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CHAPTER IV

Natural Selection; or the Survival of the Fittest

Natural Selection — Its Power compared with Man's Selection — Its Power on Charac-
ters 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 Favorable and Unfavorable to the Results of 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 Naturalization — Action
of Natural Selection, through Divergence of Character and Extinction, on the
Descendants from a Common Parent, explains the Grouping of all Organic
Beings — Advance in Organization — Low Forms preserved — Convergence of
Character — Indefinite Multiplication of Species — Summary.

How will the struggle for existence, briefly discussed 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 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 .j Under domestication, it may truly be
said that the whole organization becomes in some degree plastic. But the
variability, which we almost universally meet with in our domestic produc-
tions, 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 ex-
poses 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 in-
dividuals having any a dvantag e, however slight, over others, would have the
best chance of surviving and procreating their kind?. On the other hand, we
may feel sure that any variation in the least degree injiirious would be rigidly
destroyed. This preservation of favorable individual differences and varia-
tions, and the destruction of those which are injurious, I have called Natural
Selection, or the Survival of the Fittest. Variations neither useful nor in-
jurious would not be affected by natural selection, and would be left either a
fluctuating element, as perhaps we see in certain polymorphic species, or
would ultimately become fixed, owing to the nature of the organism and
the nature of the conditions.

51



52 THE ORIGIN OF SPECIES

Several writers have misapprehended or objected to the term Natural
Selection. Some have even imagined that natural selection induces varia-
bility, whereas it implies only the preservation of such variations as arise and
are beneficial to the being under its conditions of life. No one objects to
agriculturists speaking of the potent effects of man's selection; and in this
case the individual differences given by nature, which man for some object
selects, must of necessity first occur. Others have objected that the term
selection implies conscious choice in the animals which become modified;
and it has even been urged, that, as plants have no volition, natural selec-
tion is not applicable to them! In the literal sense of the word, no doubt,
natural selection is a false term; but who ever objected to chemists speak-
ing of the elective affinities of the various elements? — and yet an acid
cannot strictly be said to elect the base with which it in preference com-
bines. It has been said that I speak of natural selection as an active power
or Deity; but who objects to an author speaking of the attraction of gravity
as ruling the movements of the planets? Every one knows what is meant
and is implied by such metaphorical expressions; and they are almost neces-
sary for brevity. So again it is difficult to avoid personifying the word
Nature; but I mean by nature, only the aggregate action and product of
many natural laws, and by laws the sequence of events as ascertained by us.
With a little familiarity such superficial objections will be forgotten.

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 to-
gether, that any change in the numerical proportions of the inhabitants,
independently of the change of climate itself, would seriously aflfect 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
I 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, as shown in the first chapter, 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



NATURAL SELECTION 53

of the occurrence of profitable variations. Unless such occur, natural selec-
tion can do nothing.' Under the term of "variations," it must never be for-
gotten 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 un-
occupied 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 modi-
fications 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 con-
tinued 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 con-
quered 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 intrudei^.

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;
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 conse-



54 THE ORIGIN OF SPECIES

quently how poor will be his results, compared with those accumulated by-
Nature during whole geological periods! Can we wonder, then, that Na-
ture's productions should be far "truer" in character than man's produc-
tions; 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
I scrutinizing, throughout the world, the slightest variations; rejecting those
\ that are bad, preserving and adding up all that are good ; silently and in-
■' sensibly working, ]whenever and wherever opportunity offers, at the im-
provement 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 agesj| 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 again be 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 in-
crease 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. We have
seen how the color of hogs, which feed on the "paint-root" in Virginia,
determines whether they shall live or die. 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 the smooth-skinned fruits suffer far



NATURAL SELECTION 55

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 a 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 correla-
tion, when one part varies and the variations are accumulated through
natural selection, other modifications, often of the most unexpected nature,
will ensue.

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 shape, size, and flavor of 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 color 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 accumula-
tion of variations profitable 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 in-
creasing 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; and these modifications may affect, through correlation, the
structure of the adult. So, conversely, modifications in the adult may affect
the structure of the larva; but in all cases natural selection will insure that
they shall not be injurious: for if they were so, the species would become
extinct.

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 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
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



56 THE ORIGIN OF SPECIES

exclusively for opening the cocoon — or the hard tip to the beak of un-
hatched birds, used for breaking the eggs. 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 very short for the
bird's own advantage, the process of modification would be very slow, and
there would be simultaneously the raost 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.

It may be well here to remark that with all beings there must be much
fortuitous destruction, which can have little or no influence on the course
of natural selection. For instance, a vast number of eggs or seeds are
annually devoured, and these could be modified through natural selection
only if they varied in some manner which protected them from their
enemies. Yet many of these eggs or seeds would perhaps, if not destroyed,
have yielded individuals better adapted to their conditions of life than any
of those which happened to survive. So again a vast number of mature
animals and plants, whether or not they be the best adapted to their condi-
tions, must be annually destroyed by accidental causes, which would not be
in the least degree mitigated by certain changes of structure or constitution
which would in other ways be beneficial to the species. But let the destruc-
tion of the adults be ever so heavy, if the number which can exist in any
district be not wholly kept down by such causes — or again let the destruc-
tion of eggs or seeds be so great that only a hundredth or a thousandth part
are developed — ^yet of those which do survive, the best adapted individuals,
supposing that there is any variability in a favorable direction, will tend to
propagate their kind in larger numbers than the less well adapted. If the
numbers be wholly kept down by the causes just indicated, as will often
have been the case, natural selection will be powerless in certain beneficial
directions; but this is no valid objection to its efficiency at other times and
in other ways ; for we are far from having any reason to suppose that many
species ever undergo modification and improvement at the same time in
the same area.

SEXUAL SELECTION

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 com-
monly 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



NATURAL SELECTION 57

the possession of the otherjex. The result is not death to the unsuccessful
competitor, but few or no offspring. Sexual selection is, therefore^, less
r igoro us 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 oases 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 of 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 posses-
sion of the females; male salmons 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 confine-
ment well know that they often take individual preferences and dislikes:
thus Sir R. Heron has described how a pied peacock was eminently attrac-
tive to all his hen birds. I cannot 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; but I have not space here to enter
on this subject.



58 THE ORIGIN OF SPECIES

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 orna-
ment, such differences have been mainly caused by sexual selection : that is,
by individual males having had, in successive generations, some slight ad-
vantage 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 cannot 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.

ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR THE

SURVIVAL OF THE FITTEST

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,



Online LibraryCharles DarwinThe origin of species → online text (page 8 of 50)