chapter, act in regard to variation? Can the principle of selection,
which we have seen is so potent in the hands of man, apply under
nature? I think we shall see that it can act most efficiently. Let
the endless number of slight variations and individual differences
occurring in our domestic productions, and, in a lesser degree,
in those under nature, be borne in mind; as well as the strength
of the hereditary tendency. Under domestication, it may be truly
said that the whole organization becomes in some degree plastic.
But the variability, which we almost universally meet with in our
domestic productions, is not directly produced, as Hooker and Asa
Gray have well remarked, by man; he can neither originate varieties,
nor prevent their occurrence; he can only preserve and accumulate
such as do occur. Unintentionally he exposes organic beings to
new and changing conditions of life, and variability ensues; but
similar changes of conditions might and do occur under nature. Let
it also 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; and consequently what infinitely
varied diversities of structure might be of use to each being under
changing 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 occur in the course of many successive
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 favorable
individual differences and variations, and the destruction of those
which are injurious, I have called Natural Selection, or the Survival
of the Fittest.
We shall best understand the probable course of natural selection
by taking the case of a country undergoing some slight physical
change, for instance, of climate. The proportional numbers of its
inhabitants will almost immediately undergo a change, and some
species will probably 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 the inhabitants, independently of the change of
climate itself, would seriously affect the others. If the country
were open on its borders, new forms would certainly immigrate,
and this would likewise 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 cases, slight modifications, which in any way
favored the individuals of any species, by better adapting them to
their altered conditions, would tend to be preserved; and natural
selection would have free scope for the work of improvement.
We have good reason to believe that changes in the conditions of life
give a tendency to increased variability; and in the foregoing cases
the conditions have changed, and this would manifestly be favorable
to natural selection, by affording a better chance of the occurrence
of profitable variations. Unless such occur, natural selection can do
nothing. Under the term of “variations,” it must never be forgotten
that mere individual differences are included. As man can produce a
great result with his domestic animals and plants by adding up in any
given direction individual differences, so could natural selection,
but far more easily from having incomparably longer time for action.
Nor do I believe that any great physical change, as of climate, or
any unusual degree of isolation to check immigration, is necessary
in order that new and unoccupied places should be left for natural
selection to fill up by 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 species would often give it an advantage
over others; and still further modifications of the same kind would
often still further increase the advantage, as long as the species
continued under the same conditions of life and profited by similar
means of subsistence and defence. 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 be still better adapted or improved; for in all
countries the natives have been so far conquered by naturalized
productions that they have allowed some foreigners to take firm
possession of the land. And as foreigners have thus in every country
beaten some of the natives, we may safely conclude that the natives
might have been modified with advantage, so as to have better
resisted the intruders.
As man can produce, and certainly has produced, a great result
by his methodical and unconscious means of selection, what may
not natural selection effect? Man can act only on external and
visible characters: Nature, if I may be allowed to personify the
natural preservation or survival of the fittest, cares nothing for
appearances, except in so far as they are 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, as is implied by the
fact of their selection. 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 the eye or to
be plainly useful to him. Under Nature, the slightest differences 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 be his results, 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 metaphorically be said that natural selection is daily and
hourly scrutinizing, throughout the world, the slightest variations;
rejecting those that are bad, preserving and adding up all that
are 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 lapse of ages, and then so imperfect is our view into
long-past geological ages, that we see only that the forms of life
are now different from what they formerly were.
In order that any great amount of modification should be effected in
a species, a variety when once formed must again, perhaps after a
long interval of time, vary or present individual differences of the
same favorable nature as before; and these must be again preserved,
and so onward step by step. Seeing that individual differences of
the same kind perpetually recur, this can hardly be considered as
an unwarrantable assumption. But whether it is true, we can judge
only by seeing how far the hypothesis accords with and explains the
general phenomena of nature. On the other hand, the ordinary belief
that the amount of possible variation is a strictly limited quantity
is likewise a simple assumption.
