Chapter IV

Natural Selection

Natural Selection -- its power compared with man's selection -- its power

on characters of trifling importance -- its power at all ages and on both

sexes -- Sexual Selection -- On the generality of intercrosses between

individuals of the same species -- Circumstances favourable and

unfavourable to Natural Selection, namely, intercrossing, isolation, number

of individuals -- Slow action -- Extinction caused by Natural Selection --

Divergence of Character, related to the diversity of inhabitants of any

small area, and to naturalisation -- Action of Natural Selection, through

Divergence of Character and Extinction, on the descendants from a common

parent -- Explains the Grouping of all organic beings.

How will the struggle for existence, discussed too briefly in the last

chapter, act in regard to variation? Can the principle of selection, which

we have seen is so potent in the hands of man, apply in nature? I think we

shall see that it can act most effectually. Let it be borne in mind in

what an endless number of strange peculiarities our domestic productions,

and, in a lesser degree, those under nature, vary; and how strong the

hereditary tendency is. Under domestication, it may be truly said that the

whole organisation becomes in some degree plastic. Let it be borne in mind

how infinitely complex and close-fitting are the mutual relations of all

organic beings to each other and to their physical conditions of life. Can

it, then, be thought improbable, seeing that variations useful to man have

undoubtedly occurred, that other variations useful in some way to each

being in the great and complex battle of life, should sometimes occur in

the course of thousands of generations? If such do occur, can we doubt

(remembering that many more individuals are born than can possibly survive)

that individuals having any advantage, however slight, over others, would

have the best chance of surviving and of procreating their kind? On the

other hand, we may feel sure that any variation in the least degree

injurious would be rigidly destroyed. This preservation of favourable

variations and the rejection of injurious variations, I call Natural

Selection. Variations neither useful nor injurious would not be affected

by natural selection, and would be left a fluctuating element, as perhaps

we see in the species called polymorphic.

We shall best understand the probable course of natural selection by taking

the case of a country undergoing some physical change, for instance, of

climate. The proportional numbers of its inhabitants would almost

immediately undergo a change, and some species might become extinct. We

may conclude, from what we have seen of the intimate and complex manner in

which the inhabitants of each country are bound together, that any change

in the numerical proportions of some of the inhabitants, independently of

the change of climate itself, would most seriously affect many of the

others. If the country were open on its borders, new forms would certainly

immigrate, and this also would seriously disturb the relations of some of

the former inhabitants. Let it be remembered how powerful the influence of

a single introduced tree or mammal has been shown to be. But in the case

of an island, or of a country partly surrounded by barriers, into which new

and better adapted forms could not freely enter, we should then have places

in the economy of nature which would assuredly be better filled up, if some

of the original inhabitants were in some manner modified; for, had the area

been open to immigration, these same places would have been seized on by

intruders. In such case, every slight modification, which in the course of

ages chanced to arise, and which in any way favoured the individuals of any

of the species, by better adapting them to their altered conditions, would

tend to be preserved; and natural selection would thus have free scope for

the work of improvement.

We have reason to believe, as stated in the first chapter, that a change in

the conditions of life, by specially acting on the reproductive system,

causes or increases variability; and in the foregoing case the conditions

of life are supposed to have undergone a change, and this would manifestly

be favourable to natural selection, by giving a better chance of profitable

variations occurring; and unless profitable variations do occur, natural

selection can do nothing. Not that, as I believe, any extreme amount of

variability is necessary; as man can certainly produce great results by

adding up in any given direction mere individual differences, so could

Nature, but far more easily, from having incomparably longer time at her

disposal. Nor do I believe that any great physical change, as of climate,

or any unusual degree of isolation to check immigration, is actually

necessary to produce new and unoccupied places for natural selection to

fill up by modifying and improving some of the varying inhabitants. For as

all the inhabitants of each country are struggling together with nicely

balanced forces, extremely slight modifications in the structure or habits

of one inhabitant would often give it an advantage over others; and still

further modifications of the same kind would often still further increase

the advantage. No country can be named in which all the native inhabitants

are now so perfectly adapted to each other and to the physical conditions

under which they live, that none of them could anyhow be improved; for in

all countries, the natives have been so far conquered by naturalised

productions, that they have allowed foreigners to take firm possession of

the land. And as foreigners have thus everywhere beaten some of the

natives, we may safely conclude that the natives might have been modified

with advantage, so as to have better resisted such intruders.

As man can produce and certainly has produced a great result by his

methodical and unconscious means of selection, what may not nature effect?

Man can act only on external and visible characters: nature cares nothing

for appearances, except in so far as they may be useful to any being. She

can act on every internal organ, on every shade of constitutional

difference, on the whole machinery of life. Man selects only for his own

good; Nature only for that of the being which she tends. Every selected

character is fully exercised by her; and the being is placed under

well-suited conditions of life. Man keeps the natives of many climates in

the same country; he seldom exercises each selected character in some

peculiar and fitting manner; he feeds a long and a short beaked pigeon on

the same food; he does not exercise a long-backed or long-legged quadruped

in any peculiar manner; he exposes sheep with long and short wool to the

same climate. He does not allow the most vigorous males to struggle for

the females. He does not rigidly destroy all inferior animals, but

protects during each varying season, as far as lies in his power, all his

productions. He often begins his selection by some half-monstrous form; or

at least by some modification prominent enough to catch his eye, or to be

plainly useful to him. Under nature, the slightest difference of structure

or constitution may well turn the nicely-balanced scale in the struggle for

life, and so be preserved. How fleeting are the wishes and efforts of man!

how short his time! and consequently how poor will his products be,

compared with those accumulated by nature during whole geological periods.

Can we wonder, then, that nature's productions should be far 'truer' in

character than man's productions; that they should be infinitely better

adapted to the most complex conditions of life, and should plainly bear the

stamp of far higher workmanship?

It may be said that natural selection is daily and hourly scrutinising,

throughout the world, every variation, even the slightest; rejecting that

which is bad, preserving and adding up all that is good; silently and

insensibly working, whenever and wherever opportunity offers, at the

improvement of each organic being in relation to its organic and inorganic

conditions of life. We see nothing of these slow changes in progress,

until the hand of time has marked the long lapse of ages, and then so

imperfect is our view into long past geological ages, that we only see that

the forms of life are now different from what they formerly were.

Although natural selection can act only through and for the good of each

being, yet characters and structures, which we are apt to consider as of

very trifling importance, may thus be acted on. When we see leaf-eating

insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in

winter, the red-grouse the colour of heather, and the black-grouse that of

peaty earth, we must believe that these tints are of service to these birds

and insects in preserving them from danger. Grouse, if not destroyed at

some period of their lives, would increase in countless numbers; they are

known to suffer largely from birds of prey; and hawks are guided by

eyesight to their prey,--so much so, that on parts of the Continent persons

are warned not to keep white pigeons, as being the most liable to

destruction. Hence I can see no reason to doubt that natural selection

might be most effective in giving the proper colour to each kind of grouse,

and in keeping that colour, when once acquired, true and constant. Nor

ought we to think that the occasional destruction of an animal of any

particular colour would produce little effect: we should remember how

essential it is in a flock of white sheep to destroy every lamb with the

faintest trace of black. In plants the down on the fruit and the colour of

the flesh are considered by botanists as characters of the most trifling

importance: yet we hear from an excellent horticulturist, Downing, that in

the United States smooth-skinned fruits suffer far more from a beetle, a

curculio, than those with down; that purple plums suffer far more from a

certain disease than yellow plums; whereas another disease attacks

yellow-fleshed peaches far more than those with other coloured flesh. If,

with all the aids of art, these slight differences make a great difference

in cultivating the several varieties, assuredly, in a state of nature,

where the trees would have to struggle with other trees and with a host of

enemies, such differences would effectually settle which variety, whether a

smooth or downy, a yellow or purple fleshed fruit, should succeed.

In looking at many small points of difference between species, which, as

far as our ignorance permits us to judge, seem to be quite unimportant, we

must not forget that climate, food, &c., probably produce some slight and

direct effect. It is, however, far more necessary to bear in mind that

there are many unknown laws of correlation of growth, which, when one part

of the organisation is modified through variation, and the modifications

are accumulated by natural selection for the good of the being, will cause

other modifications, often of the most unexpected nature.

As we see that those variations which under domestication appear at any

particular period of life, tend to reappear in the offspring at the same

period;--for instance, in the seeds of the many varieties of our culinary

and agricultural plants; in the caterpillar and cocoon stages of the

varieties of the silkworm; in the eggs of poultry, and in the colour of the

down of their chickens; in the horns of our sheep and cattle when nearly

adult;--so in a state of nature, natural selection will be enabled to act

on and modify organic beings at any age, by the accumulation of profitable

variations at that age, and by their inheritance at a corresponding age.

