CHAPTER IV.

NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.

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 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 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—Advance in organisation—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. Under domestication, it may truly be said that the

whole organisation 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 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

individual differences and variations, and the destruction of those

which are injurious, I have called Natural Selection, or the Survival

of the Fittest. Variations neither useful nor injurious 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.

Several writers have misapprehended or objected to the term Natural

Selection. Some have even imagined that natural selection induces

variability, 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 selection

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

speaking 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 combines. 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 necessary 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 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

favoured 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, 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 the changed, and

this would manifestly be favourable 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 naturalised 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; 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 scrutinising, 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 long 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

favourable 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-grey; the alpine

ptarmigan white in winter, the red-grouse the colour 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 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 a lamb with the faintest trace of

black. We have seen how the colour 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 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 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, &c., 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.

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 flavour 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 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 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 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; 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 ensure 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 exclusively

for opening the cocoon—or the hard tip to the beak of unhatched 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 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.

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 conditions, 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 destruction 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 destruction 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 favourable 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 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 vigour, but 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 possession 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 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 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.

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, 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 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, 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 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.

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.

It should be observed that in the above illustration, I speak of the

slimmest individual wolves, and not of any single strongly marked

variation having been preserved. In former editions of this work I

sometimes spoke as if this latter alternative had frequently occurred.

I saw the great importance of individual differences, and this led me

fully to discuss the results of unconscious selection by man, which

depends on the preservation of all the more or less valuable

individuals, and on the destruction of the worst. I saw, also, that the

preservation in a state of nature of any occasional deviation of

structure, such as a monstrosity, would be a rare event; and that, if

at first preserved, it would generally be lost by subsequent

intercrossing with ordinary individuals. Nevertheless, until reading an

able and valuable article in the “North British Review” (1867), I did

not appreciate how rarely single variations, whether slight or strongly

marked, could be perpetuated. The author takes the case of a pair of

animals, producing during their lifetime two hundred offspring, of

which, from various causes of destruction, only two on an average

survive to pro-create their kind. This is rather an extreme estimate

for most of the higher animals, but by no means so for many of the

lower organisms. He then shows that if a single individual were born,

which varied in some manner, giving it twice as good a chance of life

as that of the other individuals, yet the chances would be strongly

against its survival. Supposing it to survive and to breed, and that

half its young inherited the favourable variation; still, as the

Reviewer goes onto show, the young would have only a slightly better

chance of surviving and breeding; and this chance would go on

decreasing in the succeeding generations. The justice of these remarks

cannot, I think, be disputed. If, for instance, a bird of some kind

could procure its food more easily by having its beak curved, and if

one were born with its beak strongly curved, and which consequently

flourished, nevertheless there would be a very poor chance of this one

individual perpetuating its kind to the exclusion of the common form;

but there can hardly be a doubt, judging by what we see taking place

under domestication, that this result would follow from the

preservation during many generations of a large number of individuals

with more or less strongly curved beaks, and from the destruction of a

still larger number with the straightest beaks.

It should not, however, be overlooked that certain rather strongly

marked variations, which no one would rank as mere individual

differences, frequently recur owing to a similar organisation being

similarly acted on—of which fact numerous instances could be given with

our domestic productions. In such cases, if the varying individual did

not actually transmit to its offspring its newly-acquired character, it

would undoubtedly transmit to them, as long as the existing conditions

remained the same, a still stronger tendency to vary in the same

manner. There can also be little doubt that the tendency to vary in the

same manner has often been so strong that all the individuals of the

same species have been similarly modified without the aid of any form

of selection. Or only a third, fifth, or tenth part of the individuals

may have been thus affected, of which fact several instances could be

given. Thus Graba estimates that about one-fifth of the guillemots in

the Faroe Islands consist of a variety so well marked, that it was

formerly ranked as a distinct species under the name of Uria lacrymans.

In cases of this kind, if the variation were of a beneficial nature,

the original form would soon be supplanted by the modified form,

through the survival of the fittest.

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.

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 favour in any degree the

transportal of the pollen, would likewise be favoured. 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 fertilisation, 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. I will

give only one, as likewise illustrating one step in the separation of

the sexes of plants. Some holly-trees bear only male flowers, which

have four stamens producing a rather 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 a few pollen-grains, and on some a

profusion. 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, which had 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 might be effected. It would take up too much space to show

the various steps, through dimorphism and other means, by which the

separation of the sexes in plants of various kinds is apparently now in

progress; but I may add that some of the species of holly in North

America are, according to Asa Gray, in an exactly intermediate

condition, or, as he expresses it, are more or less dioeciously

polygamous.

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,

&c., 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 or 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. 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

humble bees. 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 humble-bees

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 favourable to

each other.