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-gray;
the Alpine ptarmigan white in winter, the red grouse the color of
heather, 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 natural selection might
be effective in giving the proper color to each kind of grouse, and
in keeping that color, when once acquired, true and constant. Nor
ought we to think that the occasional destruction of an animal of any
particular color would produce little effect: we should remember
how essential it is in a flock of white sheep to destroy a lamb with
the faintest trace of black. In plants, the down on the fruit and
the color 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 colored 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 quite unimportant,
we must not forget that climate, food, etc., have no doubt produced
some direct effect. It is also necessary to bear in mind that, owing
to the law of correlation, when one part varies, and the variations
are accumulated through natural selection, other modifications, often
of the most unexpected nature, will ensue.
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 whole community; if the community profits by the
selected change. What natural selection can not 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 can not find one case which
will bear investigation. A structure used only once in an animal’s
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, used exclusively for opening the cocoon--or the hard tip to
the beak of unhatched birds, used for breaking the egg. It has been
asserted that of the best short-beaked tumbler-pigeons a greater
number 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 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 all 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.
Inasmuch as peculiarities often appear under domestication in one
sex and become hereditarily attached to that sex, so no doubt it
will be under nature. Thus it is rendered possible for the two sexes
to be modified through natural selection in relation to different
habits of life, as is sometimes the case; or for one sex to be
modified in relation to the other sex, as commonly occurs. This
leads me to say a few words on what I have called Sexual Selection.
This form of selection depends, not on a struggle for existence in
relation to other organic beings or to external conditions, but on
a struggle between the individuals of one sex, generally the males,
for the possession of the other sex. 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 depends
not so much on general vigor as on having special weapons, confined
to the male sex. A hornless stag or spurless cock would have a poor
chance of leaving numerous 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, in nearly the same manner as does the brutal cockfighter by the
careful selection of his best cocks. How low in the scale of nature
the 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 salmon have
been observed fighting all day long; male stag-beetles sometimes bear
wounds from the huge mandibles of other males; the males of certain
hymenopterous insects have been frequently seen by that inimitable
observer, M. Fabre, fighting for a particular female who sits by, an
apparently unconcerned beholder of the struggle, and then retires
with the conqueror. 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 of the lion and the
hooked jaw to the male salmon; for the shield may be as important for
victory as the sword or spear.
Among 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 with the
most elaborate care, and show off in the best manner, their gorgeous
plumage; they likewise 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 a pied peacock was eminently
attractive to all his hen birds. I can not here enter on the
necessary details; but if man can in a short time give beauty and an
elegant carriage 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. Some well-known laws, with respect to the plumage of male
and female birds, in comparison with the plumage of the young,
can partly be explained through the action of sexual selection on
variations occurring at different ages, and transmitted to the males
alone or to both sexes at corresponding ages.
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,
color, or ornament, such differences have been mainly caused by
sexual selection: that is, by individual males having had, in
successive generations, some slight advantage over other males,
in their weapons, means of defence, or charms, which they have
transmitted to their male offspring alone. Yet, I would not wish to
attribute all sexual differences to this agency: for we see in our
domestic animals peculiarities arising and becoming attached to the
male sex, which apparently have not been augmented through selection
by man. The tuft of hair on the breast of the wild turkey-cock can
not be of any use, and it is doubtful whether it can be ornamental
in the eyes of the female bird;--indeed, had the tuft appeared under
domestication, it would have been called a monstrosity.
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
was hardest pressed for food. Under such circumstances 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 some other period of the year, when they
were compelled to prey on other animals. I can see no more reason to
doubt that this would be the result than that man should be able to
improve the fleetness of his greyhounds by careful and methodical
selection, or by that kind of unconscious selection which follows
from each man trying to keep the best dogs without any thought of
modifying the breed. 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.