If it profit a plant to have its seeds more and more widely disseminated by

the wind, I can see no greater difficulty in this being effected through

natural selection, than in the cotton-planter increasing and improving by

selection the down in the pods on his cotton-trees. Natural selection may

modify and adapt the larva of an insect to a score of contingencies, wholly

different from those which concern the mature insect. These modifications

will no doubt affect, through the laws of correlation, the structure of the

adult; and probably in the case of those insects which live only for a few

hours, and which never feed, a large part of their structure is merely the

correlated result of successive changes in the structure of their larvae.

So, conversely, modifications in the adult will probably often affect the

structure of the larva; but in all cases natural selection will ensure that

modifications consequent on other modifications at a different period of

life, shall not be in the least degree injurious: for if they became so,

they would cause the extinction of the species.

Natural selection will modify the structure of the young in relation to the

parent, and of the parent in relation to the young. In social animals it

will adapt the structure of each individual for the benefit of the

community; if each in consequence profits by the selected change. What

natural selection cannot do, is to modify the structure of one species,

without giving it any advantage, for the good of another species; and

though statements to this effect may be found in works of natural history,

I cannot find one case which will bear investigation. A structure used

only once in an animal's whole life, if of high importance to it, might be

modified to any extent by natural selection; for instance, the great jaws

possessed by certain insects, and used exclusively for opening the

cocoon--or the hard tip to the beak of nestling birds, used for breaking

the egg. It has been asserted, that of the best short-beaked

tumbler-pigeons more perish in the egg than are able to get out of it; so

that fanciers assist in the act of hatching. Now, if nature had to make

the beak of a full-grown pigeon very short for the bird's own advantage,

the process of modification would be very slow, and there would be

simultaneously the most rigorous selection of the young birds within the

egg, which had the most powerful and hardest beaks, for all with weak beaks

would inevitably perish: or, more delicate and more easily broken shells

might be selected, the thickness of the shell being known to vary like

every other structure.

Sexual Selection. -- Inasmuch as peculiarities often appear under

domestication in one sex and become hereditarily attached to that sex, the

same fact probably occurs under nature, and if so, natural selection will

be able to modify one sex in its functional relations to the other sex, or

in relation to wholly different habits of life in the two sexes, as is

sometimes the case with insects. And this leads me to say a few words on

what I call Sexual Selection. This depends, not on a struggle for

existence, but on a struggle between the males for possession of the

females; the result is not death to the unsuccessful competitor, but few or

no offspring. Sexual selection is, therefore, less rigorous than natural

selection. Generally, the most vigorous males, those which are best fitted

for their places in nature, will leave most progeny. But in many cases,

victory will depend not on general vigour, but on having special weapons,

confined to the male sex. A hornless stag or spurless cock would have a

poor chance of leaving offspring. Sexual selection by always allowing the

victor to breed might surely give indomitable courage, length to the spur,

and strength to the wing to strike in the spurred leg, as well as the

brutal cock-fighter, who knows well that he can improve his breed by

careful selection of the best cocks. How low in the scale of nature this

law of battle descends, I know not; male alligators have been described as

fighting, bellowing, and whirling round, like Indians in a war-dance, for

the possession of the females; male salmons have been seen fighting all day

long; male stag-beetles often bear wounds from the huge mandibles of other

males. The war is, perhaps, severest between the males of polygamous

animals, and these seem oftenest provided with special weapons. The males

of carnivorous animals are already well armed; though to them and to

others, special means of defence may be given through means of sexual

selection, as the mane to the lion, the shoulder-pad to the boar, and the

hooked jaw to the male salmon; for the shield may be as important for

victory, as the sword or spear.

Amongst birds, the contest is often of a more peaceful character. All

those who have attended to the subject, believe that there is the severest

rivalry between the males of many species to attract by singing the

females. The rock-thrush of Guiana, birds of Paradise, and some others,

congregate; and successive males display their gorgeous plumage and perform

strange antics before the females, which standing by as spectators, at last

choose the most attractive partner. Those who have closely attended to

birds in confinement well know that they often take individual preferences

and dislikes: thus Sir R. Heron has described how one pied peacock was

eminently attractive to all his hen birds. It may appear childish to

attribute any effect to such apparently weak means: I cannot here enter on

the details necessary to support this view; but if man can in a short time

give elegant carriage and beauty to his bantams, according to his standard

of beauty, I can see no good reason to doubt that female birds, by

selecting, during thousands of generations, the most melodious or beautiful

males, according to their standard of beauty, might produce a marked

effect. I strongly suspect that some well-known laws with respect to the

plumage of male and female birds, in comparison with the plumage of the

young, can be explained on the view of plumage having been chiefly modified

by sexual selection, acting when the birds have come to the breeding age or

during the breeding season; the modifications thus produced being inherited

at corresponding ages or seasons, either by the males alone, or by the

males and females; but I have not space here to enter on this subject.

Thus it is, as I believe, that when the males and females of any animal

have the same general habits of life, but differ in structure, colour, or

ornament, such differences have been mainly caused by sexual selection;

that is, individual males have had, in successive generations, some slight

advantage over other males, in their weapons, means of defence, or charms;

and have transmitted these advantages to their male offspring. Yet, I

would not wish to attribute all such sexual differences to this agency:

for we see peculiarities arising and becoming attached to the male sex in

our domestic animals (as the wattle in male carriers, horn-like

protuberances in the cocks of certain fowls, &c.), which we cannot believe

to be either useful to the males in battle, or attractive to the females.

We see analogous cases under nature, for instance, the tuft of hair on the

breast of the turkey-cock, which can hardly be either useful or ornamental

to this bird;--indeed, had the tuft appeared under domestication, it would

have been called a monstrosity.

Illustrations of the action of Natural Selection. -- In order to make it

clear how, as I believe, natural selection acts, I must beg permission to

give one or two imaginary illustrations. Let us take the case of a wolf,

which preys on various animals, securing some by craft, some by strength,

and some by fleetness; and let us suppose that the fleetest prey, a deer

for instance, had from any change in the country increased in numbers, or

that other prey had decreased in numbers, during that season of the year

when the wolf is hardest pressed for food. I can under such circumstances

see no reason to doubt that the swiftest and slimmest wolves would have the

best chance of surviving, and so be preserved or selected,--provided always

that they retained strength to master their prey at this or at some other

period of the year, when they might be compelled to prey on other animals.

I can see no more reason to doubt this, than that man can improve the

fleetness of his greyhounds by careful and methodical selection, or by that

unconscious selection which results from each man trying to keep the best

dogs without any thought of modifying the breed.

Even without any change in the proportional numbers of the animals on which

our wolf preyed, a cub might be born with an innate tendency to pursue

certain kinds of prey. Nor can this be thought very improbable; for we

often observe great differences in the natural tendencies of our domestic

animals; one cat, for instance, taking to catch rats, another mice; one

cat, according to Mr. St. John, bringing home winged game, another hares or

rabbits, and another hunting on marshy ground and almost nightly catching

woodcocks or snipes. The tendency to catch rats rather than mice is known

to be inherited. Now, if any slight innate change of habit or of structure

benefited an individual wolf, it would have the best chance of surviving

and of leaving offspring. Some of its young would probably inherit the

same habits or structure, and by the repetition of this process, a new

variety might be formed which would either supplant or coexist with the

parent-form of wolf. Or, again, the wolves inhabiting a mountainous

district, and those frequenting the lowlands, would naturally be forced to

hunt different prey; and from the continued preservation of the individuals

best fitted for the two sites, two varieties might slowly be formed. These

varieties would cross and blend where they met; but to this subject of

intercrossing we shall soon have to return. I may add, that, according to

Mr. Pierce, there are two varieties of the wolf inhabiting the Catskill

Mountains in the United States, one with a light greyhound-like form, which

pursues deer, and the other more bulky, with shorter legs, which more

frequently attacks the shepherd's flocks.