I am well aware that this doctrine of natural selection, exemplified in

the above imaginary instances, is open to the same objections which

were first urged against Sir Charles Lyell’s noble views on “the modern

changes of the earth, as illustrative of geology;” but we now seldom

hear the agencies which we see still at work, spoken of as trifling and

insignificant, when used in explaining the excavation of the deepest

valleys or the formation of long lines of inland cliffs. Natural

selection acts only by the preservation and accumulation of 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

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 (with the exception of the curious and not well

understood cases of parthenogenesis) unite for each birth; but in the

case of hermaphrodites this is far from obvious. Nevertheless there is

reason to believe that with all hermaphrodites two individuals, either

occasionally or habitually, concur for the reproduction of their kind.

This view was long ago doubtfully suggested by Sprengel, Knight and

Kölreuter. 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, and made

so many experiments, 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 that no organic being fertilises itself for a perpetuity of

generations; but that a cross with another individual is

occasionally—perhaps at long intervals of time—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! If an occasional cross be indispensable, notwithstanding

that the plant’s own anthers and pistil stand so near each other as

almost to ensure self-fertilisation, the fullest freedom for the

entrance of pollen from another individual will explain the above state

of exposure of the organs. Many flowers, on the other hand, have their

organs of fructification closely enclosed, as in the great

papilionaceous or pea-family; but these almost invariably present

beautiful and curious adaptations in relation to the visits of insects.

So necessary are the visits of bees to many papilionaceous flowers,

that their fertility is greatly diminished if these visits be

prevented. Now, it is scarcely possible for insects to fly from flower

to flower, and not to carry pollen from one to the other, to the great

good of the plant. Insects act like a camel-hair pencil, and it is

sufficient, to ensure fertilisation, just to touch with the same brush

the anthers of one flower and then the stigma of another; but it must

not be supposed that bees would thus produce a multitude of hybrids

between distinct species; for if a plant’s own pollen and that from

another species are placed on the same stigma, the former is so

prepotent that it invariably and completely destroys, as has been shown

by Gärtner, the influence of 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 in this very genus, which seems to have a special

contrivance for self-fertilisation, it is well known that, if

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 numerous other cases, far from self-fertilisation being

favoured, there are special contrivances which effectually prevent the

stigma receiving pollen from its own flower, as I could show from the

works of Sprengel and others, as well as from my own observations: for

instance, in Lobelia fulgens, there is a really beautiful and elaborate

contrivance by which all 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

raise plenty of seedlings. Another species of Lobelia, which is visited

by bees, seeds freely in my garden. In very many other cases, though

there is no special mechanical contrivance to prevent the stigma

receiving pollen from the same flower, yet, as Sprengel, and more

recently Hildebrand and others have 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 so-named dichogamous plants have in fact separated sexes, and

must habitually be crossed. So it is with the reciprocally dimorphic

and trimorphic plants previously alluded to. 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 be in so many cases 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 of the

seedlings thus raised turn out, as I found, 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; and the pollen of

each flower readily gets on its stigma without insect agency; for I

have found that plants carefully protected from insects produce the

full number of pods. How, then, comes it that such a vast number of the

seedlings are mongrelized? It must arise from the pollen of a distinct

_variety_ having a prepotent effect over the 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 reversed, 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 large 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 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,

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 informs me that the rule

does not hold good in Australia: but if most of the Australian trees

are dichogamous, the same result would follow as if they bore flowers

with separated sexes. I have made these few remarks on trees simply to

call attention to the subject.

Turning for a brief space to animals: various terrestrial species are

hermaphrodites, such as the land-mollusca and earth-worms; but these

all pair. As yet I have not found a single terrestrial animal which can

fertilise itself. This remarkable fact, which offers so strong a

contrast with terrestrial plants, is intelligible on the view of an

occasional cross being indispensable; for owing to the nature of the

fertilising element there are no means, analogous to the action of

insects and of the wind with 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 the currents of water offer an obvious means

for an occasional cross. 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 hermaphrodite animal with the

organs of reproduction so perfectly enclosed 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,

under this point of view, a case of great difficulty; but I have been

enabled, by a fortunate chance, 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, both

with animals and plants, some species of the same family and even of

the same genus, though agreeing closely with each other in their whole

organisation, are hermaphrodites, and some unisexual. But if, in fact,

all hermaphrodites do occasionally intercross, the difference between

them and unisexual species is, as far as function is concerned, very

small.

From these several considerations and from the many special facts which

I have collected, but which I am unable here to give, it appears that

with animals and plants an occasional intercross between distinct

individuals is a very general, if not universal, law of nature.

_Circumstances favourable for the production of new forms through

Natural Selection._

This is an extremely intricate subject. A great amount of variability,

under which term individual differences are always included, will

evidently be favourable. A large number of individuals, by giving a

better chance within any given period for the appearance of profitable

variations, will compensate for a lesser amount of variability in each

individual, and is, I believe, a highly important element of success.

Though nature grants long periods of time for the work of natural

selection, she does not grant an indefinite period; for as all organic

beings are striving 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 be exterminated.

Unless favourable variations be inherited by some at least of the

offspring, nothing can be effected by natural selection. The tendency

to reversion may often check or prevent the work; but as this tendency

has not prevented man from forming by selection numerous domestic

races, why should it prevail against natural selection?

