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CHAPTER IV 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 lapse 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
larvæ. 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 Leguminosæ, 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
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 Gärtner, 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 Kölreuter
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 mongrelized? 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 Europæo-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 one
species of grass, and a similar plot 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 m2 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 a10.
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 m10, 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 a10;
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 a14 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. |
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