Let us now take a more complex case. Certain plants excrete a sweet juice,

apparently for the sake of eliminating something injurious from their sap:

this is effected by glands at the base of the stipules in some Leguminosae,

and at the back of the leaf of the common laurel. This juice, though small

in quantity, is greedily sought by insects. Let us now suppose a little

sweet juice or nectar to be excreted by the inner bases of the petals of a

flower. In this case insects in seeking the nectar would get dusted with

pollen, and would certainly often transport the pollen from one flower to

the stigma of another flower. The flowers of two distinct individuals of

the same species would thus get crossed; and the act of crossing, we have

good reason to believe (as will hereafter be more fully alluded to), would

produce very vigorous seedlings, which consequently would have the best

chance of flourishing and surviving. Some of these seedlings would

probably inherit the nectar-excreting power. Those individual flowers

which had the largest glands or nectaries, and which excreted most nectar,

would be oftenest visited by insects, and would be oftenest crossed; and so

in the long-run would gain the upper hand. Those flowers, also, which had

their stamens and pistils placed, in relation to the size and habits of the

particular insects which visited them, so as to favour in any degree the

transportal of their pollen from flower to flower, would likewise be

favoured or selected. We might have taken the case of insects visiting

flowers for the sake of collecting pollen instead of nectar; and as pollen

is formed for the sole object of fertilisation, its destruction appears a

simple loss to the plant; yet if a little pollen were carried, at first

occasionally and then habitually, by the pollen-devouring insects from

flower to flower, and a cross thus effected, although nine-tenths of the

pollen were destroyed, it might still be a great gain to the plant; and

those individuals which produced more and more pollen, and had larger and

larger anthers, would be selected.

When our plant, by this process of the continued preservation or natural

selection of more and more attractive flowers, had been rendered highly

attractive to insects, they would, unintentionally on their part, regularly

carry pollen from flower to flower; and that they can most effectually do

this, I could easily show by many striking instances. I will give only

one--not as a very striking case, but as likewise illustrating one step in

the separation of the sexes of plants, presently to be alluded to. Some

holly-trees bear only male flowers, which have four stamens producing

rather a small quantity of pollen, and a rudimentary pistil; other

holly-trees bear only female flowers; these have a full-sized pistil, and

four stamens with shrivelled anthers, in which not a grain of pollen can be

detected. Having found a female tree exactly sixty yards from a male tree,

I put the stigmas of twenty flowers, taken from different branches, under

the microscope, and on all, without exception, there were pollen-grains,

and on some a profusion of pollen. As the wind had set for several days

from the female to the male tree, the pollen could not thus have been

carried. The weather had been cold and boisterous, and therefore not

favourable to bees, nevertheless every female flower which I examined had

been effectually fertilised by the bees, accidentally dusted with pollen,

having flown from tree to tree in search of nectar. But to return to our

imaginary case: as soon as the plant had been rendered so highly

attractive to insects that pollen was regularly carried from flower to

flower, another process might commence. No naturalist doubts the advantage

of what has been called the 'physiological division of labour;' hence we

may believe that it would be advantageous to a plant to produce stamens

alone in one flower or on one whole plant, and pistils alone in another

flower or on another plant. In plants under culture and placed under new

conditions of life, sometimes the male organs and sometimes the female

organs become more or less impotent; now if we suppose this to occur in

ever so slight a degree under nature, then as pollen is already carried

regularly from flower to flower, and as a more complete separation of the

sexes of our plant would be advantageous on the principle of the division

of labour, individuals with this tendency more and more increased, would be

continually favoured or selected, until at last a complete separation of

the sexes would be effected.

Let us now turn to the nectar-feeding insects in our imaginary case: we

may suppose the plant of which we have been slowly increasing the nectar by

continued selection, to be a common plant; and that certain insects

depended in main part on its nectar for food. I could give many facts,

showing how anxious bees are to save time; for instance, their habit of

cutting holes and sucking the nectar at the bases of certain flowers, which

they can, with a very little more trouble, enter by the mouth. Bearing

such facts in mind, I can see no reason to doubt that an accidental

deviation in the size and form of the body, or in the curvature and length

of the proboscis, &c., far too slight to be appreciated by us, might profit

a bee or other insect, so that an individual so characterised would be able

to obtain its food more quickly, and so have a better chance of living and

leaving descendants. Its descendants would probably inherit a tendency to

a similar slight deviation of structure. The tubes of the corollas of the

common red and incarnate clovers (Trifolium pratense and incarnatum) do not

on a hasty glance appear to differ in length; yet the hive-bee can easily

suck the nectar out of the incarnate clover, but not out of the common red

clover, which is visited by humble-bees alone; so that whole fields of the

red clover offer in vain an abundant supply of precious nectar to the

hive-bee. Thus it might be a great advantage to the hive-bee to have a

slightly longer or differently constructed proboscis. On the other hand, I

have found by experiment that the fertility of clover greatly depends on

bees visiting and moving parts of the corolla, so as to push the pollen on

to the stigmatic surface. Hence, again, if humble-bees were to become rare

in any country, it might be a great advantage to the red clover to have a

shorter or more deeply divided tube to its corolla, so that the hive-bee

could visit its flowers. Thus I can understand how a flower and a bee

might slowly become, either simultaneously or one after the other, modified

and adapted in the most perfect manner to each other, by the continued

preservation of individuals presenting mutual and slightly favourable

deviations of structure.

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

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

first urged against Sir Charles Lyell's noble views on 'the modern changes

of the earth, as illustrative of geology;' but we now very seldom hear the

action, for instance, of the coast-waves, called a trifling and

insignificant cause, when applied to the excavation of gigantic valleys or

to the formation of the longest lines of inland cliffs. Natural selection

can act only by the preservation and accumulation of infinitesimally small

inherited modifications, each profitable to the preserved being; and as

modern geology has almost banished such views as the excavation of a great

valley by a single diluvial wave, so will natural selection, if it be a

true principle, banish the belief of the continued creation of new organic

beings, or of any great and sudden modification in their structure.

On the Intercrossing of Individuals. -- I must here introduce a short

digression. In the case of animals and plants with separated sexes, it is

of course obvious that two individuals must always unite for each birth;

but in the case of hermaphrodites this is far from obvious. Nevertheless I

am strongly inclined to believe that with all hermaphrodites two

individuals, either occasionally or habitually, concur for the reproduction

of their kind. This view, I may add, was first suggested by Andrew Knight.

We shall presently see its importance; but I must here treat the subject

with extreme brevity, though I have the materials prepared for an ample

discussion. All vertebrate animals, all insects, and some other large

groups of animals, pair for each birth. Modern research has much

diminished the number of supposed hermaphrodites, and of real

hermaphrodites a large number pair; that is, two individuals regularly

unite for reproduction, which is all that concerns us. But still there are

many hermaphrodite animals which certainly do not habitually pair, and a

vast majority of plants are hermaphrodites. What reason, it may be asked,

is there for supposing in these cases that two individuals ever concur in

reproduction? As it is impossible here to enter on details, I must trust

to some general considerations alone.

In the first place, I have collected so large a body of facts, showing, in

accordance with the almost universal belief of breeders, that with animals

and plants a cross between different varieties, or between individuals of

the same variety but of another strain, gives vigour and fertility to the

offspring; and on the other hand, that close interbreeding diminishes

vigour and fertility; that these facts alone incline me to believe that it

is a general law of nature (utterly ignorant though we be of the meaning of

the law) that no organic being self-fertilises itself for an eternity of

generations; but that a cross with another individual is

occasionally--perhaps at very long intervals--indispensable.

On the belief that this is a law of nature, we can, I think, understand

several large classes of facts, such as the following, which on any other

view are inexplicable. Every hybridizer knows how unfavourable exposure to

wet is to the fertilisation of a flower, yet what a multitude of flowers

have their anthers and stigmas fully exposed to the weather! but if an

occasional cross be indispensable, the fullest freedom for the entrance of

pollen from another individual will explain this state of exposure, more

especially as the plant's own anthers and pistil generally stand so close

together that self-fertilisation seems almost inevitable. Many flowers, on

the other hand, have their organs of fructification closely enclosed, as in

the great papilionaceous or pea-family; but in several, perhaps in all,

such flowers, there is a very curious adaptation between the structure of

the flower and the manner in which bees suck the nectar; for, in doing

this, they either push the flower's own pollen on the stigma, or bring

pollen from another flower. So necessary are the visits of bees to

papilionaceous flowers, that I have found, by experiments published

elsewhere, that their fertility is greatly diminished if these visits be

prevented. Now, it is scarcely possible that bees should fly from flower

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

as I believe, of the plant. Bees will act like a camel-hair pencil, and it

is quite sufficient just to touch the anthers of one flower and then the

stigma of another with the same brush to ensure fertilisation; but it must

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

distinct species; for if you bring on the same brush a plant's own pollen

and pollen from another species, the former will have such a prepotent

effect, that it will invariably and completely destroy, as has been shown

by Gartner, any influence from the foreign pollen.

