we find in an animal or plant no trace
of an organ, which analogy would lead us to expect to find, and which
is occasionally found in monstrous individuals of the species. Thus in
the snapdragon (antirrhinum) we generally do not find a rudiment of a
fifth stamen; but this may sometimes be seen. In tracing the
homologies of the same part in different members of a class, nothing
is more common, or more necessary, than the use and discovery of
rudiments. This is well shown in the drawings given by Owen of the
bones of the leg of the horse, ox, and rhinoceros.
It is an important fact that rudimentary organs, such as teeth in
the upper jaws of whales and ruminants, can often be detected in the
embryo, but afterwards wholly disappear. It is also, I believe, a
universal
rule, that a rudimentary part or organ is of greater size
relatively to the adjoining parts in the embryo, than in the adult; so
that the organ at this early age is less rudimentary, or even cannot
be said to be in any degree rudimentary. Hence, also, a rudimentary
organ in the adult, is often said to have retained its embryonic
condition.
I have now given the leading facts with respect to rudimentary
organs. In reflecting on them, every one must be struck with
astonishment: for the same reasoning power which tells us plainly that
most parts and organs are exquisitely adapted for certain purposes,
tells us with equal plainness that these rudimentary or atrophied
organs, are imperfect and useless. In works on natural history
rudimentary organs are generally said to have been created `for the
sake of symmetry,' or in order `to complete the scheme of nature;' but
this seems to me no explanation, merely a restatement of the fact.
Would it be thought sufficient to say that because planets revolve in
elliptic courses round the sun, satellites follow the same course
round the planets, for the sake of symmetry, and to complete the
scheme of nature? An eminent physiologist accounts for the presence of
rudimentary organs, by supposing that they serve to
excrete matter in excess, or injurious to the system; but can we
suppose that the minute papilla, which often represents the pistil in
male flowers, and which is formed merely of cellular tissue, can thus
act? Can we suppose that the formation of rudimentary teeth which are
subsequently absorbed, can be of any service to the rapidly growing
embryonic calf by the excretion of precious phosphate of lime? When a
man's fingers have been amputated, imperfect nails sometimes appear on
the stumps: I could as soon believe that these vestiges of nails have
appeared, not from unknown laws
of growth, but in order to excrete
horny matter, as that the rudimentary nails on the fin of the manatee
were formed for this purpose.
On my view of descent with modification, the origin of rudimentary
organs is simple. We have plenty of cases of rudimentary organs in our
domestic productions, — as the stump of a tail in tailless
breeds, — the vestige of an ear in earless breeds, — the
reappearance of minute dangling horns in hornless breeds of cattle,
more especially, according to Youatt, in young animals, — and
the state of the whole flower in the cauliflower. We often see
rudiments of various parts in monsters. But I doubt whether any of
these cases throw light on the origin of rudimentary organs in a state
of nature, further than by showing that rudiments can be produced; for
I doubt whether species under nature ever undergo abrupt changes. I
believe that disuse has been the main agency; that it has led in
successive generations to the gradual reduction of various organs,
until they have become rudimentary, — as in the case of the eyes
of animals inhabiting dark caverns, and of the wings of birds
inhabiting oceanic islands, which have seldom been forced to take
flight, and have ultimately lost the power of flying. Again, an organ
useful under certain conditions, might become injurious under others,
as with the wings of beetles living on small and exposed islands; and
in this case natural selection would continue slowly to reduce the
organ, until it was rendered harmless and rudimentary.
Any change in function, which can be effected by insensibly small
steps, is within the power of natural selection; so that an organ
rendered, during changed habits of life, useless or injurious for one purpose, might easily be modified and used for
another purpose. Or an organ might be retained for one alone of its
former functions. An organ, when rendered useless, may well be
variable, for its variations cannot be checked by natural selection.
At whatever period of life disuse or selection reduces an organ, and
this will generally be when the being has come to maturity and to its
full powers of action, the principle of inheritance at corresponding
ages will reproduce the organ in its reduced state at the same age,
and consequently will seldom affect or reduce it in the embryo. Thus
we can understand the greater relative size of rudimentary organs in
the embryo, and their lesser relative size in the adult. But if each
step of the process of reduction were to be inherited, not at the
corresponding age, but at an extremely early period of life (as we
have good reason to believe to be possible) the rudimentary part would
tend to be wholly lost, and we should have a case of complete
abortion. The principle, also, of economy, explained in a former
chapter, by which the materials forming any part or structure, if not
useful to the possessor, will be saved as far as is possible, will
probably often come into play; and this will tend to cause the entire
obliteration of a rudimentary organ.
As the presence of rudimentary organs is thus due to the tendency
in every part of the organisation, which has long existed, to be
inherited — we can understand, on the genealogical view of
classification, how it is that systematists have found rudimentary
parts as useful as, or even sometimes more useful than, parts of high
physiological importance. Rudimentary organs may be compared with the
letters in a word, still retained in the spelling, but become useless
in the pronunciation, but which serve as a clue in seeking for its
derivation. On the view of descent with modification, we may conclude
that the existence of organs in a rudimentary, imperfect, and useless
condition, or quite aborted, far
from presenting a strange difficulty,
as they assuredly do on the ordinary doctrine of creation, might even
have been anticipated, and can be accounted for by the laws of
inheritance.
Summary.
In this chapter I have
attempted to show, that the subordination of group to
group in all organisms throughout all time; that the nature of the
relationship, by which all living and extinct beings are united by
complex, radiating, and circuitous lines of affinities into one grand
system; the rules followed and the difficulties encountered by
naturalists in their classifications; the value set upon characters,
if constant and prevalent, whether of high vital importance, or of the
most trifling importance, or, as in rudimentary organs, of no
importance; the wide opposition in value between analogical or
adaptive characters, and characters of true affinity; and other such
rules; — all naturally follow on the view of the common
parentage of those forms which are considered by naturalists as
allied, together with their modification through natural selection,
with its contingencies of extinction and divergence of character. In
considering this view of classification, it should be borne in mind
that the element of descent has been universally used in ranking
together the sexes, ages, and acknowledged varieties of the same
species, however different they may be in structure. If we extend the
use of this element of descent, — the only certainly known cause
of similarity in organic beings, — we shall understand what is
meant by the natural system: it is genealogical in its attempted
arrangement, with the grades of acquired difference marked by the
terms varieties, species, genera, families, orders, and classes.
On this same view of descent with modification, all the great facts
in Morphology become intelligible, —
whether we look to the same
pattern displayed in the homologous organs, to whatever purpose
applied, of the different species of a class; or to the homologous
parts constructed on the same pattern in each individual animal and
plant.
On the principle of successive slight variations, not necessarily
or generally supervening at a very early period of life, and being
inherited at a corresponding period, we can understand the great
leading facts in Embryology; namely, the resemblance in an individual
embryo of the homologous parts, which when matured will become widely
different from each other in structure and function; and the
resemblance in different species of a class of the homologous parts or
organs, though fitted in the adult members for purposes
as different as possible. Larvae are active embryos, which have become
specially modified in relation to their habits of life, through the
principle of modifications being inherited at corresponding ages. On
this same principle — and bearing in mind, that when organs are
reduced in size, either from disuse or selection, it will generally be
at that period of life when the being has to provide for its own
wants, and bearing in mind how strong is the principle of inheritance
-- the occurrence of rudimentary organs and their final abortion,
present to us no inexplicable difficulties; on the contrary, their
presence might have been even anticipated. The importance of
embryological characters and of rudimentary organs in classification
is intelligible, on the view that an arrangement is only so far
natural as it is genealogical.
Finally, the several classes of facts which have been considered in
this chapter, seem to me to proclaim so plainly, that the innumerable
species, genera, and families of organic beings, with which this world
is
peopled, have all descended, each within its own class or group,
from common parents, and have all been modified in the course of
descent, that I should without hesitation adopt this view, even if it
were unsupported by other facts or arguments.
RECAPITULATION AND CONCLUSION
- Recapitulation of the difficulties on the theory
of Natural Selection
- Recapitulation of the general and special
circumstances in its favour
- Causes of the general belief in
the immutability of species
- How far the theory of natural selection may be extended
- Effects of its adoption on the study of Natural history
- Concluding remarks.
As this whole volume is
one long argument, it may be convenient to the reader to have the
leading facts and inferences briefly recapitulated.
That many and grave objections may be advanced against the theory
of descent with modification through natural selection, I do not deny.
I have endeavoured to give to them their full force. Nothing at first
can appear more difficult to believe than that the more complex organs
and instincts should have been perfected not by means superior to,
though analogous with, human reason, but by the accumulation of
innumerable slight variations, each good for the individual possessor.
Nevertheless, this difficulty, though appearing to our imagination
insuperably great, cannot be considered real if we admit the following
propositions, namely, — that gradations in the perfection of any
organ or instinct, which we may consider, either do now exist or could
have existed, each good of its kind, — that all organs and
instincts are, in ever so slight a degree, variable, — and,
lastly, that there is a struggle for existence leading to the
preservation of each profitable deviation of structure or instinct.
The truth of these propositions cannot, I think, be disputed.
It is, no doubt, extremely difficult even to conjecture by what
gradations many structures have been perfected, more especially
amongst broken and failing groups of organic beings; but we see so
many strange gradations in nature, as is proclaimed by the canon,
`Natura non facit saltum,' that we ought to be extremely
cautious in saying that any organ or instinct, or any whole being,
could not have arrived at its present state by many graduated steps.
There are, it must be admitted, cases of special difficulty on the
theory of natural selection; and one of the most curious of these is
the existence of two or three defined castes of workers or sterile
females in the same community of ants but I have attempted to show how
this difficulty can be mastered. With respect to the almost universal
sterility of species when first crossed, which forms so remarkable a
contrast with the almost universal fertility of varieties when
crossed, I must refer the reader to the recapitulation of the facts
given at the end of the eighth chapter, which seem to me conclusively
to show that this sterility is no more a special endowment than is the
incapacity of two trees to be grafted together, but that it is
incidental on constitutional differences in the reproductive systems
of the intercrossed species. We see the truth of this conclusion in
the vast difference in the result, when the same two species are
crossed reciprocally; that is, when one species is first used as the
father and then as the mother.
