THE INDIRECT REACTION OF THE COLOUR-SENSE UPON THE ANIMAL INTEGUMENTS.
In the last chapter we dealt with those cases in which the colour-sense of animals directly reacts upon the species themselves which possess it, by causing the more brilliantly-coloured among them to be specially favoured as parents of future generations. In the present chapter we must examine that other class of cases in which the colour-sense of one species indirectly reacts upon the appearance of other species, by causing all those individuals which present certain tints or spots to be destroyed, and only sparing those which present certain other tints or spots. In other words, the last chapter dealt with sexual selection; the present chapter deals with natural selection. In the first case certain special hues are favoured and, therefore, perpetuated; in the second case certain special hues are disadvantageous and, therefore, weeded out. Hence the action of the former cause is direct, the action of the latter indirect. Sexual selection actively chooses the beautiful, natural selection passively permits the fittest to survive.
Many of the cases which fall under the present head have already been cited elsewhere as proofs of the existence of a colour-sense in insects or vertebrates. Nevertheless, we may once more recapitulate them here, partly for the sake of formal completeness, but partly also to exhibit their mutual relations in a new and more systematic light. We shall thus be enabled with greater clearness to perceive how strong is the cumulative evidence which they afford for the general diffusion of a colour-sense throughout the animal world. At the same time, as this part of the subject has ere now been fully investigated by Mr. Darwin, Mr. Wallace, and numerous other well-known naturalists, I shall only attempt to give a very brief reasoned résumé of their labours, without references or details; referring those readers who wish for fuller information on the point to the original works from which my selection is made.
The colours produced (or rather spared) by natural selection fall under two groups, the Imitative and the Prohibitive.
By imitative colours we imply those which resemble the hues of some other body in such a manner as to insure protection or some other benefit for the species which possesses them. They may be useful for either of two purposes, — to escape the notice of enemies, or to deceive prey. In the first case, they enable the animal to avoid being itself devoured; in the second case, they enable it to devour others more easily, and so to secure a larger amount of food than less deceptively-coloured compeers. In the former instance, we must suppose that the majority of the original species which did not possess the imitative colouring have been discovered and devoured by enemies endowed with a colour-sense, while those which did possess the imitative colouring have continually survived. In the second instance, we must suppose that the individuals which had no imitative colouring have failed to secure sufficient food, through betraying their presence too readily to their prey, while those which had such colouring have successfully deceived their quarry, and so continually survived. We might compare the first case to that of a man who disguises himself in order to escape the observation of his enemies; and the second case to that of a man who hides himself under boughs and leaves to get a nearer shot at game. Practically, however, it is often hard to say for which of these two purposes a particular colour has been developed; and often the same colouring must enable the animal both to deceive its enemies and to escape the observation of its prey. We shall not, therefore, attempt in the sequel to distinguish between them.
One large class of imitative colours consists of a general resemblance to the whole surrounding environment. Of this we have cases in the soles and other flat-fish which exactly imitate the colour and speckled appearance of the sand on which they lie — so much so that even a careful human observer is often deceived at a distance of a few feet. Other instances are those of the birds, reptiles, and insects of Sahara, all of which, as Canon Tristram observes, copy closely the grey hue of the desert around them. Arctic animals are almost universally white. The fishes and Crustacea which live among the sargasso weed have a general yellow tint which renders them indistinguishable from the surrounding masses of algæ. Large marine animals, as Mr. Darwin points out, have their backs dark and their bellies whitish, which exactly corresponds to the general distribution of light and shade, as a spectator looks up or down in the water. Forestine birds and reptiles have ordinarily a ground-tint of green; and small green snakes and lizards are commonly found among grass or low herbage. Geckos are marbled like the walls and rocks on which they run. Some sea-side butterflies have sand-coloured wings. Aphides and many leaf-eating caterpillars are bright green in hue. Other instances are too numerous for insertion here. It is worth notice, however, that we find the general tendency to imitative colouring, in accordance with the whole environment, most strongly displayed where the environment is most uniform in its hues — as in Sahara, the Arctic snows, the sargasso sea, or the sands of the sea-bottom; because, in such circumstances, any variation of tint would be especially noticeable. Where the general distribution of colour is most varied, as in tropical forests, we find the greatest variety of animal colours; while the imitative devices are usually far more specialised, so as to resemble some particular object in the environment, not the prevailing hue of the environment as a whole.
In a second class of cases the resemblance, though still general, shows some more specialised features than those noted above. Thus, many caterpillars have spots which mimic the distribution of light and shade among the leaves on which they feed; and Sir John Lubbock refers to a like cause the colouration of those great cats which, like the leopard and jaguar, live among trees. Similarly, the same naturalist points out that the large grass-frequenting caterpillars have longitudinal lines, corresponding with those of the herbage around; while “those which live on large-veined leaves have oblique lines, like the oblique ribs of the leaves.” The jungle cats, too, such as the tiger, have perpendicular stripes, “rendering them very difficult to see among the brown grass which they frequent;” while “the ground cats, such as the lion and puma,” falling, of course, under our previous class, “are brownish or sand-colour, like the open places they inhabit.” Here, as before, only a few typical instances can be quoted, out of many hundreds collected by various careful observers.