When the stamens of a flower suddenly spring towards the pistil, or slowly

move one after the other towards it, the contrivance seems adapted solely

to ensure self-fertilisation; and no doubt it is useful for this end: but,

the agency of insects is often required to cause the stamens to spring

forward, as Kolreuter has shown to be the case with the barberry; and

curiously in this very genus, which seems to have a special contrivance for

self-fertilisation, it is well known that if very closely-allied forms or

varieties are planted near each other, it is hardly possible to raise pure

seedlings, so largely do they naturally cross. In many other cases, far

from there being any aids for self-fertilisation, there are special

contrivances, as I could show from the writings of C. C. Sprengel and from

my own observations, which effectually prevent the stigma receiving pollen

from its own flower: for instance, in Lobelia fulgens, there is a really

beautiful and elaborate contrivance by which every one of the infinitely

numerous pollen-granules are swept out of the conjoined anthers of each

flower, before the stigma of that individual flower is ready to receive

them; and as this flower is never visited, at least in my garden, by

insects, it never sets a seed, though by placing pollen from one flower on

the stigma of another, I raised plenty of seedlings; and whilst another

species of Lobelia growing close by, which is visited by bees, seeds

freely. In very many other cases, though there be no special mechanical

contrivance to prevent the stigma of a flower receiving its own pollen,

yet, as C. C. Sprengel has shown, and as I can confirm, either the anthers

burst before the stigma is ready for fertilisation, or the stigma is ready

before the pollen of that flower is ready, so that these plants have in

fact separated sexes, and must habitually be crossed. How strange are

these facts! How strange that the pollen and stigmatic surface of the same

flower, though placed so close together, as if for the very purpose of

self-fertilisation, should in so many cases be mutually useless to each

other! How simply are these facts explained on the view of an occasional

cross with a distinct individual being advantageous or indispensable!

If several varieties of the cabbage, radish, onion, and of some other

plants, be allowed to seed near each other, a large majority, as I have

found, of the seedlings thus raised will turn out mongrels: for instance,

I raised 233 seedling cabbages from some plants of different varieties

growing near each other, and of these only 78 were true to their kind, and

some even of these were not perfectly true. Yet the pistil of each

cabbage-flower is surrounded not only by its own six stamens, but by those

of the many other flowers on the same plant. How, then, comes it that such

a vast number of the seedlings are mongrelized? I suspect that it must

arise from the pollen of a distinct variety having a prepotent effect over

a flower's own pollen; and that this is part of the general law of good

being derived from the intercrossing of distinct individuals of the same

species. When distinct species are crossed the case is directly the

reverse, for a plant's own pollen is always prepotent over foreign pollen;

but to this subject we shall return in a future chapter.

In the case of a gigantic tree covered with innumerable flowers, it may be

objected that pollen could seldom be carried from tree to tree, and at most

only from flower to flower on the same tree, and that flowers on the same

tree can be considered as distinct individuals only in a limited sense. I

believe this objection to be valid, but that nature has largely provided

against it by giving to trees a strong tendency to bear flowers with

separated sexes. When the sexes are separated, although the male and

female flowers may be produced on the same tree, we can see that pollen

must be regularly carried from flower to flower; and this will give a

better chance of pollen being occasionally carried from tree to tree. That

trees belonging to all Orders have their sexes more often separated than

other plants, I find to be the case in this country; and at my request Dr.

Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those of the

United States, and the result was as I anticipated. On the other hand, Dr.

Hooker has recently informed me that he finds that the rule does not hold

in Australia; and I have made these few remarks on the sexes of trees

simply to call attention to the subject.

Turning for a very brief space to animals: on the land there are some

hermaphrodites, as land-mollusca and earth-worms; but these all pair. As

yet I have not found a single case of a terrestrial animal which fertilises

itself. We can understand this remarkable fact, which offers so strong a

contrast with terrestrial plants, on the view of an occasional cross being

indispensable, by considering the medium in which terrestrial animals live,

and the nature of the fertilising element; for we know of no means,

analogous to the action of insects and of the wind in the case of plants,

by which an occasional cross could be effected with terrestrial animals

without the concurrence of two individuals. Of aquatic animals, there are

many self-fertilising hermaphrodites; but here currents in the water offer

an obvious means for an occasional cross. And, as in the case of flowers,

I have as yet failed, after consultation with one of the highest

authorities, namely, Professor Huxley, to discover a single case of an

hermaphrodite animal with the organs of reproduction so perfectly enclosed

within the body, that access from without and the occasional influence of a

distinct individual can be shown to be physically impossible. Cirripedes

long appeared to me to present a case of very great difficulty under this

point of view; but I have been enabled, by a fortunate chance, elsewhere to

prove that two individuals, though both are self-fertilising

hermaphrodites, do sometimes cross.

It must have struck most naturalists as a strange anomaly that, in the case

of both animals and plants, species of the same family and even of the same

genus, though agreeing closely with each other in almost their whole

organisation, yet are not rarely, some of them hermaphrodites, and some of

them unisexual. But if, in fact, all hermaphrodites do occasionally

intercross with other individuals, the difference between hermaphrodites

and unisexual species, as far as function is concerned, becomes very small.

From these several considerations and from the many special facts which I

have collected, but which I am not here able to give, I am strongly

inclined to suspect that, both in the vegetable and animal kingdoms, an

occasional intercross with a distinct individual is a law of nature. I am

well aware that there are, on this view, many cases of difficulty, some of

which I am trying to investigate. Finally then, we may conclude that in

many organic beings, a cross between two individuals is an obvious

necessity for each birth; in many others it occurs perhaps only at long

intervals; but in none, as I suspect, can self-fertilisation go on for

perpetuity.

Circumstances favourable to Natural Selection. -- This is an extremely

intricate subject. A large amount of inheritable and diversified

variability is favourable, but I believe mere individual differences

suffice for the work. A large number of individuals, by giving a better

chance for the appearance within any given period of profitable variations,

will compensate for a lesser amount of variability in each individual, and

is, I believe, an extremely important element of success. Though nature

grants vast periods of time for the work of natural selection, she does not

grant an indefinite period; for as all organic beings are striving, it may

be said, to seize on each place in the economy of nature, if any one

species does not become modified and improved in a corresponding degree

with its competitors, it will soon be exterminated.

In man's methodical selection, a breeder selects for some definite object,

and free intercrossing will wholly stop his work. But when many men,

without intending to alter the breed, have a nearly common standard of

perfection, and all try to get and breed from the best animals, much

improvement and modification surely but slowly follow from this unconscious

process of selection, notwithstanding a large amount of crossing with

inferior animals. Thus it will be in nature; for within a confined area,

with some place in its polity not so perfectly occupied as might be,

natural selection will always tend to preserve all the individuals varying

in the right direction, though in different degrees, so as better to fill

up the unoccupied place. But if the area be large, its several districts

will almost certainly present different conditions of life; and then if

natural selection be modifying and improving a species in the several

districts, there will be intercrossing with the other individuals of the

same species on the confines of each. And in this case the effects of

intercrossing can hardly be counterbalanced by natural selection always

tending to modify all the individuals in each district in exactly the same

manner to the conditions of each; for in a continuous area, the conditions

will generally graduate away insensibly from one district to another. The

intercrossing will most affect those animals which unite for each birth,

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

animals of this nature, for instance in birds, varieties will generally be

confined to separated countries; and this I believe to be the case. In

hermaphrodite organisms which cross only occasionally, and likewise in

animals which unite for each birth, but which wander little and which can

increase at a very rapid rate, a new and improved variety might be quickly

formed on any one spot, and might there maintain itself in a body, so that

whatever intercrossing took place would be chiefly between the individuals

of the same new variety. A local variety when once thus formed might

subsequently slowly spread to other districts. On the above principle,

nurserymen always prefer getting seed from a large body of plants of the

same variety, as the chance of intercrossing with other varieties is thus

lessened.

Even in the case of slow-breeding animals, which unite for each birth, we

must not overrate the effects of intercrosses in retarding natural

selection; for I can bring a considerable catalogue of facts, showing that

within the same area, varieties of the same animal can long remain

distinct, from haunting different stations, from breeding at slightly

different seasons, or from varieties of the same kind preferring to pair

together.

Intercrossing plays a very important part in nature in keeping the

individuals of the same species, or of the same variety, true and uniform

in character. It will obviously thus act far more efficiently with those

animals which unite for each birth; but I have already attempted to show

that we have reason to believe that occasional intercrosses take place with

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

intervals, I am convinced that the young thus produced will gain so much in

vigour and fertility over the offspring from long-continued

self-fertilisation, that they will have a better chance of surviving and

propagating their kind; and thus, in the long run, the influence of

intercrosses, even at rare intervals, will be great. If there exist

organic beings which never intercross, uniformity of character can be

retained amongst them, as long as their conditions of life remain the same,

only through the principle of inheritance, and through natural selection

destroying any which depart from the proper type; but if their conditions

of life change and they undergo modification, uniformity of character can

be given to their modified offspring, solely by natural selection

preserving the same favourable variations.