The fertility of varieties when intercrossed and of their mongrel
offspring cannot be considered as universal; nor is their very general
fertility surprising when we remember that it is not likely that
either their constitutions or their reproductive systems should have
been profoundly modified. Moreover, most of the
varieties which have
been experimentised on have been produced under domestication; and as
domestication apparently tends to eliminate sterility, we ought not to
expect it also to produce sterility.
The sterility of hybrids is a very different case from that of
first crosses, for their reproductive organs are more or less
functionally impotent; whereas in first crosses the organs on both
sides are in a perfect condition. As we continually see that organisms
of all kinds are rendered in some degree sterile from their
constitutions having been disturbed by slightly different and new
conditions of life, we need not feel surprise at hybrids being in some
degree sterile, for their constitutions can hardly fail to have been
disturbed from being compounded of two distinct
organisations. This parallelism is supported by another parallel, but
directly opposite, class of facts; namely, that the vigour and
fertility of all organic beings are increased by slight changes in
their conditions of life, and that the offspring of slightly modified
forms or varieties acquire from being crossed increased vigour and
fertility. So that, on the one hand, considerable changes in the
conditions of life and crosses between greatly modified forms, lessen
fertility; and on the other hand, lesser changes in the conditions of
life and crosses between less modified forms, increase fertility.
Turning to geographical distribution, the difficulties encountered
on the theory of descent with modification are grave enough. All the
individuals of the same species, and all the species of the same
genus, or even higher group, must have descended from common parents;
and therefore, in however distant and isolated parts of the world they
are now found, they must in the course of successive generations have
passed from some one part to the others. We are often wholly unable
even to conjecture how this could have been effected. Yet, as we have
reason to believe that some species have retained the same specific
form for very long periods, enormously long as measured by years, too
much stress ought not to be laid on the occasional wide diffusion of
the same species; for during very long periods of time there will
always be a good chance for wide migration by many means. A broken or
interrupted range may often be accounted for by the extinction of the
species in the intermediate regions. It cannot be denied that we are
as yet very ignorant of the full extent of the various climatal and
geographical changes which have affected the earth during modern
periods; and such changes will obviously have greatly facilitated
migration. As an example, I have attempted to show how potent has been
the influence of the Glacial period on the distribution both of the
same and of representative species throughout the world. We are as yet
profoundly ignorant of the many occasional means of transport. With
respect to distinct species of the same genus inhabiting very distant
and isolated regions, as the process of modification has necessarily
been slow, all the means of migration will have been possible during a
very long period; and consequently the difficulty of the
wide diffusion of species of the same genus is in some degree
lessened.
As on the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species
in each group by gradations as fine as our present varieties, it may
be asked, Why do we not see these linking forms all around us? Why are
not all organic beings blended together in an inextricable chaos? With
respect to existing forms, we should remember that we have no right to
expect (excepting in rare cases) to discover
directly connecting
links
between them, but only between each and some extinct and supplanted
form. Even on a wide area, which has during a long period remained
continuous, and of which the climate and other conditions of life
change insensibly in going from a district occupied by one species
into another district occupied by a closely allied species, we have no
just right to expect often to find intermediate varieties in the
intermediate zone. For we have reason to believe that only a few
species are undergoing change at any one period; and all changes are
slowly effected. I have also shown that the intermediate varieties
which will at first probably exist in the intermediate zones, will be
liable to be supplanted by the allied forms on either hand; and the
latter, from existing in greater numbers, will generally be modified
and improved at a quicker rate than the intermediate varieties, which
exist in lesser numbers; so that the intermediate varieties will, in
the long run, be supplanted and exterminated.
On this doctrine of the extermination of an infinitude of
connecting links, between the living and extinct inhabitants of the
world, and at each successive period between the extinct and still
older species, why is not every geological formation charged with such
links? Why does not every collection of fossil remains afford plain
evidence of the gradation and mutation of the forms of life? We meet
with no such evidence, and this is the most obvious and forcible of
the many objections which may be urged against my theory. Why, again,
do whole groups of allied species appear, though certainly they often
falsely appear, to have come in suddenly on the several geological
stages? Why do we not find great piles of strata beneath the Silurian
system, stored with the remains of the progenitors of the
Silurian groups of fossils? For certainly on my theory such
strata
must somewhere have been deposited at these ancient and utterly
unknown epochs in the world's history.
I can answer these questions and grave objections only on the
supposition that the geological record is far more imperfect than most
geologists believe. It cannot be objected that there has not been time
sufficient for any amount of organic change; for the lapse of time has
been so great as to be utterly inappreciable by the human intellect.
The number of specimens in all our museums is absolutely as nothing
compared with the countless generations of countless species which
certainly have existed. We should not be able to recognise a species
as the parent of any one or more species if we were to examine them
ever so closely, unless we likewise possessed many of the intermediate
links between their past or parent and present states; and these many
links we could hardly ever expect to discover, owing to the
imperfection of the geological record. Numerous existing doubtful
forms could be named which are probably varieties; but who will
pretend that in future ages so many fossil links will be discovered,
that naturalists will be able to decide, on the common view, whether
or not these doubtful forms are varieties? As long as most of the
links between any two species are unknown, if any one link or
intermediate variety be discovered, it will simply be classed as
another and distinct species. Only a small portion of the world has
been geologically explored. Only organic beings of certain classes can
be preserved in a fossil condition, at least in any great number.
Widely ranging species vary most, and varieties are often at first
local, — both causes rendering the discovery of intermediate
links less likely. Local varieties will not spread into other and
distant regions until they are considerably modified and improved;
and
when they do spread, if discovered in a geological formation, they
will appear as if suddenly created there, and will be simply classed
as new species. Most formations have been intermittent in their
accumulation; and their duration, I am inclined to believe, has been
shorter than the average duration of specific forms. Successive
formations are separated from each other by enormous
blank intervals of time; for fossiliferous formations, thick enough to
resist future degradation, can be accumulated only where much sediment
is deposited on the subsiding bed of the sea. During the alternate
periods of elevation and of stationary level the record will be blank.
During these latter periods there will probably be more variability in
the forms of life; during periods of subsidence, more extinction.
With respect to the absence of fossiliferous formations beneath
the lowest Silurian strata, I can only recur to the hypothesis given
in the ninth chapter. That the geological record is imperfect all will
admit; but that it is imperfect to the degree which I require, few
will be inclined to admit. If we look to long enough intervals of
time, geology plainly declares that all species have changed; and they
have changed in the manner which my theory requires, for they have
changed slowly and in a graduated manner. We clearly see this in the
fossil remains from consecutive formations invariably being much more
closely related to each other, than are the fossils from formations
distant from each other in time.
Such is the sum of the several chief objections and difficulties
which may justly be urged against my theory; and I have now briefly
recapitulated the answers and explanations which can be given to them.
I have felt these difficulties far too heavily during many years to
doubt their weight. But it deserves especial notice that the more
important objections relate to questions on which we are confessedly
ignorant; nor do we know how ignorant we are. We do not know all the
possible transitional gradations between the simplest and the most
perfect organs; it cannot be pretended that we know all the varied
means of Distribution during the long lapse of years, or that we know
how imperfect the Geological Record is. Grave as these several
difficulties are, in my judgement they do not overthrow the theory of
descent with modification.
Now let us turn to the other side of the argument. Under
domestication we see much variability. This seems to be mainly due to
the reproductive system being eminently susceptible to changes in the
conditions of life so that this system, when not rendered
impotent, fails to reproduce offspring exactly like the parent-form.
Variability is governed by many complex laws, — by correlation
of growth, by use and disuse, and by the direct action of the physical
conditions of life. There is much difficulty in ascertaining how much
modification our domestic productions have undergone; but we may
safely infer that the amount has been large, and that modifications
can be inherited for long periods. As long as the conditions of life
remain the same, we have reason to believe that a modification, which
has already been inherited for many generations, may continue to be
inherited for an almost infinite number of generations. On the other
hand we have evidence that variability, when it has once come into
play, does not wholly cease; for new varieties are still occasionally
produced by our most anciently domesticated productions.
Man does not actually produce variability; he only
unintentionally
exposes organic beings to new conditions of life, and then nature acts
on the organisation, and causes variability. But man can and does
select the variations given to him by nature, and thus accumulate them
in any desired manner. He thus adapts animals and plants for his own
benefit or pleasure. He may do this methodically, or he may do it
unconsciously by preserving the individuals most useful to him at the
time, without any thought of altering the breed. It is certain that he
can largely influence the character of a breed by selecting, in each
successive generation, individual differences so slight as to be quite
inappreciable by an uneducated eye. This process of selection has been
the great agency in the production of the most distinct and useful
domestic breeds. That many of the breeds produced by man have to a
large extent the character of natural species, is shown by the
inextricable doubts whether very many of them are varieties or
aboriginal species.
There is no obvious reason why the principles which have acted so
efficiently under domestication should not have acted under nature. In
the preservation of favoured individuals and races, during the
constantly-recurrent Struggle for Existence, we see the most powerful
and ever-acting means of selection. The struggle for existence
inevitably follows from the high geometrical ratio of
increase which is common to all organic beings. This high rate of
increase is proved by calculation, by the effects of a succession of
peculiar seasons, and by the results of naturalisation, as explained
in the third chapter. More individuals are born than can possibly
survive. A grain in the balance will determine which individual shall
live and which shall die, — which variety or species shall
increase in number, and which shall decrease, or finally become
extinct. As the individuals
of the same species come in all respects
into the closest competition with each other, the struggle will
generally be most severe between them; it will be almost equally
severe between the varieties of the same species, and next in severity
between the species of the same genus. But the struggle will often be
very severe between beings most remote in the scale of nature. The
slightest advantage in one being, at any age or during any season,
over those with which it comes into competition, or better adaptation
in however slight a degree to the surrounding physical conditions,
will turn the balance.