In the case of methodical selection, a breeder selects for some

definite object, and if the individuals be allowed freely to

intercross, his work will completely fail. But when many men, without

intending to alter the breed, have a nearly common standard of

perfection, and all try to procure and breed from the best animals,

improvement surely but slowly follows from this unconscious process of

selection, notwithstanding that there is no separation of selected

individuals. Thus it will be under nature; for within a confined area,

with some place in the natural polity not perfectly occupied, all the

individuals varying in the right direction, though in different

degrees, will tend to be preserved. But if the area be large, its

several districts will almost certainly present different conditions of

life; and then, if the same species undergoes modification in different

districts, the newly formed varieties will intercross on the confines

of each. But we shall see in the sixth chapter that intermediate

varieties, inhabiting intermediate districts, will in the long run

generally be supplanted by one of the adjoining varieties.

Intercrossing will chiefly affect those animals which unite for each

birth and wander much, and which do not breed at a very quick rate.

Hence with animals of this nature, for instance birds, varieties will

generally be confined to separated countries; and this I find to be the

case. With hermaphrodite organisms which cross only occasionally, and

likewise with animals which unite for each birth, but which wander

little and can increase at a rapid rate, a new and improved variety

might be quickly formed on any one spot, and might there maintain

itself in a body and afterward spread, so that the individuals of the

new variety would chiefly cross together. On this principle nurserymen

always prefer saving seed from a large body of plants, as the chance of

intercrossing is thus lessened.

Even with animals which unite for each birth, and which do not

propagate rapidly, we must not assume that free intercrossing would

always eliminate the effects of natural selection; for I can bring

forward a considerable body of facts showing that within the same area

two varieties of the same animal may long remain distinct, from

haunting different stations, from breeding at slightly different

seasons, or from the individuals of each variety preferring to pair

together.

Intercrossing plays a very important part in nature by 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, as already stated,

we have reason to believe that occasional intercrosses take place with

all animals and plants. Even if these take place only at long intervals

of time, 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 crosses, even at rare

intervals, will be great. With respect to organic beings extremely low

in the scale, which do not propagate sexually, nor conjugate, and which

cannot possibly intercross, uniformity of character can be retained by

them under the same conditions of life, only through the principle of

inheritance, and through natural selection which will destroy any

individuals departing from the proper type. If the conditions of life

change and the form undergoes modification, uniformity of character can

be given to the modified offspring, solely by natural selection

preserving similar favourable variations.

Isolation also is an important element in the modification of species

through natural selection. In a confined or isolated area, if not very

large, the organic and inorganic conditions of life will generally be

almost uniform; so that natural selection will tend to modify all the

varying individuals of the same species in the same manner.

Intercrossing with the inhabitants of the surrounding districts, will

also be thus prevented. Moritz Wagner has lately published an

interesting essay on this subject, and has shown that the service

rendered by isolation in preventing crosses between newly-formed

varieties is probably greater even than I supposed. But from reasons

already assigned I can by no means agree with this naturalist, that

migration and isolation are necessary elements for the formation of new

species. The importance of isolation is likewise great in preventing,

after any physical change in the conditions, such as of climate,

elevation of the land, &c., the immigration of better adapted

organisms; and thus new places in the natural economy of the district

will be left open to be filled up by the modification of the old

inhabitants. Lastly, isolation will give time for a new variety to be

improved at a slow rate; and this may sometimes be of much importance.

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 inhabitants will be small; and this

will retard the production of new species through natural selection, by

decreasing the chances of favourable variations arising.

The mere lapse of time by itself does nothing, either for or against

natural selection. I state this because it has been erroneously

asserted that the element of time has been assumed by me to play an

all-important part in modifying species, as if all the forms of life

were necessarily undergoing change through some innate law. Lapse of

time is only so far important, and its importance in this respect is

great, that it gives a better chance of beneficial variations arising

and of their being selected, accumulated, and fixed. It likewise tends

to increase the direct action of the physical conditions of life, in

relation to the constitution of each organism.

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 number

of the species inhabiting it is 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 in the world. Hence an oceanic island at first sight seems to have

been highly favourable for the production of new species. But we may

thus 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 isolation is of great importance in the production of new

species, on the whole I am inclined to believe that largeness of area

is still more important, especially for the production of species which

shall 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 much more 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 other forms.

Moreover, great areas, though now continuous, will often, owing to

former oscillations of level, have 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

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, and will

give rise to the greatest number of new varieties and species. They

will thus play a more important part in the changing history of the

organic world.

In accordance with this view, we can, perhaps, understand some facts

which will be again alluded to in our chapter on Geographical

Distribution; for instance, the fact of the productions of the smaller

continent of Australia 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, we can

understand how it is that the flora of Madeira, according to Oswald

Heer, resembles to a certain extent 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. Consequently, the

competition between fresh water productions will have been less severe

than elsewhere; new forms will have been more slowly produced, and old

forms more slowly exterminated. And it is in fresh water basins 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 at present widely separated

in the natural scale. These anomalous forms may be called living

fossils; they have endured to the present day, from having inhabited a

confined area, and from having been exposed to less varied, and

therefore less severe, competition.