Isolation, also, is an important element in the process of natural

selection. In a confined or isolated area, if not very large, the organic

and inorganic conditions of life will generally be in a great degree

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

of a varying species throughout the area in the same manner in relation to

the same conditions. Intercrosses, also, with the individuals of the same

species, which otherwise would have inhabited the surrounding and

differently circumstanced districts, will be prevented. But isolation

probably acts more efficiently in checking the immigration of better

adapted organisms, after any physical change, such as of climate or

elevation of the land, &c.; and thus new places in the natural economy of

the country are left open for the old inhabitants to struggle for, and

become adapted to, through modifications in their structure and

constitution. Lastly, isolation, by checking immigration and consequently

competition, will give time for any new variety to be slowly improved; and

this may sometimes be of importance in the production of new species. If,

however, an isolated area be very small, either from being surrounded by

barriers, or from having very peculiar physical conditions, the total

number of the individuals supported on it will necessarily be very small;

and fewness of individuals will greatly retard the production of new

species through natural selection, by decreasing the chance of the

appearance of favourable variations.

If we turn to nature to test the truth of these remarks, and look at any

small isolated area, such as an oceanic island, although the total number

of the species inhabiting it, will be found to be small, as we shall see in

our chapter on geographical distribution; yet of these species a very large

proportion are endemic,--that is, have been produced there, and nowhere

else. Hence an oceanic island at first sight seems to have been highly

favourable for the production of new species. But we may thus greatly

deceive ourselves, for to ascertain whether a small isolated area, or a

large open area like a continent, has been most favourable for the

production of new organic forms, we ought to make the comparison within

equal times; and this we are incapable of doing.

Although I do not doubt that isolation is of considerable importance in the

production of new species, on the whole I am inclined to believe that

largeness of area is of more importance, more especially in the production

of species, which will prove capable of enduring for a long period, and of

spreading widely. Throughout a great and open area, not only will there be

a better chance of favourable variations arising from the large number of

individuals of the same species there supported, but the conditions of life

are infinitely complex from the large number of already existing species;

and if some of these many species become modified and improved, others will

have to be improved in a corresponding degree or they will be exterminated.

Each new form, also, as soon as it has been much improved, will be able to

spread over the open and continuous area, and will thus come into

competition with many others. Hence more new places will be formed, and

the competition to fill them will be more severe, on a large than on a

small and isolated area. Moreover, great areas, though now continuous,

owing to oscillations of level, will often have recently existed in a

broken condition, so that the good effects of isolation will generally, to

a certain extent, have concurred. Finally, I conclude that, although small

isolated areas probably have been in some respects highly favourable for

the production of new species, yet that the course of modification will

generally have been more rapid on large areas; and what is more important,

that the new forms produced on large areas, which already have been

victorious over many competitors, will be those that will spread most

widely, will give rise to most new varieties and species, and will thus

play an important part in the changing history of the organic world.

We can, perhaps, on these views, understand some facts which will be again

alluded to in our chapter on geographical distribution; for instance, that

the productions of the smaller continent of Australia have formerly

yielded, and apparently are now yielding, before those of the larger

Europaeo-Asiatic area. Thus, also, it is that continental productions have

everywhere become so largely naturalised on islands. On a small island,

the race for life will have been less severe, and there will have been less

modification and less extermination. Hence, perhaps, it comes that the

flora of Madeira, according to Oswald Heer, resembles the extinct tertiary

flora of Europe. All fresh-water basins, taken together, make a small area

compared with that of the sea or of the land; and, consequently, the

competition between fresh-water productions will have been less severe than

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

slowly exterminated. And it is in fresh water that we find seven genera of

Ganoid fishes, remnants of a once preponderant order: and in fresh water

we find some of the most anomalous forms now known in the world, as the

Ornithorhynchus and Lepidosiren, which, like fossils, connect to a certain

extent orders now widely separated in the natural scale. These anomalous

forms may almost be called living fossils; they have endured to the present

day, from having inhabited a confined area, and from having thus been

exposed to less severe competition.

To sum up the circumstances favourable and unfavourable to natural

selection, as far as the extreme intricacy of the subject permits. I

conclude, looking to the future, that for terrestrial productions a large

continental area, which will probably undergo many oscillations of level,

and which consequently will exist for long periods in a broken condition,

will be the most favourable for the production of many new forms of life,

likely to endure long and to spread widely. For the area will first have

existed as a continent, and the inhabitants, at this period numerous in

individuals and kinds, will have been subjected to very severe competition.

When converted by subsidence into large separate islands, there will still

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

on the confines of the range of each species will thus be checked: after

physical changes of any kind, immigration will be prevented, so that new

places in the polity of each island will have to be filled up by

modifications of the old inhabitants; and time will be allowed for the

varieties in each to become well modified and perfected. When, by renewed

elevation, the islands shall be re-converted into a continental area, there

will again be severe competition: the most favoured or improved varieties

will be enabled to spread: there will be much extinction of the less

improved forms, and the relative proportional numbers of the various

inhabitants of the renewed continent will again be changed; and again there

will be a fair field for natural selection to improve still further the

inhabitants, and thus produce new species.

That natural selection will always act with extreme slowness, I fully

admit. Its action depends on there being places in the polity of nature,

which can be better occupied by some of the inhabitants of the country

undergoing modification of some kind. The existence of such places will

often depend on physical changes, which are generally very slow, and on the

immigration of better adapted forms having been checked. But the action of

natural selection will probably still oftener depend on some of the

inhabitants becoming slowly modified; the mutual relations of many of the

other inhabitants being thus disturbed. Nothing can be effected, unless

favourable variations occur, and variation itself is apparently always a

very slow process. The process will often be greatly retarded by free

intercrossing. Many will exclaim that these several causes are amply

sufficient wholly to stop the action of natural selection. I do not

believe so. On the other hand, I do believe that natural selection will

always act very slowly, often only at long intervals of time, and generally

on only a very few of the inhabitants of the same region at the same time.

I further believe, that this very slow, intermittent action of natural

selection accords perfectly well with what geology tells us of the rate and

manner at which the inhabitants of this world have changed.

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

his powers of artificial selection, I can see no limit to the amount of

change, to the beauty and infinite complexity of the coadaptations between

all organic beings, one with another and with their physical conditions of

life, which may be effected in the long course of time by nature's power of

selection.

Extinction. -- This subject will be more fully discussed in our chapter on

Geology; but it must be here alluded to from being intimately connected

with natural selection. Natural selection acts solely through the

preservation of variations in some way advantageous, which consequently

endure. But as from the high geometrical powers of increase of all organic

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

that as each selected and favoured form increases in number, so will the

less favoured forms decrease and become rare. Rarity, as geology tells us,

is the precursor to extinction. We can, also, see that any form

represented by few individuals will, during fluctuations in the seasons or

in the number of its enemies, run a good chance of utter extinction. But

we may go further than this; for as new forms are continually and slowly

being produced, unless we believe that the number of specific forms goes on

perpetually and almost indefinitely increasing, numbers inevitably must

become extinct. That the number of specific forms has not indefinitely

increased, geology shows us plainly; and indeed we can see reason why they

should not have thus increased, for the number of places in the polity of

nature is not indefinitely great,--not that we have any means of knowing

that any one region has as yet got its maximum of species. Probably no

region is as yet fully stocked, for at the Cape of Good Hope, where more

species of plants are crowded together than in any other quarter of the

world, some foreign plants have become naturalised, without causing, as far

as we know, the extinction of any natives.

Furthermore, the species which are most numerous in individuals will have

the best chance of producing within any given period favourable variations.

We have evidence of this, in the facts given in the second chapter, showing

that it is the common species which afford the greatest number of recorded

varieties, or incipient species. Hence, rare species will be less quickly

modified or improved within any given period, and they will consequently be

beaten in the race for life by the modified descendants of the commoner

species.

From these several considerations I think it inevitably follows, that as

new species in the course of time are formed through natural selection,

others will become rarer and rarer, and finally extinct. The forms which

stand in closest competition with those undergoing modification and

improvement, will naturally suffer most. And we have seen in the chapter

on the Struggle for Existence that it is the most closely-allied

forms,--varieties of the same species, and species of the same genus or of

related genera,--which, from having nearly the same structure,

constitution, and habits, generally come into the severest competition with

each other. Consequently, each new variety or species, during the progress

of its formation, will generally press hardest on its nearest kindred, and

tend to exterminate them. We see the same process of extermination amongst

our domesticated productions, through the selection of improved forms by

man. Many curious instances could be given showing how quickly new breeds

of cattle, sheep, and other animals, and varieties of flowers, take the

place of older and inferior kinds. In Yorkshire, it is historically known

that the ancient black cattle were displaced by the long-horns, and that

these 'were swept away by the short-horns' (I quote the words of an

agricultural writer) 'as if by some murderous pestilence.'