With animals having separated sexes there will in most cases be a
struggle between the males for possession of the females. The most
vigorous individuals, or those which have most successfully struggled
with their conditions of life, will generally leave most progeny. But
success will often depend on having special weapons or means of
defence, or on the charms of the males; and the slightest advantage
will lead to victory.
As geology plainly proclaims that each land has undergone great
physical changes, we might have expected that organic beings would
have varied under nature, in the same way as they generally have
varied under the changed conditions of domestication. And if there be
any variability under nature, it would be an unaccountable fact if
natural selection had not come into play. It has often been asserted,
but the assertion is quite incapable of proof, that the amount of
variation under nature is a strictly limited quantity. Man, though
acting on external characters alone and often capriciously, can
produce within a short period a great result by adding up mere
individual differences in his domestic productions; and every one
admits that there are at least individual differences in species under
nature. But, besides such differences, all naturalists
have admitted the existence of varieties, which they think
sufficiently distinct to be worthy of record in systematic works. No
one can draw any clear distinction between individual differences and
slight varieties; or between more plainly marked varieties and
subspecies, and species. Let it be observed how naturalists differ in
the rank which they assign to the many representative forms in Europe
and North America.
If then we have under nature variability and a powerful agent
always ready to act and select, why should we doubt that variations in
any way useful to beings, under their excessively complex relations of
life, would be preserved, accumulated, and inherited? Why, if man can
by patience select variations most useful to himself, should nature
fail in selecting variations useful, under changing conditions of
life, to her living products? What limit can be put to this power,
acting during long ages and rigidly scrutinising the whole
constitution, structure, and habits of each creature, --
favouring the good and rejecting the bad? I can see no limit to this
power, in slowly and beautifully adapting each form to the most
complex relations of life. The theory of natural selection, even if we
looked no further than this, seems to me to be in itself probable. I
have already recapitulated, as fairly as I could, the opposed
difficulties and objections: now let us turn to the special facts and
arguments in favour of the theory.
On the view that species are only strongly marked and permanent
varieties, and that each species first existed as a variety, we can
see why it is that no line of demarcation can be drawn between
species, commonly supposed to have been produced by special acts of
creation, and varieties which are acknowledged to have been produced
by secondary laws. On this same view we can understand how it is that
in each region
where many species of a genus have been produced, and
where they now flourish, these same species should present many
varieties; for where the manufactory of species has been active, we
might expect, as a general rule, to find it still in action; and this
is the case if varieties be incipient species. Moreover, the species
of the large genera, which afford the greater number of varieties or
incipient species, retain to a certain degree the
character of varieties; for they differ from each other by a less
amount of difference than do the species of smaller genera. The
closely allied species also of the larger genera apparently have
restricted ranges, and they are clustered in little groups round other
species — in which respects they resemble varieties. These are
strange relations on the view of each species having been
independently created, but are intelligible if all species first
existed as varieties.
As each species tends by its geometrical ratio of reproduction to
increase inordinately in number; and as the modified descendants of
each species will be enabled to increase by so much the more as they
become more diversified in habits and structure, so as to be enabled
to seize on many and widely different places in the economy of nature,
there will be a constant tendency in natural selection to preserve the
most divergent offspring of any one species. Hence during a
long-continued course of modification, the slight differences,
characteristic of varieties of the same species, tend to be augmented
into the greater differences characteristic of species of the same
genus. New and improved varieties will inevitably supplant and
exterminate the older, less improved and intermediate varieties; and
thus species are rendered to a large extent defined and distinct
objects. Dominant species belonging to the larger groups tend to give
birth to new and dominant
forms; so that each large group tends to
become still larger, and at the same time more divergent in character.
But as all groups cannot thus succeed in increasing in size, for the
world would not hold them, the more dominant groups beat the less
dominant. This tendency in the large groups to go on increasing in
size and diverging in character, together with the almost inevitable
contingency of much extinction, explains the arrangement of all the
forms of life, in groups subordinate to groups, all within a few great
classes, which we now see everywhere around us, and which has
prevailed throughout all time. This grand fact of the grouping of all
organic beings seems to me utterly inexplicable on the theory of
creation.
As natural selection acts solely by accumulating slight,
successive, favourable variations, it can produce no great or sudden
modification; it can act only by very short and slow steps. Hence the canon of `Natura non facit saltum,' which every
fresh addition to our knowledge tends to make more strictly correct,
is on this theory simply intelligible. We can plainly see why nature
is prodigal in variety, though niggard in innovation. But why this
should be a law of nature if each species has been independently
created, no man can explain.
Many other facts are, as it seems to me, explicable on this theory.
How strange it is that a bird, under the form of woodpecker, should
have been created to prey on insects on the ground; that upland geese,
which never or rarely swim, should have been created with webbed feet;
that a thrush should have been created to dive and feed on sub-aquatic
insects; and that a petrel should have been created with habits and
structure fitting it for the life of an auk or grebe! and so on in
endless other cases. But on the view of each
species constantly trying
to increase in number, with natural selection always ready to adapt
the slowly varying descendants of each to any unoccupied or
ill-occupied place in nature, these facts cease to be strange, or
perhaps might even have been anticipated.
As natural selection acts by competition, it adapts the inhabitants
of each country only in relation to the degree of perfection of their
associates; so that we need feel no surprise at the inhabitants of any
one country, although on the ordinary view supposed to have been
specially created and adapted for that country, being beaten and
supplanted by the naturalised productions from another land. Nor ought
we to marvel if all the contrivances in nature be not, as far as we
can judge, absolutely perfect; and if some of them be abhorrent to our
ideas of fitness. We need not marvel at the sting of the bee causing
the bee's own death; at drones being produced in such vast numbers for
one single act, and being then slaughtered by their sterile sisters;
at the astonishing waste of pollen by our fir-trees; at the
instinctive hatred of the queen bee for her own fertile daughters; at
ichneumonidae feeding within the live bodies of caterpillars; and at
other such cases. The wonder indeed is, on the theory of natural
selection, that more cases of the want of absolute perfection have not
been observed.
The complex and little known laws governing variation are the same, as far as we can see, with the laws which have
governed the production of so-called specific forms. In both cases
physical conditions seem to have produced but little direct effect;
yet when varieties enter any zone, they occasionally assume some of
the characters of the species proper to that zone. In both varieties
and species, use and disuse seem to have produced some effect; for it
is difficult to resist this conclusion
when we look, for instance, at
the logger-headed duck, which has wings incapable of flight, in nearly
the same condition as in the domestic duck; or when we look at the
burrowing tucutucu, which is occasionally blind, and then at certain
moles, which are habitually blind and have their eyes covered with
skin; or when we look at the blind animals inhabiting the dark caves
of America and Europe. In both varieties and species correction of
growth seems to have played a most important part, so that when one
part has been modified other parts are necessarily modified. In both
varieties and species reversions to long-lost characters occur. How
inexplicable on the theory of creation is the occasional appearance of
stripes on the shoulder and legs of the several species of the
horse-genus and in their hybrids! How simply is this fact explained
if we believe that these species have descended from a striped
progenitor, in the same manner as the several domestic breeds of
pigeon have descended from the blue and barred rock-pigeon!
On the ordinary view of each species having been independently
created, why should the specific characters, or those by which the
species of the same genus differ from each other, be more variable
than the generic characters in which they all agree? Why, for
instance, should the colour of a flower be more likely to vary in any
one species of a genus, if the other species, supposed to have been
created independently, have differently coloured flowers, than if all
the species of the genus have the same coloured flowers? If species
are only well-marked varieties, of which the characters have become in
a high degree permanent, we can understand this fact; for they have
already varied since they branched off from a common progenitor in
certain characters, by which they have come to be specifically
distinct from each other;
and therefore these same characters would be
more likely still to be variable than the generic
characters which have been inherited without change for an enormous
period. It is inexplicable on the theory of creation why a part
developed in a very unusual manner in any one species of a genus, and
therefore, as we may naturally infer, of great importance to the
species, should be eminently liable to variation; but, on my view,
this part has undergone, since the several species branched off from a
common progenitor, an unusual amount of variability and modification,
and therefore we might expect this part generally to be still
variable. But a part may be developed in the most unusual manner, like
the wing of a bat, and yet not be more variable than any other
structure, if the part be common to many subordinate forms, that is,
if it has been inherited for a very long period; for in this case it
will have been rendered constant by long-continued natural selection.
Glancing at instincts, marvellous as some are, they offer no
greater difficulty than does corporeal structure on the theory of the
natural selection of successive, slight, but profitable modifications.
We can thus understand why nature moves by graduated steps in endowing
different animals of the same class with their several instincts. I
have attempted to show how much light the principle of gradation
throws on the admirable architectural powers of the hive-bee. Habit
no doubt sometimes comes into play in modifying instincts; but it
certainly is not indispensable, as we see, in the case of neuter
insects, which leave no progeny to inherit the effects of
long-continued habit. On the view of all the species of the same genus
having descended from a common parent, and having inherited much in
common, we can understand how it is that allied species, when placed
under considerably different conditions of life,
yet should follow
nearly the same instincts; why the thrush of South America, for
instance, lines her nest with mud like our British species. On the
view of instincts having been slowly acquired through natural
selection we need not marvel at some instincts being apparently not
perfect and liable to mistakes, and at many instincts causing other
animals to suffer.
If species be only well-marked and permanent varieties, we can at
once see why their crossed offspring should follow the
same complex laws in their degrees and kinds of resemblance to their
parents, — in being absorbed into each other by successive
crosses, and in other such points, — as do the crossed offspring
of acknowledged varieties. On the other hand, these would be strange
facts if species have been independently created, and varieties have
been produced by secondary laws.
