The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

with some of the normal variety. The following year the total 100,000

pairs will consist of 99,984 of the normal, and only 16 of the abnormal

variety; and the probabilities, of course, are, that the whole of these

latter will pair with some of the enormous preponderance of normal

individuals, and, their unions being sterile, the physiological variety

will become extinct in the third year.
If now in the second and each succeeding year a similar proportion as at

first (10 per cent) of the physiological variety is produced afresh from

the ranks of the normal variety, the same rate of diminution will go on,

and it will be found that, on the most favourable estimate, the

physiological variety can never exceed 12,000 to the 88,000 of the

normal form of the species, as shown by the following table:--

1st Year. 10,000 of physiological variety to 90,000 of normal variety.

2d " 1,220 + 10,000 again produced.

3d " 16 + 1,220 + 10,000 do. = 11,236

4th " O + 16 + 1,220 + 10,000 do. = 11,236

5th " O + 16 + 1,220 + 10,000 = 11,236

and so on for any number of generations.

In the preceding discussion we have given the theory the advantage of

the large proportion of 10 per cent of this very exceptional variety

arising in its midst year by year, and we have seen that, even under

these favourable conditions, it is unable to increase its numbers much

above its starting-point, and that it remains wholly dependent on the

continued renewal of the variety for its existence beyond a few years.

It appears, then, that this form of inter-specific sterility cannot be

increased by natural or any other known form of selection, but that it

contains within itself its own principle of destruction. If it is

proposed to get over the difficulty by postulating a larger percentage

of the variety annually arising within the species, we shall not affect

the law of decrease until we approach equality in the numbers of the two

varieties. But with any such increase of the physiological variety the

species itself would inevitably suffer by the large proportion of

sterile unions in its midst, and would thus be at a great disadvantage

in competition with other species which were fertile throughout. Thus,

natural selection will always tend to weed out any species with too

great a tendency to sterility among its own members, and will therefore

prevent such sterility from becoming the general characteristic of

varying species, which this theory demands should be the case.

sterility between the individuals of a species, can be increased by

natural selection unless correlated with some useful variation, while

all infertility not so correlated has a constant tendency to effect its

own elimination. But the opposite property, fertility, is of vital

importance to every species, and gives the offspring of the individuals

which possess it, in consequence of their superior numbers, a greater

chance of survival in the battle of life. It is, therefore, directly

under the control of natural selection, which acts both by the

self-preservation of fertile and the self-destruction of infertile

stocks--except always where correlated as above, when they become

useful, and therefore subject to be increased by natural selection.

_Summary and Concluding Remarks on Hybridity._
The facts which are of the greatest importance to a comprehension of

this very difficult subject are those which show the extreme

susceptibility of the reproductive system both in plants and animals. We

have seen how both these classes of organisms may be rendered infertile,

by a change of conditions which does not affect their general health, by

captivity, or by too close interbreeding. We have seen, also, that

infertility is frequently correlated with a difference of colour, or

with other characters; that it is not proportionate to divergence of

structure; that it varies in reciprocal crosses between pairs of the

same species; while in the cases of dimorphic and trimorphic plants the

different crosses between the same pair of individuals may be fertile or

sterile at the same time. It appears as if fertility depended on such a

delicate adjustment of the male and female elements to each other, that,

unless constantly kept up by the preservation of the most fertile

individuals, sterility is always liable to arise. This preservation

always occurs within the limits of each species, both because fertility

is of the highest importance to the continuance of the race, and also

because sterility (and to a less extent infertility) is self-destructive

as well as injurious to the species.
So long therefore as a species remains undivided, and in occupation of a

continuous area, its fertility is kept up by natural selection; but the

moment it becomes separated, either by geographical or selective

isolation, or by diversity of station or of habits, then, while each

portion must be kept fertile _inter se_, there is nothing to prevent

infertility arising between the two separated portions. As the two

portions will necessarily exist under somewhat different conditions of

life, and will usually have acquired some diversity of form and

colour--both which circumstances we know to be either the cause of

infertility or to be correlated with it,--the fact of some degree of

infertility usually appearing between closely allied but locally or

physiologically segregated species is exactly what we should expect.