Divergence of Character. -- The principle, which I have designated by this

term, is of high importance on my theory, and explains, as I believe,

several important facts. In the first place, varieties, even

strongly-marked ones, though having somewhat of the character of

species--as is shown by the hopeless doubts in many cases how to rank

them--yet certainly differ from each other far less than do good and

distinct species. Nevertheless, according to my view, varieties are

species in the process of formation, or are, as I have called them,

incipient species. How, then, does the lesser difference between varieties

become augmented into the greater difference between species? That this

does habitually happen, we must infer from most of the innumerable species

throughout nature presenting well-marked differences; whereas varieties,

the supposed prototypes and parents of future well-marked species, present

slight and ill-defined differences. Mere chance, as we may call it, might

cause one variety to differ in some character from its parents, and the

offspring of this variety again to differ from its parent in the very same

character and in a greater degree; but this alone would never account for

so habitual and large an amount of difference as that between varieties of

the same species and species of the same genus.

As has always been my practice, let us seek light on this head from our

domestic productions. We shall here find something analogous. A fancier

is struck by a pigeon having a slightly shorter beak; another fancier is

struck by a pigeon having a rather longer beak; and on the acknowledged

principle that 'fanciers do not and will not admire a medium standard, but

like extremes,' they both go on (as has actually occurred with

tumbler-pigeons) choosing and breeding from birds with longer and longer

beaks, or with shorter and shorter beaks. Again, we may suppose that at an

early period one man preferred swifter horses; another stronger and more

bulky horses. The early differences would be very slight; in the course of

time, from the continued selection of swifter horses by some breeders, and

of stronger ones by others, the differences would become greater, and would

be noted as forming two sub-breeds; finally, after the lapse of centuries,

the sub-breeds would become converted into two well-established and

distinct breeds. As the differences slowly become greater, the inferior

animals with intermediate characters, being neither very swift nor very

strong, will have been neglected, and will have tended to disappear. Here,

then, we see in man's productions the action of what may be called the

principle of divergence, causing differences, at first barely appreciable,

steadily to increase, and the breeds to diverge in character both from each

other and from their common parent.

But how, it may be asked, can any analogous principle apply in nature? I

believe it can and does apply most efficiently, from the simple

circumstance that the more diversified the descendants from any one species

become in structure, constitution, and habits, by so much will they be

better enabled to seize on many and widely diversified places in the polity

of nature, and so be enabled to increase in numbers.

We can clearly see this in the case of animals with simple habits. Take

the case of a carnivorous quadruped, of which the number that can be

supported in any country has long ago arrived at its full average. If its

natural powers of increase be allowed to act, it can succeed in increasing

(the country not undergoing any change in its conditions) only by its

varying descendants seizing on places at present occupied by other animals:

some of them, for instance, being enabled to feed on new kinds of prey,

either dead or alive; some inhabiting new stations, climbing trees,

frequenting water, and some perhaps becoming less carnivorous. The more

diversified in habits and structure the descendants of our carnivorous

animal became, the more places they would be enabled to occupy. What

applies to one animal will apply throughout all time to all animals--that

is, if they vary--for otherwise natural selection can do nothing. So it

will be with plants. It has been experimentally proved, that if a plot of

ground be sown with several distinct genera of grasses, a greater number of

plants and a greater weight of dry herbage can thus be raised. The same

has been found to hold good when first one variety and then several mixed

varieties of wheat have been sown on equal spaces of ground. Hence, if any

one species of grass were to go on varying, and those varieties were

continually selected which differed from each other in at all the same

manner as distinct species and genera of grasses differ from each other, a

greater number of individual plants of this species of grass, including its

modified descendants, would succeed in living on the same piece of ground.

And we well know that each species and each variety of grass is annually

sowing almost countless seeds; and thus, as it may be said, is striving its

utmost to increase its numbers. Consequently, I cannot doubt that in the

course of many thousands of generations, the most distinct varieties of any

one species of grass would always have the best chance of succeeding and of

increasing in numbers, and thus of supplanting the less distinct varieties;

and varieties, when rendered very distinct from each other, take the rank

of species.

The truth of the principle, that the greatest amount of life can be

supported by great diversification of structure, is seen under many natural

circumstances. In an extremely small area, especially if freely open to

immigration, and where the contest between individual and individual must

be severe, we always find great diversity in its inhabitants. For

instance, I found that a piece of turf, three feet by four in size, which

had been exposed for many years to exactly the same conditions, supported

twenty species of plants, and these belonged to eighteen genera and to

eight orders, which shows how much these plants differed from each other.

So it is with the plants and insects on small and uniform islets; and so in

small ponds of fresh water. Farmers find that they can raise most food by

a rotation of plants belonging to the most different orders: nature

follows what may be called a simultaneous rotation. Most of the animals

and plants which live close round any small piece of ground, could live on

it (supposing it not to be in any way peculiar in its nature), and may be

said to be striving to the utmost to live there; but, it is seen, that

where they come into the closest competition with each other, the

advantages of diversification of structure, with the accompanying

differences of habit and constitution, determine that the inhabitants,

which thus jostle each other most closely, shall, as a general rule, belong

to what we call different genera and orders.

The same principle is seen in the naturalisation of plants through man's

agency in foreign lands. It might have been expected that the plants which

have succeeded in becoming naturalised in any land would generally have

been closely allied to the indigenes; for these are commonly looked at as

specially created and adapted for their own country. It might, also,

perhaps have been expected that naturalised plants would have belonged to a

few groups more especially adapted to certain stations in their new homes.

But the case is very different; and Alph. De Candolle has well remarked in

his great and admirable work, that floras gain by naturalisation,

proportionally with the number of the native genera and species, far more

in new genera than in new species. To give a single instance: in the last

edition of Dr. Asa Gray's 'Manual of the Flora of the Northern United

States,' 260 naturalised plants are enumerated, and these belong to 162

genera. We thus see that these naturalised plants are of a highly

diversified nature. They differ, moreover, to a large extent from the

indigenes, for out of the 162 genera, no less than 100 genera are not there

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

these States.

By considering the nature of the plants or animals which have struggled

successfully with the indigenes of any country, and have there become

naturalised, we can gain some crude idea in what manner some of the natives

would have had to be modified, in order to have gained an advantage over

the other natives; and we may, I think, at least safely infer that

diversification of structure, amounting to new generic differences, would

have been profitable to them.

The advantage of diversification in the inhabitants of the same region is,

in fact, the same as that of the physiological division of labour in the

organs of the same individual body--a subject so well elucidated by Milne

Edwards. No physiologist doubts that a stomach by being adapted to digest

vegetable matter alone, or flesh alone, draws most nutriment from these

substances. So in the general economy of any land, the more widely and

perfectly the animals and plants are diversified for different habits of

life, so will a greater number of individuals be capable of there

supporting themselves. A set of animals, with their organisation but

little diversified, could hardly compete with a set more perfectly

diversified in structure. It may be doubted, for instance, whether the

Australian marsupials, which are divided into groups differing but little

from each other, and feebly representing, as Mr. Waterhouse and others have

remarked, our carnivorous, ruminant, and rodent mammals, could successfully

compete with these well-pronounced orders. In the Australian mammals, we

see the process of diversification in an early and incomplete stage of

development.

After the foregoing discussion, which ought to have been much amplified, we

may, I think, assume that the modified descendants of any one species will

succeed by so much the better as they become more diversified in structure,

and are thus enabled to encroach on places occupied by other beings. Now

let us see how this principle of great benefit being derived from

divergence of character, combined with the principles of natural selection

and of extinction, will tend to act.

The accompanying diagram will aid us in understanding this rather

perplexing subject. Let A to L represent the species of a genus large in

its own country; these species are supposed to resemble each other in

unequal degrees, as is so generally the case in nature, and as is

represented in the diagram by the letters standing at unequal distances. I

have said a large genus, because we have seen in the second chapter, that

on an average more of the species of large genera vary than of small

genera; and the varying species of the large genera present a greater

number of varieties. We have, also, seen that the species, which are the

commonest and the most widely-diffused, vary more than rare species with

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

species, belonging to a genus large in its own country. The little fan of

diverging dotted lines of unequal lengths proceeding from (A), may

represent its varying offspring. The variations are supposed to be

extremely slight, but of the most diversified nature; they are not supposed

all to appear simultaneously, but often after long intervals of time; nor

are they all supposed to endure for equal periods. Only those variations

which are in some way profitable will be preserved or naturally selected.