If we admit that the geological record is imperfect in an extreme
degree, then such facts as the record gives, support the theory of
descent with modification. New species have come on the stage slowly
and at successive intervals; and the amount of change, after equal
intervals of time, is widely different in different groups. The
extinction of species and of whole groups of species, which has played
so conspicuous a part in the history of the organic world, almost
inevitably follows on the principle of natural selection; for old
forms will be supplanted by new and improved forms. Neither single
species nor groups of species reappear when the chain of ordinary
generation has once been broken. The gradual diffusion of dominant
forms, with the slow modification of their descendants, causes the
forms of life, after long intervals of time, to appear as if they had
changed simultaneously throughout the world. The fact of the fossil
remains of each formation being in some degree intermediate in
character between the
fossils in the formations above and below, is
simply explained by their intermediate position in the chain of
descent. The grand fact that all extinct organic beings belong to the
same system with recent beings, falling either into the same or into
intermediate groups, follows from the living and the extinct being the
offspring of common parents. As the groups which have descended from
an ancient progenitor have generally diverged in character, the
progenitor with its early descendants will often be intermediate in
character in comparison with its later descendants; and thus we can
see why the more ancient a fossil is, the oftener it stands in some
degree intermediate between existing and allied groups. Recent forms
are generally looked at as being, in some vague sense, higher than
ancient and extinct forms; and they are in so far higher as the later
and more improved forms have conquered the older and less improved
organic beings in the struggle for life. Lastly, the law of the long endurance of allied forms on the same continent, --
of marsupials in Australia, of edentata in America, and other such
cases, — is intelligible, for within a confined country, the
recent and the extinct will naturally be allied by descent.
Looking to geographical distribution, if we admit that there has
been during the long course of ages much migration from one part of
the world to another, owing to former climatal and geographical
changes and to the many occasional and unknown means of dispersal,
then we can understand, on the theory of descent with modification,
most of the great leading facts in Distribution. We can see why there
should be so striking a parallelism in the distribution of organic
beings throughout space, and in their geological succession throughout
time; for in both cases the beings have been connected by the bond of
ordinary generation, and the means of
modification have been the same.
We see the full meaning of the wonderful fact, which must have struck
every traveller, namely, that on the same continent, under the most
diverse conditions, under heat and cold, on mountain and lowland, on
deserts and marshes, most of the inhabitants within each great class
are plainly related; for they will generally be descendants of the
same progenitors and early colonists. On this same principle of former
migration, combined in most cases with modification, we can
understand, by the aid of the Glacial period, the identity of some few
plants, and the close alliance of many others, on the most distant
mountains, under the most different climates; and likewise the close
alliance of some of the inhabitants of the sea in the northern and
southern temperate zones, though separated by the whole intertropical
ocean. Although two areas may present the same physical conditions of
life, we need feel no surprise at their inhabitants being widely
different, if they have been for a long period completely separated
from each other; for as the relation of organism to organism is the
most important of all relations, and as the two areas will have
received colonists from some third source or from each other, at
various periods and in different proportions, the course of
modification in the two areas will inevitably be different.
On this view of migration, with subsequent modification, we can see why oceanic islands should be inhabited by few
species, but of these, that many should be peculiar. We can clearly
see why those animals which cannot cross wide spaces of ocean, as
frogs and terrestrial mammals, should not inhabit oceanic islands; and
why, on the other hand, new and peculiar species of bats, which can
traverse the ocean, should so often be found on islands far distant
from any continent. Such facts
as the presence of peculiar species of
bats, and the absence of all other mammals, on oceanic islands, are
utterly inexplicable on the theory of independent acts of creation.
The existence of closely allied or representative species in any
two areas, implies, on the theory of descent with modification, that
the same parents formerly inhabited both areas; and we almost
invariably find that wherever many closely allied species inhabit two
areas, some identical species common to both still exist. Wherever
many closely allied yet distinct species occur, many doubtful forms
and varieties of the same species likewise occur. It is a rule of high
generality that the inhabitants of each area are related to the
inhabitants of the nearest source whence immigrants might have been
derived. We see this in nearly all the plants and animals of the
Galapagos archipelago, of Juan Fernandez, and of the other American
islands being related in the most striking manner to the plants and
animals of the neighbouring American mainland; and those of the Cape
de Verde archipelago and other African islands to the African
mainland. It must be admitted that these facts receive no explanation
on the theory of creation.
The fact, as we have seen, that all past and present organic beings
constitute one grand natural system, with group subordinate to group,
and with extinct groups often falling in between recent groups, is
intelligible on the theory of natural selection with its contingencies
of extinction and divergence of character. On these same principles we
see how it is, that the mutual affinities of the species and genera
within each class are so complex and circuitous. We see why certain
characters are far more serviceable than others for classification;
-- why adaptive characters, though of paramount importance to the
being, are of hardly any
importance in classification; why characters
derived from rudimentary parts, though of no service to
the being, are often of high classificatory value; and why
embryological characters are the most valuable of all. The real
affinities of all organic beings are due to inheritance or community
of descent. The natural system is a genealogical arrangement, in which
we have to discover the lines of descent by the most permanent
characters, however slight their vital importance may be.
The framework of bones being the same in the hand of a man, wing of
a bat, fin of the porpoise, and leg of the horse, — the same
number of vertebrae forming the neck of the giraffe and of the
elephant, — and innumerable other such facts, at once explain
themselves on the theory of descent with slow and slight successive
modifications. The similarity of pattern in the wing and leg of a bat,
though used for such different purposes, — in the jaws and legs
of a crab, — in the petals, stamens, and pistils of a flower, is
likewise intelligible on the view of the gradual modification of parts
or organs, which were alike in the early progenitor of each class. On
the principle of successive variations not always supervening at an
early age, and being inherited at a corresponding not early period of
life, we can clearly see why the embryos of mammals, birds, reptiles,
and fishes should be so closely alike, and should be so unlike the
adult forms. We may cease marvelling at the embryo of an air-breathing
mammal or bird having branchial slits and arteries running in loops,
like those in a fish which has to breathe the air dissolved in water,
by the aid of well-developed branchiae.
Disuse, aided sometimes by natural selection, will often tend to
reduce an organ, when it has become useless by changed habits or under
changed conditions
of life; and we can clearly understand on this view
the meaning of rudimentary organs. But disuse and selection will
generally act on each creature, when it has come to maturity and has
to play its full part in the struggle for existence, and will thus
have little power of acting on an organ during early life; hence the
organ will not be much reduced or rendered rudimentary at this early
age. The calf, for instance, has inherited teeth, which never cut
through the gums of the upper jaw, from an early progenitor having
well-developed teeth; and we may believe, that the teeth in the mature
animal were reduced, during successive generations, by
disuse or by the tongue and palate having been fitted by natural
selection to browse without their aid; whereas in the calf, the teeth
have been left untouched by selection or disuse, and on the principle
of inheritance at corresponding ages have been inherited from a remote
period to the present day. On the view of each organic being and each
separate organ having been specially created, how utterly inexplicable
it is that parts, like the teeth in the embryonic calf or like the
shrivelled wings under the soldered wing-covers of some beetles,
should thus so frequently bear the plain stamp of inutility! Nature
may be said to have taken pains to reveal, by rudimentary organs and
by homologous structures, her scheme of modification, which it seems
that we wilfully will not understand.
I have now recapitulated the chief facts and considerations which
have thoroughly convinced me that species have changed, and are still
slowly changing by the preservation and accumulation of successive
slight favourable variations. Why, it may be asked, have all the most
eminent living naturalists and geologists rejected this view of the
mutability of species? It cannot be
asserted that organic beings in a
state of nature are subject to no variation; it cannot be proved that
the amount of variation in the course of long ages is a limited
quantity; no clear distinction has been, or can be, drawn between
species and well-marked varieties. It cannot be maintained that
species when intercrossed are invariably sterile, and varieties
invariably fertile; or that sterility is a special endowment and sign
of creation. The belief that species were immutable productions was
almost unavoidable as long as the history of the world was thought to
be of short duration; and now that we have acquired some idea of the
lapse of time, we are too apt to assume, without proof, that the
geological record is so perfect that it would have afforded us plain
evidence of the mutation of species, if they had undergone mutation.
But the chief cause of our natural unwillingness to admit that one
species has given birth to other and distinct species, is that we are
always slow in admitting any great change of which we do
not see the intermediate steps. The difficulty is the same as that
felt by so many geologists, when Lyell first insisted that long lines
of inland cliffs had been formed, and great valleys excavated, by the
slow action of the coast-waves. The mind cannot possibly grasp the
full meaning of the term of a hundred million years; it cannot add up
and perceive the full effects of many slight variations, accumulated
during an almost infinite number of generations.
Although I am fully convinced of the truth of the views given in
this volume under the form of an abstract, I by no means expect to
convince experienced naturalists whose minds are stocked with a
multitude of facts all viewed, during a long course of years, from a
point of view directly opposite to mine. It is so easy
to hide our
ignorance under such expressions as the `plan of creation,' `unity of
design,' c., and to think that we give an explanation when we
only restate a fact. Any one whose disposition leads him to attach
more weight to unexplained difficulties than to the explanation of a
certain number of facts will certainly reject my theory. A few
naturalists, endowed with much flexibility of mind, and who have
already begun to doubt on the immutability of species, may be
influenced by this volume; but I look with confidence to the future,
to young and rising naturalists, who will be able to view both sides
of the question with impartiality. Whoever is led to believe that
species are mutable will do good service by conscientiously expressing
his conviction; for only thus can the load of prejudice by which this
subject is overwhelmed be removed.
Several eminent naturalists have of late published their belief
that a multitude of reputed species in each genus are not real
species; but that other species are real, that is, have been
independently created. This seems to me a strange conclusion to arrive
at. They admit that a multitude of forms, which till lately they
themselves thought were special creations, and which are still thus
looked at by the majority of naturalists, and which consequently have
every external characteristic feature of true species, — they
admit that these have been produced by variation, but they refuse to
extend the same view to other and very slightly different forms.