To sum up, as far as the extreme intricacy of the subject permits, the

circumstances favourable and unfavourable for the production of new

species through natural selection. I conclude that for terrestrial

productions a large continental area, which has undergone many

oscillations of level, will have been the most favourable for the

production of many new forms of life, fitted to endure for a long time

and to spread widely. While the area existed as a continent the

inhabitants will have been numerous in individuals and kinds, and will

have been subjected to severe competition. When converted by subsidence

into large separate islands there will still have existed many

individuals of the same species on each island: intercrossing on the

confines of the range of each new species will have been checked: after

physical changes of any kind immigration will have been prevented, so

that new places in the polity of each island will have had to be filled

up by the modification of the old inhabitants; and time will have been

allowed for the varieties in each to become well modified and

perfected. When, by renewed elevation, the islands were reconverted

into a continental area, there will again have been very severe

competition; the most favoured or improved varieties will have been

enabled to spread; there will have been much extinction of the less

improved forms, and the relative proportional numbers of the various

inhabitants of the reunited continent will again have been changed; and

again there will have been a fair field for natural selection to

improve still further the inhabitants, and thus to produce new species.

That natural selection generally act with extreme slowness I fully

admit. It can act only when there are places in the natural polity of a

district which can be better occupied by the modification of some of

its existing inhabitants. The occurrence of such places will often

depend on physical changes, which generally take place very slowly, and

on the immigration of better adapted forms being prevented. As some few

of the old inhabitants become modified the mutual relations of others

will often be disturbed; and this will create new places, ready to be

filled up by better adapted forms; but all this will take place very

slowly. Although all the individuals of the same species differ in some

slight degree from each other, it would often be long before

differences of the right nature in various parts of the organisation

might occur. The result would often be greatly retarded by free

intercrossing. Many will exclaim that these several causes are amply

sufficient to neutralise the power of natural selection. I do not

believe so. But I do believe that natural selection will generally act

very slowly, only at long intervals of time, and only on a few of the

inhabitants of the same region. I further believe that these slow,

intermittent results accord well with what geology tells us of the rate

and manner at which the inhabitants of the world have changed.

Slow though the process of selection may be, if feeble man can do much

by artificial selection, I can see no limit to the amount of change, to

the beauty and complexity of the coadaptations between all organic

beings, one with another and with their physical conditions of life,

which may have been effected in the long course of time through

nature’s power of selection, that is by the survival of the fittest.

_Extinction caused by Natural Selection._

This subject will be more fully discussed in our chapter on Geology;

but it must here be 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. Owing to the high geometrical rate of increase of all organic

beings, each area is already fully stocked with inhabitants, and it

follows from this, that as the favoured forms increase in number, so,

generally, will the less favoured decrease and become rare. Rarity, as

geology tells us, is the precursor to extinction. We can see that any

form which is represented by few individuals will run a good chance of

utter extinction, during great fluctuations in the nature or the

seasons, or from a temporary increase in the number of its enemies. But

we may go further than this; for as new forms are produced, unless we

admit that specific forms can go on indefinitely increasing in number,

many old forms must become extinct. That the number of specific forms

has not indefinitely increased, geology plainly tells us; and we shall

presently attempt to show why it is that the number of species

throughout the world has not become immeasurably great.

We have seen that the species which are most numerous in individuals

have the best chance of producing favourable variations within any

given period. We have evidence of this, in the facts stated in the

second chapter, showing that it is the common and diffused or dominant

species which offer the greatest number of recorded varieties. Hence,

rare species will be less quickly modified or improved within any given

period; they will consequently be beaten in the race for life by the

modified and improved 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 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 among 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, 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 far less from each

other 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 a degree of difference as that between the species of the

same genus.

As has always been my practice, I have sought light on this head from

our domestic productions. We shall here find something analogous. It

will be admitted that the production of races so different as

short-horn and Hereford cattle, race and cart horses, the several

breeds of pigeons, &c., could never have been effected by the mere

chance accumulation of similar variations during many successive

generations. In practice, a fancier is, for instance, 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 the

sub-breeds of the tumbler-pigeon) 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 of history, the men of one nation

or district required swifter horses, while those of another required

stronger and bulkier horses. The early differences would be very

slight; but, in the course of time, from the continued selection of

swifter horses in the one case, and of stronger ones in the other, the

differences would become greater, and would be noted as forming two

sub-breeds. Ultimately after the lapse of centuries, these sub-breeds

would become converted into two well-established and distinct breeds.