To the effects of intercrossing in eliminating variations of all
kinds, I shall have to recur; but it may be here remarked that most
animals and plants keep to their proper homes, and do not needlessly
wander about; we see this even with migratory birds, which almost
always return to the same spot. Consequently each newly formed
variety would generally be at first local, as seems to be the common
rule with varieties in a state of nature; so that similarly modified
individuals would soon exist in a small body together, and would
often breed together. If the new variety were successful in its
battle for life, it would slowly spread from a central district,
competing with and conquering the unchanged individuals on the
margins of an ever-increasing circle.
[Illustration: Beautiful and Curious Shells
1, Strombus; 2, Chama; 3, Struthiolaria; 4, Tridacna; 5, Hippopus; 6,
Ranella; 7, Pteroceras; 8, 10, Murex; 9, Terebra]
It may be worth while to give another and more complex illustration
of the action of natural selection. Certain plants excrete sweet
juice, apparently for the sake of eliminating something injurious
from the sap: this is effected, for instance, by glands at the
base of the stipules in some Leguminosæ, and at the backs of the
leaves of the common laurel. This juice, though small in quantity,
is greedily sought by insects; but their visits do not in any way
benefit the plant Now, let us suppose that the juice or nectar was
excreted from the inside of the flowers of a certain number of plants
of any species. Insects in seeking the nectar would get dusted with
pollen, and would often transport it from one flower to another.
The flowers of two distinct individuals of the same species would
thus get crossed; and the act of crossing, as can be fully proved,
gives rise to vigorous seedlings, which consequently would have the
best chance of flourishing and surviving. The plants which produced
flowers with the largest glands or nectaries, excreting most nectar,
would oftenest be visited by insects, and would oftenest be crossed;
and so in the long run would gain the upper hand and form a local
variety. The flowers, also, which had their stamens and pistils
placed, in relation to the size and habits of the particular insect
which visited them, so as to favor in any degree the transportal
of the pollen, would likewise be favored. 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 purpose of
fertilization, its destruction appears to be 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 to be thus
robbed; and the individuals which produced more and more pollen, and
had larger anthers, would be selected.
When our plant, by the above process long continued, had been
rendered highly attractive to insects, they would, unintentionally on
their part, regularly carry pollen from flower to flower; and that
they do this effectually, I could easily show by many striking facts.
Let us now turn to the nectar-feeding insects; 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 with a very little more trouble they can enter
by the mouth. Bearing such facts in mind, it may be believed that
under certain circumstances individual differences in the curvature
or length of the proboscis, etc., too slight to be appreciated by
us, might profit a bee or other insect, so that certain individuals
would be able to obtain their food more quickly than others; and thus
the communities to which they belonged would flourish and throw
off many swarms inheriting the same peculiarities. The tubes of the
corolla 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
humblebees alone; so that whole fields of the red clover offer in
vain an abundant supply of precious nectar to the hive-bee. That
this nectar is much liked by the hive-bee is certain; for I have
repeatedly seen, but only in the autumn, many hive-bees sucking the
flowers through holes bitten in the base of the tube by humblebees.
The difference in the length of the corolla in the two kinds of
clover, which determines the visits of the hive-bee, must be very
trifling; for I have been assured that when red clover has been mown,
the flowers of the second crop are somewhat smaller, and that these
are visited by many hive-bees. I do not know whether this statement
is accurate; nor whether another published statement can be trusted,
namely, that the Ligurian bee, which is generally considered a mere
variety of the common hive-bee, and which freely crosses with it,
is able to reach and suck the nectar of the red clover. Thus, in
a country where this kind of clover abounded, it might be a great
advantage to the hive-bee to have a slightly longer or differently
constructed proboscis. On the other hand, as the fertility of this
clover absolutely depends on bees visiting the flowers, if humblebees
were to become rare in any country, it might be a great advantage
to the plant to have a shorter or more deeply divided corolla, so
that the hive-bees should be enabled to suck 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 to each
other in the most perfect manner, by the continued preservation of
all the individuals which presented slight deviations of structure
mutually favorable to each other.