Nevertheless they do not pretend that they can define, or
even conjecture, which are the created forms of life, and which are
those produced by secondary laws. They admit variation as a vera causa in one case, they arbitrarily reject
it in another, without assigning any distinction in the two cases. The
day will come when this will be given as a curious illustration of
the
blindness of preconceived opinion. These authors seem no more startled
at a miraculous act of creation than at an ordinary birth. But do they
really believe that at innumerable periods in the earth's history
certain elemental atoms have been commanded suddenly to flash into
living tissues? Do they believe that at each supposed act of creation
one individual or many were produced? Were all the infinitely numerous
kinds of animals and plants created as eggs or seed, or as full grown?
and in the case of mammals, were they created bearing the false marks
of nourishment from the mother's womb? Although naturalists very
properly demand a full explanation of every difficulty from those who
believe in the mutability of species, on their own side they ignore
the whole subject of the first appearance of species in what they
consider reverent silence.
It may be asked how far I extend the doctrine of the modification
of species. The question is difficult to answer, because the more
distinct the forms are which we may consider, by so much the arguments
fall away in force. But some arguments of the greatest weight extend
very far. All the members of whole classes can be connected together
by chains of affinities, and all can be classified on the same
principle, in groups subordinate to groups. Fossil remains sometimes
tend to fill up very wide intervals between existing orders. Organs
in a rudimentary condition plainly show that an early progenitor had
the organ in a fully developed state; and this in some instances
necessarily implies an enormous amount of modification in the
descendants. Throughout whole classes various structures are formed on
the same pattern, and at an embryonic age the species closely resemble
each other. Therefore I cannot doubt that the theory of descent with
modification
embraces all the members of the same class. I believe
that animals have descended from at most only four or five
progenitors, and plants from an equal or lesser number.
Analogy would lead me one step further, namely, to the
belief that all animals and plants have descended from some one
prototype. But analogy may be a deceitful guide. Nevertheless all
living things have much in common, in their chemical composition,
their germinal vesicles, their cellular structure, and their laws of
growth and reproduction. We see this even in so trifling a
circumstance as that the same poison often similarly affects plants
and animals; or that the poison secreted by the gall-fly produces
monstrous growths on the wild rose or oak-tree. Therefore I should
infer from analogy that probably all the organic beings which have
ever lived on this earth have descended from some one primordial form,
into which life was first breathed.
When the views entertained in this volume on the origin of species,
or when analogous views are generally admitted, we can dimly foresee
that there will be a considerable revolution in natural history.
Systematists will be able to pursue their labours as at present; but
they will not be incessantly haunted by the shadowy doubt whether this
or that form be in essence a species. This I feel sure, and I speak
after experience, will be no slight relief. The endless disputes
whether or not some fifty species of British brambles are true species
will cease. Systematists will have only to decide (not that this will
be easy) whether any form be sufficiently constant and distinct from
other forms, to be capable of definition; and if definable, whether
the differences be sufficiently important to deserve a specific name.
This latter point will become a far more essential consideration
than
it is at present; for differences, however slight, between any two
forms, if not blended by intermediate gradations, are looked at by
most naturalists as sufficient to raise both forms to the rank of
species. Hereafter we shall be compelled to acknowledge that the only
distinction between species and well-marked varieties is, that the
latter are known, or believed, to be connected at the present day by
intermediate gradations, whereas species were formerly thus connected.
Hence, without quite rejecting the consideration of the present
existence of intermediate gradations between any two forms, we shall
be led to weigh more carefully and to value higher the
actual amount of difference between them. It is quite possible that
forms now generally acknowledged to be merely varieties may hereafter
be thought worthy of specific names, as with the primrose and cowslip;
and in this case scientific and common language will come into
accordance. In short, we shall have to treat species in the same
manner as those naturalists treat genera, who admit that genera are
merely artificial combinations made for convenience. This may not be a
cheering prospect; but we shall at least be freed from the vain search
for the undiscovered and undiscoverable essence of the term species.
The other and more general departments of natural history will rise
greatly in interest. The terms used by naturalists of affinity,
relationship, community of type, paternity, morphology, adaptive
characters, rudimentary and aborted organs, c., will cease to be
metaphorical, and will have a plain signification. When we no longer
look at an organic being as a savage looks at a ship, as at something
wholly beyond his comprehension; when we regard every production of
nature as one which has had a history; when we contemplate every
complex structure
and instinct as the summing up of many contrivances,
each useful to the possessor, nearly in the same way as when we look
at any great mechanical invention as the summing up of the labour, the
experience, the reason, and even the blunders of numerous workmen;
when we thus view each organic being, how far more interesting, I
speak from experience, will the study of natural history become!
A grand and almost untrodden field of inquiry will be opened, on
the causes and laws of variation, on correlation of growth, on the
effects of use and disuse, on the direct action of external
conditions, and so forth. The study of domestic productions will rise
immensely in value. A new variety raised by man will be a far more
important and interesting subject for study than one more species
added to the infinitude of already recorded species. Our
classifications will come to be, as far as they can be so made,
genealogies; and will then truly give what may be called the plan of
creation. The rules for classifying will no doubt become simpler when
we have a definite object in view. We possess no
pedigrees or armorial bearings; and we have to discover and trace the
many diverging lines of descent in our natural genealogies, by
characters of any kind which have long been inherited. Rudimentary
organs will speak infallibly with respect to the nature of long-lost
structures. Species and groups of species, which are called aberrant,
and which may fancifully be called living fossils, will aid us in
forming a picture of the ancient forms of life. Embryology will reveal
to us the structure, in some degree obscured, of the prototypes of
each great class.
When we can feel assured that all the individuals of the same
species, and all the closely allied species of most genera, have
within a not very remote period descended
from one parent, and have
migrated from some one birthplace; and when we better know the many
means of migration, then, by the light which geology now throws, and
will continue to throw, on former changes of climate and of the level
of the land, we shall surely be enabled to trace in an admirable
manner the former migrations of the inhabitants of the whole world.
Even at present, by comparing the differences of the inhabitants of
the sea on the opposite sides of a continent, and the nature of the
various inhabitants of that continent in relation to their apparent
means of immigration, some light can be thrown on ancient geography.
The noble science of Geology loses glory from the extreme
imperfection of the record. The crust of the earth with its embedded
remains must not be looked at as a well-filled museum, but as a poor
collection made at hazard and at rare intervals. The accumulation of
each great fossiliferous formation will be recognised as having
depended on an unusual concurrence of circumstances, and the blank
intervals between the successive stages as having been of vast
duration. But we shall be able to gauge with some security the
duration of these intervals by a comparison of the preceding and
succeeding organic forms. We must be cautious in attempting to
correlate as strictly contemporaneous two formations, which include
few identical species, by the general succession of their forms of
life. As species are produced and exterminated by slowly acting and
still existing causes, and not by miraculous acts of creation and by
catastrophes; and as the most important of all causes of organic change is one which is almost independent of altered and
perhaps suddenly altered physical conditions, namely, the mutual
relation of organism to organism, — the improvement of one being
entailing the improvement or the extermination of
others; it follows,
that the amount of organic change in the fossils of consecutive
formations probably serves as a fair measure of the lapse of actual
time. A number of species, however, keeping in a body might remain for
a long period unchanged, whilst within this same period, several of
these species, by migrating into new countries and coming into
competition with foreign associates, might become modified; so that we
must not overrate the accuracy of organic change as a measure of time.
During early periods of the earth's history, when the forms of life
were probably fewer and simpler, the rate of change was probably
slower; and at the first dawn of life, when very few forms of the
simplest structure existed, the rate of change may have been slow in
an extreme degree. The whole history of the world, as at present
known, although of a length quite incomprehensible by us, will
hereafter be recognised as a mere fragment of time, compared with the
ages which have elapsed since the first creature, the progenitor of
innumerable extinct and living descendants, was created.
In the distant future I see open fields for far more important
researches. Psychology will be based on a new foundation, that of the
necessary acquirement of each mental power and capacity by gradation.
Light will be thrown on the origin of man and his history.
Authors of the highest eminence seem to be fully satisfied with the
view that each species has been independently created. To my mind it
accords better with what we know of the laws impressed on matter by
the Creator, that the production and extinction of the past and
present inhabitants of the world should have been due to secondary
causes, like those determining the birth and death of the individual.
When I view all beings not as special creations, but as the lineal
descendants of some few beings which lived long before the
first bed
of the Silurian system was deposited, they seem to me to become
ennobled. Judging from the past, we may safely infer that
not one living species will transmit its unaltered likeness to a
distant futurity. And of the species now living very few will transmit
progeny of any kind to a far distant futurity; for the manner in which
all organic beings are grouped, shows that the greater number of
species of each genus, and all the species of many genera, have left
no descendants, but have become utterly extinct. We can so far take a
prophetic glance into futurity as to foretel that it will be the
common and widely-spread species, belonging to the larger and dominant
groups, which will ultimately prevail and procreate new and dominant
species. As all the living forms of life are the lineal descendants of
those which lived long before the Silurian epoch, we may feel certain
that the ordinary succession by generation has never once been broken,
and that no cataclysm has desolated the whole world. Hence we may look
with some confidence to a secure future of equally inappreciable
length. And as natural selection works solely by and for the good of
each being, all corporeal and mental endowments will tend to progress
towards perfection.
It is interesting to contemplate an entangled bank, clothed with
many plants of many kinds, with birds singing on the bushes, with
various insects flitting about, and with worms crawling through the
damp earth, and to reflect that these elaborately constructed forms,
so different from each other, and dependent on each other in so
complex a manner, have all been produced by laws acting around us.