As the differences became greater, the inferior animals with

intermediate characters, being neither very swift nor very strong,

would not have been used for breeding, and will thus 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 (though it was a long

time before I saw how), 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 discern 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 power of increase be allowed to act, it can succeed in

increasing (the country not undergoing any change in 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 animals become, the more places they

will 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 effect 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 be raised in the latter than in the former case. The

same has been found to hold good when one variety and 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 the varieties were

continually selected which differed from each other in the same manner,

though in a very slight degree, as do the distinct species and genera

of grasses, a greater number of individual plants of this species,

including its modified descendants, would succeed in living on the same

piece of ground. And we know that each species and each variety of

grass is annually sowing almost countless seeds; and is thus striving,

as it may be said, to the utmost to increase in number. Consequently,

in the course of many thousand generations, the most distinct varieties

of any one species of grass would 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 very 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: also in small ponds of fresh water. Farmers find

that they can raise more 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 its nature not to be

in any way peculiar), 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, 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 would succeed 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 naturalised genera, no less than 100 genera are not

there indigenous, and thus a large proportional addition is made to the

genera now living in the United States.

By considering the nature of the plants or animals which have in any

country struggled successfully with the indigenes, and have there

become naturalised, we may gain some crude idea in what manner some of

the natives would have had to be modified in order to gain an advantage

over their compatriots; and we may at least infer that diversification

of structure, amounting to new generic differences, would be profitable

to them.

The advantage of diversification of structure 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-developed orders. In the Australian mammals, we see the

process of diversification in an early and incomplete stage of

development.

_The Probable Effects of the Action of Natural Selection through

Divergence of Character and Extinction, on the Descendants of a Common

Ancestor._

After the foregoing discussion, which has been much compressed, we may

assume that the modified descendants of any one species will succeed 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 benefit being derived from divergence of

character, combined with the principles of natural selection and of

extinction, tends 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 as we saw in the second

chapter, on an average more species vary in large genera than in 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 most widely-diffused, vary more than do the rare and

restricted species. Let (A) be a common, widely-diffused, and varying

species, belonging to a genus large in its own country. The branching

and 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

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 form it into a fairly

well-marked variety, such as would be thought worthy of record in a

systematic work.

[Illustration]

The intervals between the horizontal lines in the diagram, may

represent each a thousand or more generations. After a thousand

generations, species (A) is supposed to have produced two fairly

well-marked varieties, namely _a_1 and _m_1. These two varieties will

generally still be exposed to the same conditions which made their

parents variable, and the tendency to variability is in itself

hereditary; consequently they will likewise tend to vary, and commonly

in nearly the same manner as did their parents. Moreover, these two

varieties, being only slightly modified forms, will tend to inherit

those advantages which made their parent (A) more numerous than most of

the other inhabitants of the same country; they will also partake of

those more general advantages which made the genus to which the

parent-species belonged, a large genus in its own country. And all

these circumstances are 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 _a_2, which will, owing to the

principle of divergence, differ more from (A) than did variety _a_1.

Variety _m_1 is supposed to have produced two varieties, namely _m_2

and _s_2, 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 of 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, nor that it goes on continuously; it is far more

probable that each form remains for long periods unaltered, and then

again undergoes modification. Nor do I suppose that the most divergent

varieties are invariably preserved: 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 increase. 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 allow 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 no doubt the process of modification will be confined to a single

line of descent, and the number of modified descendants will not be

increased; although the amount of divergent modification may have been

augmented. This case would be represented in the diagram, if all the

lines proceeding from (A) were removed, excepting that from _a_1 to

_a_10. In the same way the English racehorse 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, _a_10, _f_10, and _m_10, which, from having

diverged in character during the successive generations, will have come

to differ largely, but perhaps unequally, from each other and from

their common parent. If we suppose the amount of change between each

horizontal line in our diagram to be excessively small, these three

forms may still be only well-marked varieties; 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 doubtful or at

least 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 _a_14 and _m_14, 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 (_w_10 and _z_10) 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 _n_14 to _z_14, are supposed to have been produced. In any

genus, the species which are already very different in character from

each other, will generally tend to produce the greatest number of

modified descendants; for these will have the best chance of seizing on

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 long but unequal periods

continue to transmit unaltered descendants; and this is shown in the

diagram by the dotted lines unequally prolonged upwards.

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 progenitor. 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 states of a the same 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. If,

however, the modified offspring of a species get into some distinct

country, or become quickly adapted to some quite new station, in which

offspring and progenitor 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, being replaced by eight new species (_a_14 to _m_14);

and species (I) will be replaced by six (_n_14 to _z_14) 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, 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 (E and F) 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 _a_14 and _z_14 will be much greater than that between

the most distinct 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 _a_14, _q_14, _p_14,

will be nearly related from having recently branched off from _a_10;

_b_14 and _f_14, from having diverged at an earlier period from _a_5,

will be in some degree distinct from the three first-named species; and

lastly, _o_14, _e_14, and _m_14, 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 a distinct genus.

