CHAPTER I.

INTRODUCTION.

Before Darwin published his remarkable and memorable work on the Origin of Species, the decoration of animals and plants was a mystery as much hidden to the majority as the beauty of the rainbow ere Newton analysed the light. That the world teemed with beauty in form and colour was all we knew; and the only guess that could be made as to its uses was the vague and unsatisfactory suggestion that it was appointed for the delight of man.

Why, if such was the case, so many flowers were "born to blush unseen," so many insects hidden in untrodden forests, so many bright-robed creatures buried in the depths of the sea, no man could tell. It seemed but a poor display of creative intelligence to lavish for thousands of years upon heedless savage eyes such glories as are displayed by the forests of Brazil; and the mind recoiled from the suggestion that such could ever have been the prime intention.

But with the dawn of the new scientific faith, light began to shine upon these and kindred questions; nature ceased to appear a mass of useless, unconnected facts, and ornamentation appeared in its true guise as of extreme importance to the beings possessing it. It was the theory of descent with modification that threw this light upon nature.

This theory, reduced to its simplest terms, is that species, past and present, have arisen from the accumulation by inheritance of minute differences of form, structure, colour, or habit, giving to the individual a better chance, in the struggle for existence, of obtaining food or avoiding danger. It is based on a few well-known and universally admitted facts or laws of nature: namely, the law of multiplication in geometrical progression causing the birth of many more individuals than can survive, leading necessarily to the struggle for existence; the law of heredity, in virtue of which the offspring resembles its parents; the law of variation, in virtue of which the offspring has an individual character slightly differing from its parents.

To illustrate these laws roughly we will take the case of a bird, say, the thrush. The female lays on the average five eggs, and if all these are hatched, and the young survive, thrushes would be as seven to two times as numerous in the next year. Let two of these be females, and bring up each five young; in the second year we shall have seventeen thrushes, in the third thirty-seven, in the fourth seventy-seven, and so on. Now common experience tells us not merely that such a vast increase of individuals does not take place, but can never do so, as in a very few years the numbers would be so enormously increased that food would be exhausted.

On the other hand, we know that the numbers of individuals remain practically the same. It follows, then, that of every five eggs four fail to arrive at maturity; and this rigorous destruction of individuals is what is known as the struggle for existence. If, instead of a bird, we took an insect, laying hundreds of eggs, a fish, laying thousands, or a plant, producing still greater quantities of seed, we should find the extermination just as rigorous, and the numbers of individuals destroyed incomparably greater. Darwin has calculated that from a single pair of elephants nearly nineteen millions would be alive in 750 years if each elephant born arrived at maturity, lived a hundred years, and produced six young--and the elephant is the slowest breeder of all animals.

The struggle for existence, then, is a real and potent fact, and it follows that if, from any cause whatever, a being possesses any power or peculiarity that will give it a better chance of survival over its fellows--be that power ever so slight--it will have a very decided advantage.

Now it can be shown that no two individuals are exactly alike, in other words, that variation is constantly taking place, and that no animal or plant preserves its characters unmodified. This we might have expected if we attentively consider how impossible it is for any two individuals to be subjected to exactly the same conditions of life and habit. But for the proofs of variability we have not to rely upon theoretical reasoning. No one can study, even superficially, any class or species without daily experiencing the conviction that no two individuals are alike, and that variation takes place in almost every conceivable direction.

Granted then the existence of the struggle for existence and the variability of individuals, and granting also that if any variation gives its possessor a firmer hold upon life, it follows as a necessity that the most favoured individuals will have the best chance of surviving and leaving descendants, and by the law of heredity, we know these offspring will tend to inherit the characters of their parents. This action is often spoken of as the preservation of favoured races, and as the survival of the fittest.

The gradual accumulation of beneficial characters will give rise in time to new varieties and species; and in this way primarily has arisen the wonderful diversity of life that now exists. Such, in barest outline, is the theory of descent with modification.

Let us now see in what way this theory has been applied to colouration. The colours, or, more strictly, the arrangement of colours, in patterns is of several kinds, viz.:--

1. _General Colouration_, or such as appears to have no very special function _as_ colour. We find this most frequently in the vegetable kingdom, as, for instance, the green hue of leaves, which, though it has a most valuable function chemically has no particular use as colour, so far as we can see.

2. _Distinctive Colouration_, or the arrangement of colours in different patterns or tints corresponding to each species. This is the most usual style of colouring, and the three following kinds are modifications of it. It is this which gives each species its own design, whether in animals or plants.

3. _Protective Resemblance_, or the system of colouring which conceals the animal from its prey, or hides the prey from its foe. Of this class are the green hues of many caterpillars, the brown tints of desert birds, and the more remarkable resemblances of insects to sticks and leaves.

4. _Mimetic Colouration_, or the resemblance of one animal to another. It is always the resemblance of a rare species, which is the favourite food of some creature, to a common species nauseous to the mimicker's foe. Of this character are many butterflies.

5. _Warning Colours_, or distinctive markings and tints rendering an animal conspicuous, and, as it were, proclaiming _noli me tangere_ to its would-be attackers.

6. _Sexual Colours_, or particular modifications of colour in the two sexes, generally taking the form of brilliancy in the male, as in the peacock and birds of paradise.

Under one or other of these headings most schemes of colouration will be found to arrange themselves.