These laws, taken in the largest sense, being Growth with
Reproduction; inheritance which is almost implied by reproduction;
Variability from the indirect and direct action of the external
conditions
of life, and from use and disuse; a Ratio of Increase so
high as to lead to a Struggle for Life, and as a consequence to
Natural Selection, entailing Divergence of Character and the
Extinction of less-improved forms. Thus, from the war of nature, from
famine and death, the most exalted object which we are capable of
conceiving, namely, the production of the higher animals, directly
follows. There is grandeur in this view of life, with its several
powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on
according to the fixed law of gravity, from so simple a beginning
endless forms most beautiful and most wonderful have been, and are
being, evolved.
GLOSSARY*
*I am indebted to the kindness of Mr. W. S. Dallas for this
Glossary, which has been given because several readers have complained
to me that some of the terms used were unintelligible to them. Mr.
Dallas has endeavoured to give the explanations of the terms in as
popular a form as possible.
EDITOR'S NOTE: This
glossary did not appear in the first edition and has been reproduced
here directly from the sixth edition
ABERRANT.----Forms or groups of animals or plants which
deviate in important characters from their nearest allies, so as not
to be easily included in the same group with them, are said to be
aberrant.
ABERRATION (in Optics).----In the refraction of light by
a convex lens the rays passing through different parts of the lens are
brought to a focus at slightly different distances,----this
is called spherical aberration; at the same
time the coloured rays are separated by the prismatic action of the
lens and likewise brought to a focus at different
distances,----this is chromatic
aberration.
ABNORMAL.----Contrary to the general rule.
ABORTED.----An organ is said to be aborted, when its
development has been arrested at a very early stage.
ALBINISM.----Albinos are animals in which the usual
colouring matters characteristic of the species have not been produced
in the skin and its appendages. Albinism is the state of being an
albino.
ALGAE.----A class of plants including the ordinary
sea-weeds and the filamentous fresh-water weeds.
ALTERNATION OF GENERATIONS.----This term is applied to a
peculiar mode of reproduction which prevails among many of the lower
animals, in which the egg produces a living form quite different from
its parent, but from which the parent-form is reproduced by a process
of budding, or by the division of the substance of the first product
of the egg.
AMMONITES.----A group of fossil, spiral, chambered
shells, allied to the existing pearly Nautilus, but having the
partitions between the chambers waved in complicated patterns at their
junction with the outer wall of the shell.
ANALOGY.----That resemblance of structures which depends
upon similarity of function, as in the wings of insects and birds.
Such structures are said to be analogous,
and to be analogues of each other.
ANIMALCULE.----A minute animal: generally applied to
those visible only by the microscope.
ANNELIDS.----A class of worms in which the surface of the
body exhibits a more or less distinct division into rings or segments,
generally provided with appendages for locomotion and with gills. It
includes the ordinary marine worms, the earthworms, and the leeches.
ANTENN.----Jointed organs appended to the head in
Insects, Crustacea and Centipedes, and not belonging to the mouth.
ANTHERS.----The summits of the stamens of flowers, in
which the pollen or fertilising dust is produced.
APLACENTALIA, APLACENTATA or Aplacental Mammals. See Mammalia.
ARCHETYPAL.----Of or belonging to the Archetype, or ideal
primitive form upon which all the beings of a group seem to be
organised.
ARTICULATA.----A great division of the Animal Kingdom
characterised generally by having the surface of the body divided into
rings called segments, a greater or less number of which are furnished
with jointed legs (such as Insects, Crustaceans and Centipedes).
ASYMMETRICAL.----Having the two sides unlike.
ATROPHIED.----Arrested in development at a very early stage.
BALANUS.----The genus including the common Acorn-shells
which live in abundance on the rocks of the sea-coast.
BATRACHIANS.----A class of animals allied to the Reptiles,
but undergoing a peculiar metamorphosis, in which the young animal is
generally aquatic and breathes by gills.
(Examples, Frogs, Toads, and Newts.)
BOULDERS.---- Large transported blocks of stone generally
imbedded in clays or gravels.
BRACHIOPODA.----A class of marine Mollusca, or
soft-bodied animals, furnished with a bivalve shell, attached to
submarine objects by a stalk which passes through an aperture in one
of the valves, and furnished with fringed arms, by the action of which
food is carried to the mouth.
BRANCHI.----Gills or organs for respiration in
water.
BRANCHIAL.----Pertaining to gills or branchi.
CAMBRIAN SYSTEM.----A Series of very ancient
Palozoic rocks, between the Laurentian and the Silurian. Until
recently these were regarded as the oldest fossiliferous rocks.
CANID.----The Dog-family, including the Dog, Wolf,
Fox, Jackal, c.
CARAPACE.----The shell enveloping the anterior part of
the body in Crustaceans generally; applied also to the hard shelly
pieces of the Cirripedes.
CARBONIFEROUS.----This term is applied to the great
formation which includes, among other rocks, the coal-measures. It
belongs to the oldest, or Palozoic, system of formations.
CAUDAL.----Of or belonging to the tail.
CEPHALOPODS.----The highest class of the Mollusca, or
Soft-bodied animals, characterised by having the mouth surrounded by a
greater or less number of fleshy arms or tentacles, which, in most
living species, are furnished with sucking-cups. (Examples, Cuttle-fish, Nautilus.)
CETACEA.----An order of Mammalia, including the Whales,
Dolphins, c., having the form of the body fish-like, the skin
naked, and only the fore-limbs developed.
CHELONIA.----An order of Reptiles including the Turtles,
Tortoises, c.
CIRRIPEDES.----An order of Crustaceans including the
Barnacles and Acorn-shells. Their young resemble those of many other
Crustaceans in form; but when mature they are always attached to other
objects, either directly or by means of a stalk, and their bodies are
enclosed by a calcareous shell composed of several pieces, two of
which can open to give issue to a bunch of curled, jointed tentacles,
which represent the limbs.
COCCUS.----The genus of Insects including the Cochineal.
In these the male is a minute, winged fly, and the female generally a
motionless, berry-like mass.
COCOON.----A case usually of
silky material, in which insects are frequently enveloped during the
second or resting-stage (pupa) of their existence. The term
`cocoon-stage' is here used as equivalent to `pupa-stage.'
CLOSPERMOUS.----A term applied to those fruits of
the Umbellifer which have the seed hollowed on the inner face.
COLEOPTERA.----Beetles, an order of Insects, having a
biting mouth and the first pair of wings more or less horny, forming
sheaths for the second pair, and usually meeting in a straight line
down the middle of the back.
COLUMN.----A peculiar organ in the flowers of Orchids, in
which the stamens, style and stigma (or the reproductive parts) are
united.
COMPOSIT or COMPOSITOUS PLANTS.---- Plants in
which the inflorescence consists of numerous small flowers (florets)
brought together into a dense head, the base of which is enclosed by a
common envelope. (Examples, the Daisy,
Dandelion, c.)
CONFERV.----The filamentous weeds of fresh water.
CONGLOMERATE.----A rock made up of fragments of rock or
pebbles, cemented together by some other material.
COROLLA.----The second envelope of a flower usually
composed of coloured, leaf-like organs (petals), which may be united
by their edges either in the basal part or throughout.
CORRELATION.----The normal coincidence of one phenomenon,
character, c., with another.
CORYMB.----A bunch of flowers in which those springing
from the lower part of the flower stalk are supported on long stalks
so as to be nearly on a level with the upper ones.
COTYLEDONS.----The first or seed-leaves of plants.
CRUSTACEANS.----A class of articulated animals, having
the skin of the body generally more or less hardened by the deposition
of calcareous matter, breathing by means of gills.
(Examples, Crab, Lobster, Shrimp, c.)
CURCULIO.----The old generic term for the Beetles known
as Weevils, characterised by their four-jointed feet, and by the head
being produced into a sort of beak, upon the sides of which the
antenn are inserted.
CUTANEOUS.----Of or belonging to the skin.
DEGRADATION.----The wearing down of land by the action of
the sea or of meteoric agencies.
DENUDATION.----The wearing away of the surface of the
land by water.
DEVONIAN SYSTEM or formation.----A series of
Palozoic rocks, including the Old Red Sandstone.
DICOTYLEDONS or DICOTYLEDONOUS PLANTS.----A class of
plants characterised by having two seed-leaves, by the formation of
new wood between the bark and the old wood (exogenous growth) and by
the reticulation of the veins of the leaves. The parts of the flowers
are generally in multiples of five.
DIFFERENTIATION.----The separation or discrimination of
parts or organs which in simpler forms of life are more or less
united.
DIMORPHIC.----Having two distinct
forms.----Dimorphism is the condition of the appearance of
the same species under two dissimilar forms.
DICIOUS.----Having the organs of the sexes upon
distinct individuals.
DIORITE.----A peculiar form of Greenstone.
DORSAL.----Of or belonging to the back.
EDENTATA.----A peculiar order of Quadrupeds,
characterised by the absence of at least the middle incisor (front)
teeth in both jaws. (Examples, the Sloths
and Armadillos.)
ELYTRA.----The hardened fore-wings of Beetles, serving as
sheaths for the membranous hind-wings, which constitute the true
organs of flight.
EMBRYO.----The young animal undergoing development within
the egg or womb.
EMBRYOLOGY.----The study of the development of the
embryo.
ENDEMIC.----Peculiar to a given locality.
ENTOMOSTRACA.----A division of the class Crustacea,
having all the segments of the body usually distinct, gills attached
to the feet or organs of the mouth, and the feet fringed with fine
hairs. They are generally of small size.
EOCENE.----The earliest of the three divisions of the
Tertiary epoch of geologists. Rocks of this age contain a small
proportion of shells identical with species now living.
EPHEMEROUS INSECTS.----Insects allied to the May-fly.
FAUNA.----The totality of the animals naturally
inhabiting a certain country or region, or which have lived during a
given geological period.
FELID.----The Cat-family.
FERAL.----Having become wild from a state of cultivation
or domestication.
FLORA.----The totality of the plants growing naturally in
a country, or during a given geological period.
FLORETS.----Flowers imperfectly developed in some
respects, and collected into a dense spike or head, as in the Grasses,
the Dandelion, c.
FTAL.----Of or belonging to the ftus, or
embryo in course of development.