A very specialised form of this adaptation to parti-coloured environments is found amongst those animals which, like the chameleon, and the chameleon-shrimp, possess the power of altering their colour, in accordance with the surface upon which they rest. But in this case it is remarkable, as Mr. Wallace observes, that only such colours can be produced as occur normally in the natural environment of the particular species.
A third class of cases with imitative colouring proceeds from general to special resemblances. Here we may place all the leaf-insects, stick-insects, and other creatures which present close similarities to various surrounding organic bodies. A sufficient number of these have been already mentioned to avoid the necessity for repetition at present.
A special case of this third class is shown in the well-known phenomena of mimicry, with which Mr. Bates and Mr. Wallace have made us familiar. These, too, have before received ample attention, and need not longer detain us now. With them we may close our first division of Imitative Colours.
The second division, that of Prohibitive Colours, embraces those cases where a colour acts as a warning of some noxious or disagreeable quality in its possessor. These colours are usually very conspicuous, as it must be supposed that they court attention, and so prove protective to the species. Among them may be noticed the bright-coloured but nauseous caterpillars and butterflies, numerous inedible reptiles and amphibia (such as Mr. Belt’s Nicaraguan frog already quoted), several birds of pugnacious habits, and perhaps some beetles and dragon-flies. Mr. E. N. Moseley believes that the colour of many marine organisms are prohibitive, and act as warnings to hungry passers-by. For my own part, however, I must confess that, when I consider the universality of colour as a means of attraction, I am almost as much inclined to doubt the reality of these explanations as Mr. Wallace is inclined to doubt the reality of sexual selection.
To sum up, we may conclude that the whole colouration of the organic world is, in the rough, perfectly explicable upon the hypothesis that the higher animals generally possess a colour-sense essentially identical with our own; while it is absolutely inexplicable if we suppose that they do not possess such a colour-sense. The inference is almost irresistible, that this hypothesis is true. Our cumulative proof has now been completed. We have seen that flowers, fruits, insects, birds, and mammals, all show us just the colouration which we should naturally expect if we believed all the more developed animals to see colours as we see them. We have also observed that many of them do undoubtedly possess such powers in a manner essentially similar to ourselves. Our hypothesis is thus a hypothesis which explains all the facts; the cause which it postulates is a vera causa, a cause otherwise known to be real and sufficient for the production of the facts; and so far as many of the cases are concerned, it is not a hypothesis at all, but a known and ascertained certainty. The grounds for believing in a common and identical colour-sense amongst all the higher animals are accordingly seen to be practically irresistible.
Note. — At the conclusion of the present portion of my work which deals with the colour-sense in lower animals, it may be well to point out what are the chief instances of organic colouration which the theories here adopted leave yet unexplained. They may be briefly summed up under three heads. The first includes the radiate animals, and such other marine creatures as the sea-slugs and some of the lower articulates. It is possible that the colours in these cases may be purely adventitious, depending entirely, like the green of leaves, on the chemical constitution of the pigmentary substance, and subserving no special function as colours. This is particularly likely in the case of deep-sea organisms, living at a depth where little or no light can ever penetrate. (See Sir Wyville Thomson’s “Depths of the Sea,” passim, and especially p, 466.) Nevertheless, animals found under such circumstances occasionally possess very large and striking eyes (see, for example, the figure of Cystosoma Neptuni in Sir W. Thomson’s “Voyage of the Challenger,” ), so that the colours may perhaps be protective. Upon this difficult subject the reader may consult Mr. Moseley’s interesting papers, where the colours of deep-sea organisms are explained as survivals of a habit originally acquired for protective purposes in shoal water. The second class includes the shells of Mollusca. At present, I see no other explanation of their colours save that they are purely adventitious; but this last refuge must only be regarded as provisional, since fresh facts or suggestions are continually coming to light, which enable us to discover some functional reason for what at first sight appeared purely accidental. The third class includes the eggs of birds. And here I am disposed to allege as a possible explanation that the colouration may act as a supplementary allurement to the instinct of incubation, just as sexual colours act as a supplementary allurement to the instinct of reproduction. This theory will seem less far-fetched when we recollect the fact that the eggs of reptiles, usually abandoned by the mother, are generally quite dingy in their coverings, while those of birds, forming objects of such great parental solicitude, are almost always more or less beautiful in their hues. And if we put these indications beside the other marks of æsthetic feeling in birds — their song, colour, dermal adjuncts, ornamental nests, bowers, and occasional habit of abstracting brilliant objects — the theory certainly gains in verisimilitude. On the other hand, it must always be remembered that the occurrence of colour never really demands an explanation in organic bodies, any more than it does in the ruby, the sapphire, or the emerald.
For further details upon the colouration of animals the reader must be referred to Mr. Wallace’s admirable work on “Tropical Nature.”