infertility is not difficult to explain. The popular conclusions on this

matter have been drawn chiefly from what occurs among domestic animals,

and we have seen that the very first essential to their becoming

domesticated was that they should continue fertile under changed

conditions of life. During the slow process of the formation of new

varieties by conscious or unconscious selection, fertility has always

been an essential character, and has thus been invariably preserved or

increased; while there is some evidence to show that domestication

itself tends to increase fertility.

experimented on than among animals, and we accordingly find numerous

cases in which distinct species of plants are perfectly fertile when

crossed, their hybrid offspring being also fertile _inter se_. We also

find some few examples of the converse fact--varieties of the same

species which when crossed are infertile or even sterile.

essential to the formation of new species, in order to prevent the

swamping effects of intercrossing, has been shown to be unsound, because

the varieties or incipient species will, in most cases, be sufficiently

isolated by having adopted different habits or by frequenting different

stations; while selective association, which is known to be general

among distinct varieties or breeds of the same species, will produce an

effective isolation even when the two forms occupy the same area.

the conclusion that the sterility or infertility of species with each

other, whether manifested in the difficulty of obtaining first crosses

between them or in the sterility of the hybrids thus obtained, is not a

constant or necessary result of specific difference, but is incidental

on unknown peculiarities of the reproductive system. These peculiarities

constantly tend to arise under changed conditions owing to the extreme

susceptibility of that system, and they are usually correlated with

variations of form or of colour. Hence, as fixed differences of form and

colour, slowly gained by natural selection in adaptation to changed

conditions, are what essentially characterise distinct species, some

amount of infertility between species is the usual result.

solution may be carried a step further. If we accept the association of

some degree of infertility, however slight, as a not unfrequent

accompaniment of the external differences which always arise in a state

of nature between varieties and incipient species, it has been shown

that natural selection _has_ power to increase that infertility just as

it has power to increase other favourable variations. Such an increase

of infertility will be beneficial, whenever new species arise in the

same area with the parent form; and we thus see how, out of the

fluctuating and very unequal amounts of infertility correlated with

physical variations, there may have arisen that larger and more constant

amount which appears usually to characterise well-marked species.

the present chapter, although from an experimental point of view very

insufficient, all point to the general conclusion we have now reached,

and afford us a not unsatisfactory solution of the great problem of

hybridism in relation to the origin of species by means of natural

selection. Further experimental research is needed in order to complete

the elucidation of the subject; but until these additional facts are

forthcoming no new theory seems required for the explanation of the

phenomena.
FOOTNOTES:
[Footnote 51: Darwin's _Animals and Plants under Domestication_, vol.

ii. pp. 163-170.]

various problems to which they give rise, the reader must consult

Darwin's volume on _The Different Forms of Flowers in Plants of the same

Species_, chaps, i.-iv.]

Introduction, p. lxiv.]

379.]
[Footnote 57: _Origin of Species_, p. 239.]

(1870), Dr. Ogle has adduced some curious physiological facts bearing on

the presence or absence of white colours in the higher animals. He

states that a dark pigment in the olfactory region of the nostrils is

essential to perfect smell, and that this pigment is rarely deficient

except when the whole animal is pure white, and the creature is then

almost without smell or taste. He observes that there is no proof that,

in any of the cases given above, the black animals actually eat the

poisonous root or plant; and that the facts are readily understood if

the senses of smell and taste are dependent on a pigment which is absent

in the white animals, who therefore eat what those gifted with normal

senses avoid. This explanation however hardly seems to cover the facts.

We cannot suppose that almost all the sheep in the world (which are

mostly white) are without smell or taste. The cutaneous disease on the

white patches of hair on horses, the special liability of white terriers

to distemper, of white chickens to the gapes, and of silkworms which

produce yellow silk to the fungus, are not explained by it. The

analogous facts in plants also indicate a real constitutional relation

with colour, not an affection of the sense of smell and taste only.]
[Footnote 60: For all these facts, see _Animals and Plants under

Domestication_, vol. ii. pp. 335-338.]