FORAMINIFERA.----A class of animals of very low
organisation, and generally of small size, having a jelly-like body,
from the Surface of which delicate filaments can be given off and
retracted for the prehension of external objects, and having a
calcareous or sandy shell, usually divided into chambers, and
perforated with small apertures.
FOSSILIFEROUS.----Containing fossils.
FOSSORIAL.----Having a faculty of digging. The Fossorial
Hymenoptera are a group of Wasp-like Insects, which burrow in sandy
soil to make nests for their young.
FRENUM (pl. FRENA).----A small band or fold of skin.
FUNGI (Sing. FUNGUS).----A class of cellular plants, of
which Mushrooms, Toadstools, and Moulds, are familiar examples.
FURCULA.----The forked bone formed by the union of the
collarbones in many birds, such as the common Fowl.
GALLINACEOUS BIRDS.----An order of Birds of which the
common Fowl, Turkey, and Pheasant, are well-known examples.
GALLUS.----The genus of birds which includes the common
Fowl.
GANGLION.----A swelling or knot from which nerves are
given off as from a centre.
GANOID FISHES.----Fishes covered with peculiar enamelled
bony scales. Most of them are extinct.
GERMINAL VESICLE.----A minute vesicle in the eggs of
animals, from which development of the embryo proceeds.
GLACIAL PERIOD.----A period of great cold and of enormous
extension of ice upon the surface of the earth. It is believed that
glacial periods have occurred repeatedly during the geological history
of the earth, but the term is generally applied to the close of the
Tertiary epoch, when nearly the whole of Europe was subjected to an
arctic climate.
GLAND.----An organ which secretes or separates some
peculiar product from the blood or sap of animals or plants.
GLOTTIS.----The opening of the windpipe into the
sophagus or gullet.
GNEISS.----A rock approaching granite in composition, but
more or less laminated, and really produced by the alteration of a
sedimentary deposit after its consolidation.
GRALLATORES.----The so-called Wading-birds (Storks,
Cranes, Snipes, c.), which are generally furnished with long
legs, bare of feathers above the heel, and have no membranes between
the toes.
GRANITE.----A rock consisting essentially of crystal of
felspar and mica in a mass of quarts.
HABITAT.----The locality in which a plant or animal
naturally lives.
HEMIPTERA.----An order or sub-order of Insects,
characterised by the possession of a jointed beak or rostrum, and by
having the fore-wings horny in the basal portion and membranous at the
extremity, where they cross each other. This group includes the
various species of Bugs.
HERMAPHRODITE.----Possessing the organs of both sexes.
HOMOLOGY.----That relation between parts which results
from their development from corresponding embryonic parts, either in
different animals, as in the case of the arm of man, the foreleg of a
quadruped, and the wing of a bird; or in the same individual, as in
the case of the fore and hind legs in quadrupeds, and the segments or
rings and their appendages of which the body of a worm, a centipede,
c., is composed. The latter is called serial
homology. The parts which stand in such a relation to each
other are said to be homologous, and one
such part or organ is called the homologue
of the other. In different plants the parts of the flower are
homologous, and in general these parts are regarded as homologous with
leaves.
HOMOPTERA.----An order or sub-order of Insects having
(like the Hemiptera) a jointed beak, but in which the fore-wings are
either wholly membranous or wholly leathery. The Cicad, Frog-hoppers, and Aphides, are well-known examples.
HYBRID.----The offspring of the union of two distinct
species.
HYMENOPTERA.----An order of insects possessing biting
jaws and usually four membranous wings in which there are a few veins.
Bees and Wasps are familiar examples of this group.
HYPERTROPHIED.---- Excessively developed.
ICHNEUMONID.----A family of Hymenopterous insects,
the members of which lay their eggs in the bodies or eggs of other
insects.
IMAGO.----The perfect (generally winged) reproductive
state of an insect.
INDIGENS.----The aboriginal animal or vegetable
inhabitants of a country or region.
INFLORESCENCE.----The mode of arrangement of the flowers
of plants.
INFUSORIA.----A class of microscopic Animalcules,
so called from their having originally been observed in infusions of
vegetable matters. They consist of a gelatinous material enclosed in
a delicate membrane, the whole or part of which is furnished with
short vibrating hairs (called cilia), by means of which the
animalcules swim through the water or convey the minute particles of
their food to the orifice of the mouth.
INSECTIVOROUS.----Feeding on Insects.
INVERTEBRATA, or INVERTEBRATE ANIMALS.----Those animals
which do not possess a backbone or spinal column.
LACUN.----Spaces left among the tissues in some of
the lower animals, and serving in place of vessels for the circulation
of the fluids of the body.
LAMELLATED.----Furnished with lamell or little
plates.
LARVA (pl.LARV).----The first condition of an
insect at its issuing from the egg, when it is usually in the form of
a grub, caterpillar, or maggot.
LARYNX.----The upper part of the windpipe opening into
the gullet.
LAURENTIAN.----A group of greatly altered and very
ancient rocks, which is greatly developed along the course of the St.
Laurence, whence the name. It is in these that the earliest known
traces of organic bodies have been found.
LEGUMINOS.----An order of plants represented by
the common Peas and Beans, having an irregular flower in which one
petal stands up like a wing, and the stamens and pistil are enclosed
in a sheath formed by two other petals. The fruit is a pod (or
legume).
LEMURID.----A group of four-handed animals,
distinct from the Monkeys and approaching the Insectivorous Quadrupeds
in some of their characters and habits. Its members have the nostrils
curved or twisted, and a claw instead of a nail upon the first finger
of the hind hands.
LEPIDOPTERA.----An order of Insects, characterised by the
possession of a spiral proboscis, and of four large more or less scaly
wings. It includes the well-known Butterflies and Moths.
LITTORAL.----Inhabiting the seashore.
LOESS.----A marly deposit of recent (Post-Tertiary) date,
which occupies a great part of the valley of the Rhine.
MALACOSTRACA.----The higher division of the Crustacea,
including the ordinary Crabs, Lobsters, Shrimps, c., together
with the Woodlice and Sand-hoppers.
MAMMALIA.----The highest class of animals, including the
ordinary hairy quadrupeds, the Whales, and Man, and characterised by
the production of living young which are nourished after birth by milk
from the teats (Mamm, Mammary
glands) of the mother. A striking difference in embryonic
development has led to the division of this class into two great
groups; in one of these, when the embryo has attained a certain stage,
a vascular connection, called the placenta,
is formed between the embryo and the mother; in the other this is
wanting, and the young are produced in a very incomplete state. The
former, including the greater part of the class, are called Placental mammals; the latter, or Aplacental mammals, include the Marsupials and
Monotremes (Ornithorhynchus).
MAMMIFEROUS. Having mamm; or teats (See MAMMALIA).
MANDIBLES, in Insects.----The first or uppermost pair of
jaws, which are generally solid, horny, biting organs. In Birds the
term is applied to both jaws with their horny coverings. In
Quadrupeds the mandible is properly the lower jaw.
MARSUPIALS.----An order of Mammalia in which the young
are born in a very incomplete state of development, and carried by the
mother, while sucking, in a ventral pouch (marsupium), such as the
Kangaroos, Opossums, c. (see MAMMALIA).
MAXILL, in Insects.----The second or lower pair of
jaws, which are composed of several joints and furnished with peculiar
jointed appendages called palpi, or feelers.
MELANISM.----The opposite of albinism; an undue
development of colouring material in the skin and its appendages.
METAMORPHIC ROCKS.----Sedimentary rocks which have
undergone alteration, generally by the action of heat, subsequently to
their deposition and consolidation.
MOLLUSCA.----One of the great divisions of the Animal
Kingdom, including those animals which have a soft body, usually
furnished with a shell, and in which the nervous ganglia, or centres,
present no definite general arrangement. They are generally known
under the denomination of `shell-fish;' the cuttle-fish, and the
common snails, whelks, oysters, mussels, and cockles, may serve as
examples of them.
MONOCOTYLEDONS, or MONOCOTYLEDONOUS PLANTS.----Plants in which
the seed sends up only a single seed-leaf (or cotyledon);
characterised by the absence of consecutive layers of wood in the stem
(endogenous growth), by the veins of the leaves being generally
straight, and by the parts of the flowers being generally in multiples
of three. (Examples, Grasses, Lilies,
Orchids, Palms, c.)
MORAINES.----The accumulations of fragments of rock
brought down by glaciers.
MORPHOLOGY.----The law of form or structure independent
of function.
MYSIS-STAGE.----A stage in the development of certain
Crustaceans (Prawns), in which they closely resemble the adults of a
genus (Mysis) belonging to a slightly lower
group.
NASCENT.----Commencing development.
NATATORY.----Adapted for the purpose of swimming.
NAUPLIUS-FORM.----The earliest stage in the development
of many Crustacea, especially belonging to the lower groups. In this
stage the animal has a short body, with indistinct indications of a
division into segments, and three pairs of fringed limbs. This form of
the common fresh-water Cyclops was
described as a distinct genus under the name of Nauplius.
NEURATION.----The arrangement of the veins or nervures in
the wings of Insects.
NEUTERS.----Imperfectly developed females of certain
social insects (such as Ants and Bees), which perform all the labours
of the community. Hence they are also called workers.
NICTITATING MEMBRANE.----A semi-transparent membrane,
which can be drawn across the eye in Birds and Reptiles, either to
moderate the effects of a strong light or to sweep particles of dust,
c., from the surface of the eye.
OCELLI----The simple eyes or stemmata of Insects, usually
situated on the crown of the head between the great compound eyes.
SOPHAGUS.----The gullet.
OOLITIC.----A great series of secondary rocks, so called
from the texture of some of its members, which appear to be made up of
a mass of small egg-like calcareous bodies.
OPERCULUM.----A calcareous plate employed by many
Mollusca to close the aperture of their shell. The opercular valves of Cirripedes are those which
close the aperture of the shell.