The six descendants from (I) will form two sub-genera or genera. But as

the original species (I) differed largely from (A), standing nearly at

the extreme end of the original genus, the six descendants from (I)

will, owing to inheritance alone, 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, except (F), extinct, and

have left no descendants. Hence the six new species descended from (I),

and the eight descendants 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 be 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 represents a

species, the supposed progenitor 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. Being descended

from a form that stood between the 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

two 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 call such cases

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

more generations; it may also represent 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, 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 various

remote 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 the diagram, we suppose the

amount of change represented by each successive group of diverging

dotted lines to be great, the forms marked _a_14 to _p_14, those marked

_b_14 and _f_14, and those marked _o_14 to _m_14, will form three very

distinct genera. We shall also have two very distinct genera descended

from (I), differing widely from the descendants of (A). These two

groups of genera will thus form two distinct families, or orders,

according to the amount of divergent modification supposed to be

represented in the diagram. And the two new families, or orders, are

descended from two species of the original genus; and these are

supposed to be descended from some still more ancient and unknown form.

We have seen that in each country it is the species belonging to 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 number, 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 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 have as yet

suffered least extinction, will, for a long period, continue to

increase. But which groups will ultimately prevail, no man can predict;

for we 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 as, according to this view,

extremely few of the more ancient species have transmitted descendants

to the present day, and, as all the descendants of the same species

form a class, we can understand how it is that there exist so few

classes in each main division of the animal and vegetable kingdoms.

Although few of the most ancient species have left modified

descendants, yet, at remote geological periods, the earth may have been

almost as well peopled with species of many genera, families, orders

and classes, as at the present day.

_On the Degree to which Organisation tends to advance._

Natural selection acts exclusively by the preservation and accumulation

of variations, which are beneficial under the organic and inorganic

conditions to which each creature is exposed at all periods of life.

The ultimate result is that each creature tends to become more and more

improved in relation to its conditions. This improvement inevitably

leads to the gradual advancement of the organisation of the greater

number of living beings throughout the world. But here we enter on a

very intricate subject, for naturalists have not defined to each

other’s satisfaction what is meant by an advance in organisation. Among

the vertebrata the degree of intellect and an approach in structure to

man clearly come into play. It might be thought that the amount of

change which the various parts and organs pass through in their

development from embryo to maturity would suffice as a standard of

comparison; but there are cases, as with certain parasitic crustaceans,

in which several parts of the structure become less perfect, so that

the mature animal cannot be called higher than its larva. Von Baer’s

standard seems the most widely applicable and the best, namely, the

amount of differentiation of the parts of the same organic being, in

the adult state, as I should be inclined to add, and their

specialisation for different functions; or, as Milne Edwards would

express it, the completeness of the division of physiological labour.

But we shall see how obscure this subject is if we look, for instance,

to fishes, among which some naturalists rank those as highest which,

like the sharks, approach nearest to amphibians; while other

naturalists rank the common bony or teleostean fishes as the highest,

inasmuch as they are most strictly fish-like, and differ most from the

other vertebrate classes. We see still more plainly the obscurity of

the subject by turning to plants, among which the standard of intellect

is of course quite excluded; and here some botanists rank those plants

as highest which have every organ, as sepals, petals, stamens and

pistils, fully developed in each flower; whereas other botanists,

probably with more truth, look at the plants which have their several

organs much modified and reduced in number as the highest.

If we take as the standard of high organisation, the amount of

differentiation and specialisation of the several organs in each being

when adult (and this will include the advancement of the brain for

intellectual purposes), natural selection clearly leads towards this

standard: for all physiologists admit that the specialisation of

organs, inasmuch as in this state they perform their functions better,

is an advantage to each being; and hence the accumulation of variations

tending towards specialisation is within the scope of natural

selection. On the other hand, we can see, bearing in mind that all

organic beings are striving to increase at a high ratio and to seize on

every unoccupied or less well occupied place in the economy of nature,

that it is quite possible for natural selection gradually to fit a

being to a situation in which several organs would be superfluous or

useless: in such cases there would be retrogression in the scale of

organisation. Whether organisation on the whole has actually advanced

from the remotest geological periods to the present day will be more

conveniently discussed in our chapter on Geological Succession.

But it may be objected that if all organic beings thus tend to rise in

the scale, how is it that throughout the world a multitude of the

lowest forms still exist; and how is it that in each great class some

forms are far more highly developed than others? Why have not the more

highly developed forms every where supplanted and exterminated the

lower? Lamarck, who believed in an innate and inevitable tendency

towards perfection in all organic beings, seems to have felt this

difficulty so strongly that he was led to suppose that new and simple

forms are continually being produced by spontaneous generation. Science

has not as yet proved the truth of this belief, whatever the future may

reveal. On our theory the continued existence of lowly organisms offers

no difficulty; for natural selection, or the survival of the fittest,

does not necessarily include progressive development—it only takes

advantage of such variations as arise and are beneficial to each

creature under its complex relations of life. And it may be asked what

advantage, as far as we can see, would it be to an infusorian

animalcule—to an intestinal worm—or even to an earth-worm, to be highly

organised. If it were no advantage, these forms would be left, by

natural selection, unimproved or but little improved, and might remain

for indefinite ages in their present lowly condition. And geology tells

us that some of the lowest forms, as the infusoria and rhizopods, have

remained for an enormous period in nearly their present state. But to

suppose that most of the many now existing low forms have not in the

least advanced since the first dawn of life would be extremely rash;

for every naturalist who has dissected some of the beings now ranked as

very low in the scale, must have been struck with their really wondrous

and beautiful organisation.