At the outset, and confining ourselves to the animal kingdom for the present, bearing in mind the fierce intensity of the struggle for life, it would seem that any scheme of colour that would enable its possessor to elude its foes or conceal itself from its prey, would be of vital importance. Hence we might infer that protective colouring would be a very usual phenomenon; and such we find to be the case. In the sea we have innumerable instances of protective colouring. Fishes that lie upon the sandy bottom are sand-coloured, like soles and plaice, in other orders we find the same hues in shrimps and crabs, and a common species on our shores (_Carcinus mænas_) has, just behind the eyes, a little light irregular patch, so like the shell fragments around that when it hides in the sand, with eyes and light spot alone showing, it is impossible to distinguish it.

The land teems with protective colours. The sombre tints of so many insects, birds and animals are cases in point, as are the golden coat of the spider that lurks in the buttercup, and the green mottlings of the underwings of the orange-tip butterfly. Where absolute hiding is impossible, as on the African desert, we find every bird and insect, without exception, assimilating the colour of the sand.

But if protective colour is thus abundant, it is no less true that colour of the most vivid description has arisen for the sole purpose of attracting notice. We observe this in the hues of many butterflies, in the gem-like humming birds, in sun-birds, birds of paradise, peacocks and pheasants. To see the shining metallic blue of a Brazilian Morpho flashing in the sun, as it lazily floats along the forest glades, is to be sure that in such cases the object of the insect is to attract notice.

These brilliant hues, when studied, appear to fall into two classes, having very diverse functions, namely Sexual and Warning Colours.

Protection is ensured in many ways, and among insects one of the commonest has been the acquisition of a nauseous flavour. This is often apparent even to our grosser senses; and the young naturalist who captures his first crimson-and-green Burnet Moth or Scarlet Tiger, becomes at once aware of the existence of a fetid greasy secretion. This the insectivorous birds know so well that not one will ever eat such insects. But unless there were some outward and visible sign of this inward and sickening taste, it would little avail the insect to be first killed and then rejected. Hence these warning colours--they as effectively signal danger as the red and green lamps on our railways.

It may here be remarked that wherever mimickry occurs in insects, the species mimicked is always an uneatable one, and the mimicker a palatable morsel. It is nature's way of writing "poison" on her jam-pots.

The other class of prominent colours--the Sexual--have given rise to two important theories, the one by Darwin, the counter-theory by Wallace.

Darwin's theory of Sexual Selection is briefly this:--He points out in much detail how the male is generally the most powerful, the most aggressive, the most ardent, and therefore the wooer, while the female is, as a rule, gentler, smaller, and is wooed or courted. He brings forward an enormous mass of well-weighed facts to show, for example, how often the males display their plumes and beauties before their loves in the pairing season, and his work is a long exposition of the truth that Tennyson proclaimed when he wrote:--

"In the spring a fuller crimson comes upon the robin's breast, In the spring the wanton lapwing gets himself another crest, In the spring a livelier iris changes on the burnished dove, In the spring the young man's fancy lightly turns to thoughts of love."

That birds are eminently capable of appreciating beauty is certain, and numerous illustrations are familiar to everyone. Suffice it here to notice the pretty Bower Birds of Australia, that adorn their love arbours with bright shells and flowers, and show as unmistakable a delight in them as the connoisseur among his art treasures.

From these and kindred facts Darwin draws the conclusion that the females are most charmed with, and select the most brilliant males, and that by continued selection of this character, the sexual hues have been gradually evolved.

To this theory Wallace takes exception. Admitting, as all must, the fact of sexually distinct ornamentation, he demurs to the conclusion that they have been produced by sexual selection.

In the first place, he insists upon the absence of all proof that the least attractive males fail to obtain partners, without which the theory must fail. Next he tells us that it was the case of the Argus pheasant, so admirably worked out by Darwin, that first shook his faith in sexual selection. Is it possible, he asks, that those exquisite eye-spots, shaded "like balls lying loose within sockets" (objects of which the birds could have had no possible experience) should have been produced ... "through thousands and tens of thousands of female birds, all preferring those males whose markings varied slightly in this one direction, this uniformity of choice continuing through thousands and tens of thousands of generations"?[1]

As an alternative explanation, he would advance no new theory, but simply apply the known laws of evolution. He points out, and dwells upon, the high importance of protection to the female while sitting on the nest. In this way he accounts for the more sombre hues of the female; and finds strong support in the fact that in those birds in which the male undertakes the household duties, he is of a domestic dun colour, and his gad-about-spouse is bedizened like a country-girl at fair time.

With regard to the brilliant hues themselves, he draws attention to the fact that depth and intensity of colour are a sign of vigour and health--that the pairing time is one of intense excitement, and that we should naturally expect to find the brightest hues then displayed. Moreover, he shows--and this is most important to us--that "the most highly-coloured and most richly varied markings occur on those parts which have undergone the greatest modification, or have acquired the most abnormal development."[2]

It is not our object to discuss these rival views; but they are here laid down in skeleton, that the nature of the problem of the principles of colouration may be easily understood.

Seeing, then, how infinitely varied is colouration, and how potently selection has modified it, the question may be asked, "Is it possible to find any general system or law which has determined the main plan of decoration, any system which underlies natural selection, and through which it works"? We venture to think there is; and the object of this work is to develop the laws we have arrived at after several years of study.

[1] Wallace, Tropical Nature, p. 206. [2] _Op. cit._, p. 206.