And here the importance of the principle of benefit being derived from

divergence of character comes in; for this will generally lead to the most

different or divergent variations (represented by the outer dotted lines)

being preserved and accumulated by natural selection. When a dotted line

reaches one of the horizontal lines, and is there marked by a small

numbered letter, a sufficient amount of variation is supposed to have been

accumulated to have formed a fairly well-marked variety, such as would be

thought worthy of record in a systematic work.

The intervals between the horizontal lines in the diagram, may represent

each a thousand generations; but it would have been better if each had

represented ten thousand generations. After a thousand generations,

species (A) is supposed to have produced two fairly well-marked varieties,

namely a1 and m1. These two varieties will generally continue to be

exposed to the same conditions which made their parents variable, and the

tendency to variability is in itself hereditary, consequently they will

tend to vary, and generally to vary in nearly the same manner as their

parents varied. Moreover, these two varieties, being only slightly

modified forms, will tend to inherit those advantages which made their

common parent (A) more numerous than most of the other inhabitants of the

same country; they will likewise partake of those more general advantages

which made the genus to which the parent-species belonged, a large genus in

its own country. And these circumstances we know to be favourable to the

production of new varieties.

If, then, these two varieties be variable, the most divergent of their

variations will generally be preserved during the next thousand

generations. And after this interval, variety a1 is supposed in the

diagram to have produced variety a2, which will, owing to the principle of

divergence, differ more from (A) than did variety a1. Variety m1 is

supposed to have produced two varieties, namely m2 and s2, differing from

each other, and more considerably from their common parent (A). We may

continue the process by similar steps for any length of time; some of the

varieties, after each thousand generations, producing only a single

variety, but in a more and more modified condition, some producing two or

three varieties, and some failing to produce any. Thus the varieties or

modified descendants, proceeding from the common parent (A), will generally

go on increasing in number and diverging in character. In the diagram the

process is represented up to the ten-thousandth generation, and under a

condensed and simplified form up to the fourteen-thousandth generation.

But I must here remark that I do not suppose that the process ever goes on

so regularly as is represented in the diagram, though in itself made

somewhat irregular. I am far from thinking that the most divergent

varieties will invariably prevail and multiply: a medium form may often

long endure, and may or may not produce more than one modified descendant;

for natural selection will always act according to the nature of the places

which are either unoccupied or not perfectly occupied by other beings; and

this will depend on infinitely complex relations. But as a general rule,

the more diversified in structure the descendants from any one species can

be rendered, the more places they will be enabled to seize on, and the more

their modified progeny will be increased. In our diagram the line of

succession is broken at regular intervals by small numbered letters marking

the successive forms which have become sufficiently distinct to be recorded

as varieties. But these breaks are imaginary, and might have been inserted

anywhere, after intervals long enough to have allowed the accumulation of a

considerable amount of divergent variation.

As all the modified descendants from a common and widely-diffused species,

belonging to a large genus, will tend to partake of the same advantages

which made their parent successful in life, they will generally go on

multiplying in number as well as diverging in character: this is

represented in the diagram by the several divergent branches proceeding

from (A). The modified offspring from the later and more highly improved

branches in the lines of descent, will, it is probable, often take the

place of, and so destroy, the earlier and less improved branches: this is

represented in the diagram by some of the lower branches not reaching to

the upper horizontal lines. In some cases I do not doubt that the process

of modification will be confined to a single line of descent, and the

number of the descendants will not be increased; although the amount of

divergent modification may have been increased in the successive

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

lines proceeding from (A) were removed, excepting that from a1 to a10. In

the same way, for instance, the English race-horse and English pointer have

apparently both gone on slowly diverging in character from their original

stocks, without either having given off any fresh branches or races.

After ten thousand generations, species (A) is supposed to have produced

three forms, a10, f10, and m10, which, from having diverged in character

during the successive generations, will have come to differ largely, but

perhaps unequally, from each other and from their common parent. If we

suppose the amount of change between each horizontal line in our diagram to

be excessively small, these three forms may still be only well-marked

varieties; or they may have arrived at the doubtful category of

sub-species; but we have only to suppose the steps in the process of

modification to be more numerous or greater in amount, to convert these

three forms into well-defined species: thus the diagram illustrates the

steps by which the small differences distinguishing varieties are increased

into the larger differences distinguishing species. By continuing the same

process for a greater number of generations (as shown in the diagram in a

condensed and simplified manner), we get eight species, marked by the

letters between a14 and m14, all descended from (A). Thus, as I believe,

species are multiplied and genera are formed.

In a large genus it is probable that more than one species would vary. In

the diagram I have assumed that a second species (I) has produced, by

analogous steps, after ten thousand generations, either two well-marked

varieties (w10 and z10) or two species, according to the amount of change

supposed to be represented between the horizontal lines. After fourteen

thousand generations, six new species, marked by the letters n14 to z14,

are supposed to have been produced. In each genus, the species, which are

already extremely different in character, will generally tend to produce

the greatest number of modified descendants; for these will have the best

chance of filling new and widely different places in the polity of nature:

hence in the diagram I have chosen the extreme species (A), and the nearly

extreme species (I), as those which have largely varied, and have given

rise to new varieties and species. The other nine species (marked by

capital letters) of our original genus, may for a long period continue

transmitting unaltered descendants; and this is shown in the diagram by the

dotted lines not prolonged far upwards from want of space.

But during the process of modification, represented in the diagram, another

of our principles, namely that of extinction, will have played an important

part. As in each fully stocked country natural selection necessarily acts

by the selected form having some advantage in the struggle for life over

other forms, there will be a constant tendency in the improved descendants

of any one species to supplant and exterminate in each stage of descent

their predecessors and their original parent. For it should be remembered

that the competition will generally be most severe between those forms

which are most nearly related to each other in habits, constitution, and

structure. Hence all the intermediate forms between the earlier and later

states, that is between the less and more improved state of a species, as

well as the original parent-species itself, will generally tend to become

extinct. So it probably will be with many whole collateral lines of

descent, which will be conquered by later and improved lines of descent.

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

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

child and parent do not come into competition, both may continue to exist.

If then our diagram be assumed to represent a considerable amount of

modification, species (A) and all the earlier varieties will have become

extinct, having been replaced by eight new species (a14 to m14); and (I)

will have been replaced by six (n14 to z14) new species.

But we may go further than this. The original species of our genus were

supposed to resemble each other in unequal degrees, as is so generally the

case in nature; species (A) being more nearly related to B, C, and D, than

to the other species; and species (I) more to G, H, K, L, than to the

others. These two species (A) and (I), were also supposed to be very

common and widely diffused species, so that they must originally have had

some advantage over most of the other species of the genus. Their modified

descendants, fourteen in number at the fourteen-thousandth generation, will

probably have inherited some of the same advantages: they have also been

modified and improved in a diversified manner at each stage of descent, so

as to have become adapted to many related places in the natural economy of

their country. It seems, therefore, to me extremely probable that they

will have taken the places of, and thus exterminated, not only their

parents (A) and (I), but likewise some of the original species which were

most nearly related to their parents. Hence very few of the original

species will have transmitted offspring to the fourteen-thousandth

generation. We may suppose that only one (F), of the two species which

were least closely related to the other nine original species, has

transmitted descendants to this late stage of descent.

The new species in our diagram descended from the original eleven species,

will now be fifteen in number. Owing to the divergent tendency of natural

selection, the extreme amount of difference in character between species

a14 and z14 will be much greater than that between the most different of

the original eleven species. The new species, moreover, will be allied to

each other in a widely different manner. Of the eight descendants from (A)

the three marked a14, q14, p14, will be nearly related from having recently

branched off from a10; b14 and f14, from having diverged at an earlier

period from a5, will be in some degree distinct from the three first-named

species; and lastly, o14, e14, and m14, will be nearly related one to the

other, but from having diverged at the first commencement of the process of

modification, will be widely different from the other five species, and may

constitute a sub-genus or even a distinct genus.

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

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

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

will, owing to inheritance, differ considerably from the eight descendants

from (A); the two groups, moreover, are supposed to have gone on diverging

in different directions. The intermediate species, also (and this is a

very important consideration), which connected the original species (A) and

(I), have all become, excepting (F), extinct, and have left no descendants.

Hence the six new species descended from (I), and the eight descended from

(A), will have to be ranked as very distinct genera, or even as distinct

sub-families.

