Darwinism (1889) Part 13

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 cla.s.ses 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 circ.u.mstances 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.

The reason why varieties do not usually exhibit a similar amount of 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.

Among plants, wild species and varieties have been more frequently 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.

The idea that either infertility or geographical isolation is absolutely 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 a.s.sociation, 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.

From the various considerations now adverted to, Mr. Darwin arrived at 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.

Here the problem was left by Mr. Darwin; but we have shown that its solution may be carried a step further. If we accept the a.s.sociation 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 great body of facts of which a condensed account has been given in 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.]

[Footnote 52: For a full account of these interesting facts and of the 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.]

[Footnote 53: See _Nature_, vol. xxi. p. 207.]

[Footnote 54: Low's _Domesticated Animals of Great Britain_, Introduction, p. lxiv.]

[Footnote 55: Low's _Domesticated Animals_, p. 28.]

[Footnote 56: _Amaryllidaceae_, by the Hon. and Rev. William Herbert, p.

379.]

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

[Footnote 58: _Origin of Species_, sixth edition, p. 9.]

[Footnote 59: In the _Medico-Chirurgical Transactions_, vol. liii.

(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 a.n.a.logous facts in plants also indicate a real const.i.tutional 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.]

[Footnote 61: _Animals and Plants under Domestication_, vol. ii. pp.

102, 103.]

[Footnote 62: As this argument is a rather difficult one to follow, 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?_

1. Let there be a species which has varied into _two forms_ each 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 acc.u.mulate 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 r.e.t.a.r.ded, 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.

6. We may fairly suppose, also, that as soon as any sterility 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.

8. The first result, then, of a partial sterility of crosses 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.

9. But when the struggle for existence becomes severe, that 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 a.s.sumes 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 b.u.t.terflies--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 s.e.xual 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 const.i.tutes 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 mult.i.tudes 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 cla.s.ses 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 const.i.tution 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 const.i.tution 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 const.i.tuting 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 gla.s.s on which extremely fine lines have been ruled; and these colours often produce the effect of metallic l.u.s.tre, and are the cause of most of the metallic hues of birds and insects.

As colour thus depends on molecular or chemical const.i.tution or on the 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 colours of red snow and other low algae and fungi, or even in the universal mantle of green which clothes so large a portion of the earth's surface. The presence of some colour, or even of many brilliant colours, in animals and plants would require no other explanation than does that of the sky or the ocean, of the ruby or the emerald--that is, it would require a purely physical explanation only. It is the wonderful individuality of the colours of animals and plants that attracts our attention--the fact that the colours are localised in definite patterns, sometimes in accordance with structural characters, sometimes altogether independent of them; while often differing in the most striking and fantastic manner in allied species. We are thus compelled to look upon colour not merely as a physical but also as a biological characteristic, which has been differentiated and specialised by natural selection, and must, therefore, find its explanation in the principle of adaptation or utility.