Nearly the same remarks are applicable, if we look to the different

grades of organisation within the same great group; for instance, in

the vertebrata, to the co-existence of mammals and fish—among mammalia,

to the co-existence of man and the ornithorhynchus—among fishes, to the

co-existence of the shark and the lancelet (Amphioxus), which latter

fish in the extreme simplicity of its structure approaches the

invertebrate classes. But mammals and fish hardly come into competition

with each other; the advancement of the whole class of mammals, or of

certain members in this class, to the highest grade would not lead to

their taking the place of fishes. Physiologists believe that the brain

must be bathed by warm blood to be highly active, and this requires

aërial respiration; so that warm-blooded mammals when inhabiting the

water lie under a disadvantage in having to come continually to the

surface to breathe. With fishes, members of the shark family would not

tend to supplant the lancelet; for the lancelet, as I hear from Fritz

Müller, has as sole companion and competitor on the barren sandy shore

of South Brazil, an anomalous annelid. The three lowest orders of

mammals, namely, marsupials, edentata, and rodents, co-exist in South

America in the same region with numerous monkeys, and probably

interfere little with each other. Although organisation, on the whole,

may have advanced and be still advancing throughout the world, yet the

scale will always present many degrees of perfection; for the high

advancement of certain whole classes, or of certain members of each

class, does not at all necessarily lead to the extinction of those

groups with which they do not enter into close competition. In some

cases, as we shall hereafter see, lowly organised forms appear to have

been preserved to the present day, from inhabiting confined or peculiar

stations, where they have been subjected to less severe competition,

and where their scanty numbers have retarded the chance of favourable

variations arising.

Finally, I believe that many lowly organised forms now exist throughout

the world, from various causes. In some cases variations or individual

differences of a favourable nature may never have arisen for natural

selection to act on and accumulate. In no case, probably, has time

sufficed for the utmost possible amount of development. In some few

cases there has been what we must call retrogression or organisation.

But the main cause lies in the fact that under very simple conditions

of life a high organisation would be of no service—possibly would be of

actual disservice, as being of a more delicate nature, and more liable

to be put out of order and injured.

Looking to the first dawn of life, when all organic beings, as we may

believe, presented the simplest structure, how, it has been asked,

could the first step in the advancement or differentiation of parts

have arisen? Mr. Herbert Spencer would probably answer that, as soon as

simple unicellular organisms came by growth or division to be

compounded of several cells, or became attached to any supporting

surface, his law “that homologous units of any order become

differentiated in proportion as their relations to incident forces

become different” would come into action. But as we have no facts to

guide us, speculation on the subject is almost useless. It is, however,

an error to suppose that there would be no struggle for existence, and,

consequently, no natural selection, until many forms had been produced:

variations in a single species inhabiting an isolated station might be

beneficial, and thus the whole mass of individuals might be modified,

or two distinct forms might arise. But, as I remarked towards the close

of the introduction, no one ought to feel surprise at much remaining as

yet unexplained on the origin of species, if we make due allowance for

our profound ignorance on the mutual relations of the inhabitants of

the world at the present time, and still more so during past ages.

_Convergence of Character._

Mr. H.C. Watson thinks that I have overrated the importance of

divergence of character (in which, however, he apparently believes),

and that convergence, as it may be called, has likewise played a part.

If two species belonging to two distinct though allied genera, had both

produced a large number of new and divergent forms, it is conceivable

that these might approach each other so closely that they would have

all to be classed under the same genus; and thus the descendants of two

distinct genera would converge into one. But it would in most cases be

extremely rash to attribute to convergence a close and general

similarity of structure in the modified descendants of widely distinct

forms. The shape of a crystal is determined solely by the molecular

forces, and it is not surprising that dissimilar substances should

sometimes assume the same form; but with organic beings we should bear

in mind that the form of each depends on an infinitude of complex

relations, namely on the variations which have arisen, these being due

to causes far too intricate to be followed out—on the nature of the

variations which have been preserved or selected, and this depends on

the surrounding physical conditions, and in a still higher degree on

the surrounding organisms with which each being has come into

competition—and lastly, on inheritance (in itself a fluctuating

element) from innumerable progenitors, all of which have had their

forms determined through equally complex relations. It is incredible

that the descendants of two organisms, which had originally differed in

a marked manner, should ever afterwards converge so closely as to lead

to a near approach to identity throughout their whole organisation. If

this had occurred, we should meet with the same form, independently of

genetic connection, recurring in widely separated geological

formations; and the balance of evidence is opposed to any such an

admission.