Thus it is, as I believe, that two or more genera are produced by descent,

with modification, from two or more species of the same genus. And the two

or more parent-species are supposed to have descended from some one species

of an earlier genus. In our diagram, this is indicated by the broken

lines, beneath the capital letters, converging in sub-branches downwards

towards a single point; this point representing a single species, the

supposed single parent of our several new sub-genera and genera.

It is worth while to reflect for a moment on the character of the new

species F14, which is supposed not to have diverged much in character, but

to have retained the form of (F), either unaltered or altered only in a

slight degree. In this case, its affinities to the other fourteen new

species will be of a curious and circuitous nature. Having descended from

a form which stood between the two parent-species (A) and (I), now supposed

to be extinct and unknown, it will be in some degree intermediate in

character between the two groups descended from these species. But as

these two groups have gone on diverging in character from the type of their

parents, the new species (F14) will not be directly intermediate between

them, but rather between types of the two groups; and every naturalist will

be able to bring some such case before his mind.

In the diagram, each horizontal line has hitherto been supposed to

represent a thousand generations, but each may represent a million or

hundred million generations, and likewise a section of the successive

strata of the earth's crust including extinct remains. We shall, when we

come to our chapter on Geology, have to refer again to this subject, and I

think we shall then see that the diagram throws light on the affinities of

extinct beings, which, though generally belonging to the same orders, or

families, or genera, with those now living, yet are often, in some degree,

intermediate in character between existing groups; and we can understand

this fact, for the extinct species lived at very ancient epochs when the

branching lines of descent had diverged less.

I see no reason to limit the process of modification, as now explained, to

the formation of genera alone. If, in our diagram, we suppose the amount

of change represented by each successive group of diverging dotted lines to

be very great, the forms marked a14 to p14, those marked b14 and f14, and

those marked o14 to m14, will form three very distinct genera. We shall

also have two very distinct genera descended from (I) and as these latter

two genera, both from continued divergence of character and from

inheritance from a different parent, will differ widely from the three

genera descended from (A), the two little groups of genera will form two

distinct families, or even orders, according to the amount of divergent

modification supposed to be represented in the diagram. And the two new

families, or orders, will have descended from two species of the original

genus; and these two species are supposed to have descended from one

species of a still more ancient and unknown genus.

We have seen that in each country it is the species of the larger genera

which oftenest present varieties or incipient species. This, indeed, might

have been expected; for as natural selection acts through one form having

some advantage over other forms in the struggle for existence, it will

chiefly act on those which already have some advantage; and the largeness

of any group shows that its species have inherited from a common ancestor

some advantage in common. Hence, the struggle for the production of new

and modified descendants, will mainly lie between the larger groups, which

are all trying to increase in number. One large group will slowly conquer

another large group, reduce its numbers, and thus lessen its chance of

further variation and improvement. Within the same large group, the later

and more highly perfected sub-groups, from branching out and seizing on

many new places in the polity of Nature, will constantly tend to supplant

and destroy the earlier and less improved sub-groups. Small and broken

groups and sub-groups will finally tend to disappear. Looking to the

future, we can predict that the groups of organic beings which are now

large and triumphant, and which are least broken up, that is, which as yet

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

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

for we well know that many groups, formerly most extensively developed,

have now become extinct. Looking still more remotely to the future, we may

predict that, owing to the continued and steady increase of the larger

groups, a multitude of smaller groups will become utterly extinct, and

leave no modified descendants; and consequently that of the species living

at any one period, extremely few will transmit descendants to a remote

futurity. I shall have to return to this subject in the chapter on

Classification, but I may add that on this view of extremely few of the

more ancient species having transmitted descendants, and on the view of all

the descendants of the same species making a class, we can understand how

it is that there exist but very few classes in each main division of the

animal and vegetable kingdoms. Although extremely few of the most ancient

species may now have living and modified descendants, yet at the most

remote geological period, the earth may have been as well peopled with many

species of many genera, families, orders, and classes, as at the present

day.

Summary of Chapter -- If during the long course of ages and under varying

conditions of life, organic beings vary at all in the several parts of

their organisation, and I think this cannot be disputed; if there be, owing

to the high geometrical powers of increase of each species, at some age,

season, or year, a severe struggle for life, and this certainly cannot be

disputed; then, considering the infinite complexity of the relations of all

organic beings to each other and to their conditions of existence, causing

an infinite diversity in structure, constitution, and habits, to be

advantageous to them, I think it would be a most extraordinary fact if no

variation ever had occurred useful to each being's own welfare, in the same

way as so many variations have occurred useful to man. But if variations

useful to any organic being do occur, assuredly individuals thus

characterised will have the best chance of being preserved in the struggle

for life; and from the strong principle of inheritance they will tend to

produce offspring similarly characterised. This principle of preservation,

I have called, for the sake of brevity, Natural Selection. Natural

selection, on the principle of qualities being inherited at corresponding

ages, can modify the egg, seed, or young, as easily as the adult. Amongst

many animals, sexual selection will give its aid to ordinary selection, by

assuring to the most vigorous and best adapted males the greatest number of

offspring. Sexual selection will also give characters useful to the males

alone, in their struggles with other males.

Whether natural selection has really thus acted in nature, in modifying and

adapting the various forms of life to their several conditions and

stations, must be judged of by the general tenour and balance of evidence

given in the following chapters. But we already see how it entails

extinction; and how largely extinction has acted in the world's history,

geology plainly declares. Natural selection, also, leads to divergence of

character; for more living beings can be supported on the same area the

more they diverge in structure, habits, and constitution, of which we see

proof by looking at the inhabitants of any small spot or at naturalised

productions. Therefore during the modification of the descendants of any

one species, and during the incessant struggle of all species to increase

in numbers, the more diversified these descendants become, the better will

be their chance of succeeding in the battle of life. Thus the small

differences distinguishing varieties of the same species, will steadily

tend to increase till they come to equal the greater differences between

species of the same genus, or even of distinct genera.

We have seen that it is the common, the widely-diffused, and widely-ranging

species, belonging to the larger genera, which vary most; and these will

tend to transmit to their modified offspring that superiority which now

makes them dominant in their own countries. Natural selection, as has just

been remarked, leads to divergence of character and to much extinction of

the less improved and intermediate forms of life. On these principles, I

believe, the nature of the affinities of all organic beings may be

explained. It is a truly wonderful fact--the wonder of which we are apt to

overlook from familiarity--that all animals and all plants throughout all

time and space should be related to each other in group subordinate to

group, in the manner which we everywhere behold--namely, varieties of the

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

closely and unequally related together, forming sections and sub-genera,

species of distinct genera much less closely related, and genera related in

different degrees, forming sub-families, families, orders, sub-classes, and

classes. The several subordinate groups in any class cannot be ranked in a

single file, but seem rather to be clustered round points, and these round

other points, and so on in almost endless cycles. On the view that each

species has been independently created, I can see no explanation of this

great fact in the classification of all organic beings; but, to the best of

my judgment, it is explained through inheritance and the complex action of

natural selection, entailing extinction and divergence of character, as we

have seen illustrated in the diagram.

The affinities of all the beings of the same class have sometimes been

represented by a great tree. I believe this simile largely speaks the

truth. The green and budding twigs may represent existing species; and

those produced during each former year may represent the long succession of

extinct species. At each period of growth all the growing twigs have tried

to branch out on all sides, and to overtop and kill the surrounding twigs

and branches, in the same manner as species and groups of species have

tried to overmaster other species in the great battle for life. The limbs

divided into great branches, and these into lesser and lesser branches,

were themselves once, when the tree was small, budding twigs; and this

connexion of the former and present buds by ramifying branches may well

represent the classification of all extinct and living species in groups

subordinate to groups. Of the many twigs which flourished when the tree

was a mere bush, only two or three, now grown into great branches, yet

survive and bear all the other branches; so with the species which lived

during long-past geological periods, very few now have living and modified

descendants. From the first growth of the tree, many a limb and branch has

decayed and dropped off; and these lost branches of various sizes may

represent those whole orders, families, and genera which have now no living

representatives, and which are known to us only from having been found in a

fossil state. As we here and there see a thin straggling branch springing

from a fork low down in a tree, and which by some chance has been favoured

and is still alive on its summit, so we occasionally see an animal like the

Ornithorhynchus or Lepidosiren, which in some small degree connects by its

affinities two large branches of life, and which has apparently been saved

from fatal competition by having inhabited a protected station. As buds

give rise by growth to fresh buds, and these, if vigorous, branch out and

overtop on all sides many a feebler branch, so by generation I believe it

has been with the great Tree of Life, which fills with its dead and broken

branches the crust of the earth, and covers the surface with its ever

branching and beautiful ramifications.