102, 103.]

while its theoretical importance is very great, I add here the following

briefer exposition of it, in a series of propositions; being, with a few

verbal alterations, a copy of what I wrote on the subject about twenty

years back. Some readers may find this easier to follow than the fuller

discussion in the text:--

_Can Sterility of Hybrids have been Produced by Natural

Selection?_

adapted to certain existing conditions better than the parent

form, which they soon supplant.
2. If these _two forms_, which are supposed to coexist in the

same district, do not intercross, natural selection will

accumulate all favourable variations till they become well

suited to their conditions of life, and form two slightly

differing species.
3. But if these _two forms_ freely intercross with each other,

and produce hybrids, which are also quite fertile _inter se_,

then the formation of the two distinct races or species will be

retarded, or perhaps entirely prevented; for the offspring of

the crossed unions will be _more vigorous_ owing to the cross,

although _less adapted_ to their conditions of life than either

of the pure breeds.
4. Now, let a partial sterility of the hybrids of some

considerable proportion of these two forms arise; and, as this

would probably be due to some special conditions of life, we may

fairly suppose it to arise in some definite portion of the area

occupied by the two forms.
5. The result will be that, in that area, the hybrids (although

continually produced by first crosses almost as freely as

before) will not themselves increase so rapidly as the two pure

forms; and as the two pure forms are, by the terms of the

problem, better suited to their several conditions of life than

the hybrids, they will inevitably increase more rapidly, and

will continually tend to supplant the hybrids altogether at

every recurrent severe struggle for existence.

appears some disinclination to _cross unions_ will appear, and

this will further tend to the diminution of the production of

hybrids.
7. In the other part of the area, however, where hybridism

occurs with perfect freedom, hybrids of various degrees may

increase till they equal or even exceed in number the pure

species--that is, the incipient species will be liable to be

swamped by intercrossing.

appearing in one part of the area occupied by the two forms,

will be--that the great majority of the individuals will there

consist of the two pure forms only, while in the remaining part

these will be in a minority,--which is the same as saying that

the new _physiological variety_ of the two forms will be better

suited to the conditions of existence than the remaining portion

which has not varied physiologically.

variety which is best adapted to the conditions of existence

always supplants that which is imperfectly adapted; therefore,

_by natural selection_ the _varieties_ which are _sterile_ when

crossed will become established as the only ones.
10. Now let variations in the _amount of sterility_ and in

the _disinclination to crossed unions_ continue to occur--also

in certain parts of the area: exactly the same result must

recur, and the progeny of this new physiological variety will in

time occupy the whole area.
11. There is yet another consideration that would facilitate the

process. It seems probable that the _sterility variations_

would, to some extent, concur with, and perhaps depend upon, the

_specific variations_; so that, just in proportion as the _two

forms_ diverged and became better adapted to the conditions of

existence, they would become more sterile when intercrossed. If

this were the case, then natural selection would act with double

strength; and those which were better adapted to survive both

structurally and physiologically would certainly do so.]
[Footnote 63: Cases of this kind are referred to at p. 155. It must,

however, be noted, that such sterility in first crosses appears to be

equally rare between different species of the same genus as between

individuals of the same species. Mules and other hybrids are freely

produced between very distinct species, but are themselves infertile or

quite sterile; and it is this infertility or sterility of the hybrids

that is the characteristic--and was once thought to be the criterion--of

species, not the sterility of their first crosses. Hence we should not

expect to find any constant infertility in the first crosses between the

distinct strains or varieties that formed the starting-point of new

species, but only a slight amount of infertility in their mongrel

offspring. It follows, that Mr. Romanes' theory of _Physiological

Selection_--which assumes sterility or infertility between first crosses

as the fundamental fact in the origin of species--does not accord with

the general phenomena of hybridism in nature.]
[Footnote 64: The exact number is 1219.51, but the fractions are omitted

for clearness.]