ORBIT.----The bony cavity for the reception of the eye.
ORGANISM.----An organised being, whether plant or animal.
ORTHOSPERMOUS.----A term applied to those fruits of the
Umbellifer which have the seed straight.
OSCULANT.----Forms or groups apparently intermediate
between and connecting other groups are said to be osculant.
OVA.----Eggs.
OVARIUM or OVARY (in plants).----The lower part of the
pistil or female organ of the flower, containing the ovules or
incipient seeds; by growth after the other organs of the flower have
fallen, it usually becomes converted into the fruit.
OVIGEROUS.----Egg-bearing.
OVULES (of plants).----The seeds in the earliest
condition.
PACHYDERMS.----A group of Mammalia, so called from their
thick skins, and including the Elephant, Rhinoceros, Hippopotamus,
c.
PALOZOIC.----The oldest system of fossiliferous
rocks.
PALPI.----Jointed appendages to some of the organs of the
mouth in Insects and Crustacea.
PAPILIONACE.----An order of Plants (see
LEGUMINOS).----The flowers of these plants are called
papilionaceous, or butterfly-like, from the
fancied resemblance of the expanded superior petals to the wings of a
butterfly.
PARASITE.----An animal or plant living upon or in, and at
the expense of, another organism.
PARTHENOGENESIS.----The production of living Organisms
from unimpregnated eggs or seeds.
PEDUNCULATED.----Supported upon a stem or stalk. The
pedunculated oak has its acorns borne upon a footstalk.
PELORIA or PELORISM.----The appearance of regularity of
structure in the flowers of plants which normally bear irregular
flowers.
PELVIS.----The bony arch to which the hind limbs of
Vertebrate animals are articulated.
PETALS.----The leaves of the corolla, or second circle of
organs in a flower. They are usually of delicate texture and brightly
coloured.
PHYLLODINEOUS.----Having flattened, leaf-like twigs or
leafstalks instead of true leaves.
PIGMENT.----The colouring material produced generally in
the superficial parts of animals. The cells secreting it are called
pigment-cells.
PINNATE.---- Bearing leaflets on each side of a central
stalk.
PISTILS.----The female organs of a flower, which occupy a
position in the centre of the other floral organs. The pistil is
generally divisible into the ovary or germen, the style and the
stigma.
PLACENTALIA, PLACENTATA, or Placental Mammals.----See
MAMMALIA.
PLANTIGRADES.----Quadrupeds which walk upon the whole
sole of the foot, like the Bears.
PLASTIC.----Readily capable of change.
PLEISTOCENE PERIOD.----The latest portion of the
Tertiary epoch.
PLUMULE (in plants).----The minute bud between the
seed-leaves of newly-germinated plants.
PLUTONIC ROCKS.----Rocks supposed to have been produced
by igneous action in the depths of the earth.
POLLEN.----The male element in flowering plants; usually
a fine dust produced by the anthers, which, by contact with the stigma
effects the fecundation of the seeds. This impregnation is brought
about by means of tubes (pollen-tubes)
which issue from the pollen-grains adhering to the stigma, and
penetrate through the tissues until they reach the ovary.
POLYANDROUS (flowers).----Flowers having many stamens.
POLYGAMOUS PLANTS.----Plants in which some flowers are
unisexual and others hermaphrodite. The unisexual (male and female)
flowers, may be on the same or on different plants.
POLYMORPHIC.----Presenting many forms.
POLYZOARY.----The common structure for
the Polyzoa, such as the well-known Sea-mats.
PREHENSILE.----Capable of grasping.
PREPOTENT.----Having a superiority of power.
PRIMARIES.----The feathers forming the tip of the wing of
a bird, and inserted upon that part which represents the hand of man.
PROCESSES.----Projecting portions of bones, usually for
the attachment of muscles, ligaments, c.
PROPOLIS.----A resinous material collected by the
Hive-Bees from the opening buds of various trees.
PROTEAN.---- Exceedingly variable.
PROTOZOA.----The lowest great division of the Animal
Kingdom. These animals are composed of a gelatinous material, and show
scarcely any trace of distinct organs. The Infusoria, Foraminifera,
and Sponges, with some other forms, belong to this division.
PUPA (pl. PUP).----The second stage in the
development of an Insect, from which it emerges in the perfect
(winged) reproductive form. In most insects the pupal stage is passed in perfect repose. The
chrysalis is the pupal state of butterflies.
RADICLE.----The minute root of an embryo plant.
RAMUS.----One half of the lower jaw in the Mammalia. The
portion which rises to articulate with the skull is called the ascending ramus.
RANGE.----The extent of country over which a plant or
animal is naturally spread. Range in time
expresses the distribution of a species or group through the
fossiliferous beds of the earth's crust.
RETINA.----The delicate inner coat of the eye, formed by
nervous filaments spreading from the optic nerve, and serving for the
perception of the impressions produced by light.
RETROGRESSION.----Backward development. When an animal,
as it approaches maturity, becomes less perfectly organised than might
be expected from its early stages and known relationships, it is said
to undergo a retrograde development or
metamorphosis.
RHIZOPODS.----A class of lowly organised animals
(protozoa), having a gelatinous body, the surface of which can be
protruded in the form of root-like processes or filaments, which serve
for locomotion and the prehension of food. The most important order
is that of the Foraminifera.
RODENTS.----The gnawing Mammalia, such as the Rats,
Rabbits, and Squirrels. They are especially characterised by the
possession of a single pair of chisel-like cutting teeth in each jaw,
between which and the grinding teeth there is a great gap.
RUBUS.----The Bramble Genus.
RUDIMENTARY.----Very imperfectly developed.
RUMINANTS.----The group of Quadrupeds which ruminate or
chew the cud, such as oxen, sheep, and deer. They have divided hoofs,
and are destitute of front teeth in the upper jaw.
SACRAL.----Belonging to the sacrum, or the bone composed
usually of two or more united vertebr to which the sides of the
pelvis in Vertebrate animals are attached.
SARCODE.----The gelatinous material of which the bodies
of the lowest animals (Protozoa) are composed.
SCUTELL.----The horny plates with which the feet
of birds are generally more or less covered, especially in front.
SEDIMENTARY FORMATIONS.----Rocks deposited as sediments
from water.
SEGMENTS.----The transverse rings of which the body of an
articulate animal or Annelid is composed.
SEPALS.----The leaves or segments of the calyx, or
outermost envelope of an ordinary flower. They are usually green, but
sometimes brightly coloured.
SERRATURES.----Teeth like those of a saw.
SESSILE.----Not supported on a stem or footstalk.
SILURIAN SYSTEM.----A Very ancient system of
fossiliferous rocks belonging to the earlier part of the
Palozoic series.
SPECIALISATION.----The setting apart of a particular
organ for the performance of a particular function.
SPINAL CHORD.----The central portion of the nervous
system in the Vertebrata, which descends from the brain through the
arches of the vertebr, and gives off nearly all the nerves to
the various organs of the body.
STAMENS.----The male organs of flowering plants, standing
in a circle within the petals. They usually consist of a filament and
an anther, the anther being the essential part in which the pollen, or
fecundating dust, is formed.
STERNUM.----The breast-bone.
STIGMA.----The apical portion of the pistil in flowering
plants.
STIPULES.----Small leafy organs placed at the base of the
footstalks of the leaves in many plants.
STYLE.----The middle portion of the perfect pistil, which
rises like a column from the ovary and supports the stigma at its
summit.
SUBCUTANEOUS.----Situated beneath the skin.
SUCTORIAL.---- Adapted for sucking.
SUTURES (in the skull).----The lines of junction of the
bones of which the skull is composed.
TARSUS (pl. TARSI).----The jointed feet of articulate
animals, such as Insects.
TELEOSTEAN FISHES.----Fishes of the kind familiar to us
in the present day, having the skeleton usually completely ossified
and the scales horny.
TENTACULA or TENTACLES.----Delicate fleshy organs of
prehension or touch possessed by many of the lower animals.
TERTIARY.----The latest geological epoch, immediately
preceding the establishment of the present order of things.
TRACHEA.----The windpipe or passage for the admission of
air to the lungs.
TRIDACTYLE.----Three-fingered, or composed of three
movable parts attached to a common base.
TRILOBITES.----A peculiar group of extinct Crustaceans,
somewhat resembling the Woodlice in external form, and, like some of
them, capable of rolling themselves up into a ball. Their remains are
found only in the Palozoic rocks, and most abundantly in those
of Silurian age.
TRIMORPHIC.---- Presenting three distinct forms.
UMBELLIFER.----An order of plants in which the
flowers, which contain five stamens and a pistil with two styles, are
supported upon footstalks which spring from the top of the flower stem
and spread out like the wires of an umbrella, so as to bring all the
flowers in the same head (umbel) nearly to
the same level. (Examples, Parsley and
Carrot).
UNGULATA.---- Hoofed quadrupeds.
UNICELLULAR.----Consisting of a single cell.
VASCULAR.----Containing blood-vessels
VERMIFORM.---- Like a worm.
VERTEBRATA: or VERTEBRATE ANIMALS.----The highest
division of the animal kingdom, so called from the presence in most
cases of a backbone composed of numerous joints or vertebr, which constitutes the centre of
the skeleton and at the same time supports and protects the central
parts of the nervous system.
WHORLS.----The circles or spiral lines in which the parts
of plants are arranged upon the axis of growth.
WORKERS.----See neuters.
ZOEA-STAGE.----The earliest stage in the development of
many of the higher Crustacea, so called from the name of Zoea applied to these young animals when they
were supposed to constitute a peculiar genus.
ZOOIDS.----In many of the lower animals (such as the
Corals, Medus, c.) reproduction takes place in two ways,
namely, by means of eggs and by a process of budding with or without
separation from the parent of the product of the latter, which is
often very different from that of the egg. The individuality of the
species is represented by the whole of the form produced between two
sexual reproductions; and these forms, which are apparently individual
animals, have been called zooids.