Mr. Watson has also objected that the continued action of natural

selection, together with divergence of character, would tend to make an

indefinite number of specific forms. As far as mere inorganic

conditions are concerned, it seems probable that a sufficient number of

species would soon become adapted to all considerable diversities of

heat, moisture, &c.; but I fully admit that the mutual relations of

organic beings are more important; and as the number of species in any

country goes on increasing, the organic conditions of life must become

more and more complex. Consequently there seems at first no limit to

the amount of profitable diversification of structure, and therefore no

limit to the number of species which might be produced. We do not know

that even the most prolific area is fully stocked with specific forms:

at the Cape of Good Hope and in Australia, which support such an

astonishing number of species, many European plants have become

naturalised. But geology shows us, that from an early part of the

tertiary period the number of species of shells, and that from the

middle part of this same period, the number of mammals has not greatly

or at all increased. What then checks an indefinite increase in the

number of species? The amount of life (I do not mean the number of

specific forms) supported on an area must have a limit, depending so

largely as it does on physical conditions; therefore, if an area be

inhabited by very many species, each or nearly each species will be

represented by few individuals; and such species will be liable to

extermination from accidental fluctuations in the nature of the seasons

or in the number of their enemies. The process of extermination in such

cases would be rapid, whereas the production of new species must always

be slow. Imagine the extreme case of as many species as individuals in

England, and the first severe winter or very dry summer would

exterminate thousands on thousands of species. Rare species, and each

species will become rare if the number of species in any country

becomes indefinitely increased, will, on the principal often explained,

present within a given period few favourable variations; consequently,

the process of giving birth to new specific forms would thus be

retarded. When any species becomes very rare, close interbreeding will

help to exterminate it; authors have thought that this comes into play

in accounting for the deterioration of the aurochs in Lithuania, of red

deer in Scotland and of bears in Norway, &c. Lastly, and this I am

inclined to think is the most important element, a dominant species,

which has already beaten many competitors in its own home, will tend to

spread and supplant many others. Alph. de Candolle has shown that those

species which spread widely tend generally to spread _very_ widely,

consequently they will tend to supplant and exterminate several species

in several areas, and thus check the inordinate increase of specific

forms throughout the world. Dr. Hooker has recently shown that in the

southeast corner of Australia, where, apparently, there are many

invaders from different quarters of the globe, the endemic Australian

species have been greatly reduced in number. How much weight to

attribute to these several considerations I will not pretend to say;

but conjointly they must limit in each country the tendency to an

indefinite augmentation of specific forms.

_Summary of Chapter._

If under changing conditions of life organic beings present individual

differences in almost every part of their structure, and this cannot be

disputed; if there be, owing to their geometrical rate of increase, a

severe struggle for life at some age, season or year, 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 life, causing an infinite diversity in structure,

constitution, and habits, to be advantageous to them, it would be a

most extraordinary fact if no variations had ever occurred useful to

each being’s own welfare, in the same manner as so many variations have

occurred useful to man. But if variations useful to any organic being

ever 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, these will tend to produce offspring

similarly characterised. This principle of preservation, or the

survival of the fittest, I have called Natural Selection. It leads to

the improvement of each creature in relation to its organic and

inorganic conditions of life; and consequently, in most cases, to what

must be regarded as an advance in organisation. Nevertheless, low and

simple forms will long endure if well fitted for their simple

conditions of life.

Natural selection, on the principle of qualities being inherited at

corresponding ages, can modify the egg, seed, or young as easily as the

adult. Among many animals sexual selection will have given 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 or rivalry with

other males; and these characters will be transmitted to one sex or to

both sexes, according to the form of inheritance which prevails.

Whether natural selection has really thus acted in adapting the various

forms of life to their several conditions and stations, must be judged

by the general tenour and balance of evidence given in the following

chapters. But we have already seen 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 the more organic beings diverge in structure, habits and

constitution, by so much the more can a large number be supported on

the area, of which we see proof by looking to the inhabitants of any

small spot, and to the productions naturalised in foreign lands.

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 the descendants become, the better

will be their chance of success in the battle for life. Thus the small

differences distinguishing varieties of the same species, steadily tend

to increase, till they 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 within each class,

which vary most; and these 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, the nature of the affinities, and

the generally well defined distinctions between the innumerable organic

beings in each class throughout the world, 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 groups, subordinate to groups,

in the manner which we everywhere behold—namely, varieties of the same

species most closely related, species of the same genus less closely

and unequally related, 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 clustered round points, and these

round other points, and so on in almost endless cycles. If species had

been independently created, no explanation would have been possible of

this kind of classification; but 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 former years 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 at all times overmastered 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 young, 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 the other

branches; so with the species which lived during long-past geological

periods, very few have left living and modified descendants. From the

first growth of the tree, many a limb and branch has decayed and

dropped off; and these fallen 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 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.