CHAPTER VIII
THE ORIGIN AND USES OF COLOUR IN ANIMALS

The Darwinian theory threw new light on organic colour--The

problem to be solved--The constancy of animal colour indicates

utility--Colour and environment--Arctic animals

white--Exceptions prove the rule--Desert, forest, nocturnal, and

oceanic animals--General theories of animal colour--Variable

protective colouring--Mr. Poulton's experiments--Special or

local colour adaptations--Imitation of particular objects--How

they have been produced--Special protective colouring of

butterflies--Protective resemblance among marine

animals--Protection by terrifying enemies--Alluring

coloration--The coloration of birds' eggs--Colour as a means of

recognition--Summary of the preceding exposition--Influence of

locality or of climate on colour--Concluding remarks.

Among the numerous applications of the Darwinian theory in the

interpretation of the complex phenomena presented by the organic world,

none have been more successful, or are more interesting, than those

which deal with the colours of animals and plants. To the older school

of naturalists colour was a trivial character, eminently unstable and

untrustworthy in the determination of species; and it appeared to have,

in most cases, no use or meaning to the objects which displayed it. The

bright and often gorgeous coloration of insect, bird, or flower, was

either looked upon as having been created for the enjoyment of mankind,

or as due to unknown and perhaps undiscoverable laws of nature.
But the researches of Mr. Darwin totally changed our point of view in

this matter. He showed, clearly, that some of the colours of animals are

useful, some hurtful to them; and he believed that many of the most

brilliant colours were developed by sexual choice; while his great

general principle, that all the fixed characters of organic beings have

been developed under the action of the law of utility, led to the

inevitable conclusion that so remarkable and conspicuous a character as

colour, which so often constitutes the most obvious distinction of

species from species, or group from group, must also have arisen from

survival of the fittest, and must, therefore, in most cases have some

relation to the wellbeing of its possessors. Continuous observation and

research, carried on by multitudes of observers during the last thirty

years, have shown this to be the case; but the problem is found to be

far more complex than was at first supposed. The modes in which colour

is of use to different classes of organisms is very varied, and have

probably not yet been all discovered; while the infinite variety and

marvellous beauty of some of its developments are such as to render it

hopeless to arrive at a complete and satisfactory explanation of every

individual case. So much, however, has been achieved, so many curious

facts have been explained, and so much light has been thrown on some of

the most obscure phenomena of nature, that the subject deserves a

prominent place in any account of the Darwinian theory.

_The Problem to be Solved._
Before dealing with the various modifications of colour in the animal

world it is necessary to say a few words on colour in general, on its

prevalence in nature, and how it is that the colours of animals and

plants require any special explanation. What we term colour is a

subjective phenomenon, due to the constitution of our mind and nervous

system; while, objectively, it consists of light-vibrations of different

wave-lengths emitted by, or reflected from, various objects. Every

visible object must be coloured, because to be visible it must send rays

of light to our eye. The kind of light it sends is modified by the

molecular constitution or the surface texture of the object. Pigments

absorb certain rays and reflect the remainder, and this reflected

portion has to our eyes a definite colour, according to the portion of

the rays constituting white light which are absorbed. Interference

colours are produced either by thin films or by very fine striae on the

surfaces of bodies, which cause rays of certain wave-lengths to

neutralise each other, leaving the remainder to produce the effects of

colour. Such are the colours of soap-bubbles, or of steel or glass on

which extremely fine lines have been ruled; and these colours often

produce the effect of metallic lustre, and are the cause of most of the

metallic hues of birds and insects.

minute surface texture of bodies, and, as the matter of which organic

beings are composed consists of chemical compounds of great complexity

and extreme instability, and is also subject to innumerable changes

during growth and development, we might naturally expect the phenomena

of colour to be more varied here than in less complex and more stable

compounds. Yet even in the inorganic world we find abundant and varied

colours; in the earth and in the water; in metals, gems, and minerals;

in the sky and in the ocean; in sunset clouds and in the many-tinted

rainbow. Here we can have no question of _use_ to the coloured object,

and almost as little perhaps in the vivid red of blood, in the brilliant