by Grant Allen
But we do not yet see why the pigments which are deposited in the dead processes of the skin, should possess one colour rather than another. The answer to this question must ultimately depend upon the habits and needs of each species. Where protective colouring proved most useful to the race, it would be acquired, as Mr. Wallace has well pointed out, among the infinite possible varieties of these very changeful substances; but where attractive hues were of greater advantage, they would be retained by sexual selection. Mr. Wallace has shown why pigment should be developed in the feathers of birds and the scales of butterflies; but it does not seem to me that he has sufficiently shown why one pigment rather than another should be developed. The black colouring of the crow, and the dusky wings of owls and moths, are just as much products of that integumentary modification on which he lays so much stress, as are the bright hues of butterflies or the plumage of parrots. In fact, it seems to me that Mr. Wallace does not sufficiently distinguish between pigment and bright pigment.
On the whole, therefore, while fully recognising the value of Mr. Wallace’s arguments with regard to the origin of pigments, the relation of ornament to activity, the protective use of subdued colour in certain cases to one or other sex, and so forth, I am still inclined to think that the general coincidence between bright food and bright colour noticed above, does really warrant a belief in such a simple form of sexual selection as that here advocated. Especially am I inclined to suppose that differences of taste thus originated may themselves be active differentiating agents for the production of new species with correlated habits.
A few further suggestions and observations of a general character may here be added, merely as hints for those who wish to pursue the subject independently.
The brightest colouration and most developed ornamental adjuncts seem to be confined to very small and active animals, such as butterflies, beetles, humming-birds, sun-birds, and flying-lizards. Those of the somewhat larger creatures, parrots, toucans, fishes, snakes, and greater reptiles or amphibia, are, on the whole, not quite so brilliant or so largely developed; and the activity of these species is less than that of the preceding group. On the other hand, the largest animals of each great division, among vertebrates at least, show a decided tendency toward very dull and inconspicuous hues; take as examples the sharks, sturgeons, sun-fish, tunny, and cod; the gigantic salamander; the crocodiles, turtles, and great snakes; the ostriches, emus, eagles, condors, storks, swans, and penguins; the whales, walruses, elephants, hippopotami, rhinoceroses, gorillas, bears, buffaloes, and elks. These are mostly slow and inactive animals, and they are also little distinguished for expanded epidermal modifications. Indeed, it may be said, roughly speaking, that all very large birds or mammals show more or less tendency to lose or minimise their feathers or hair.
Those larger animals which, like the mandrill, the cassowary, and the king-vulture, display brilliant colouring, have it disposed in small patches on a particular portion of the body, not over its whole surface.
Flying animals seem to show special æsthetic tastes. At least, they are largely provided with apparent sexual allurements. Such are the colours of birds, flying-lizards, and butterflies; the perfumes of some Lepidoptera; the stridulating organs of insects and the song of birds; and the frills, ruffs, crests, lappets, or tails of birds and butterflies. Of course, the origin of these may be largely accounted for by the causes which Mr. Wallace assigns; but their selection and persistence seems to imply an unusual æsthetic sensibility. It is among birds alone, too, that we find clear evidence of æsthetic feelings, as with the magpies and bower birds. Now, is it not possible that the comparative security which each flying race obtained on its first development, permitted the various species to indulge their taste to a greater degree than would have been the case with terrestrial creatures. Would not flying creatures be more likely to notice and follow the attraction of such a sense-stimulation than sluggish terrestrial animals? And might not the introduction of predatory species at a later date, capable of preying on these classes, afterwards modify the colouration or other sexual allurement according to varying circumstances? For example, may not the growth of insect-eating birds have affected butterflies in various ways, so as to preserve those with protective or warning colours, while yet leaving many traces of the primitive sexual colouration? or may not the presence or absence of hawks and birds of prey have determined the development of song or colour respectively as an allurement in each species of bird? Does not the abundance of coloured animals in certain isolated lands, lying close to great continents, point toward the conclusion, that where special immunity from enemies exists, the æsthetic fancy can be more implicitly followed? And may not Mr. Wallace’s own remarks upon the humming-birds of Juan Fernandez possibly bear the same construction?
A similar hint may be thrown out with regard to fish, which bear somewhat the same relation to the other inhabitants of the ocean as flying creatures bear to the terrestrial animals. Indeed, the analogy between the fins of a gurnard and the wings of a butterfly must have been forced upon any one who has seen those exquisite fish in an aquarium. Again, the whole group of higher arboreal mammals have some considerable likeness to birds and the other winged animals in the activity of their movements, and the comparative security of their elevated position, while their tendency to produce such forms as the flying squirrels, the anomalurus, the galeopithecus, and the bats, shows how close the functional and adaptive resemblance may sometimes become. Now, we have already seen that these arboreal animals are on the whole much more brightly coloured than other mammals, and we have also noted their tendency to develop hairy appendages, such as beards, ruffs, and top-knots. Moreover, some of them are also distinguished for their loud and piercing cries (produced in the case of the howlers by a special organ), which may be compared with the song of birds, and the presumably sexual noises given out by some other creatures.
Here, too, we see a striking analogy between the development of the sense of sight in general and of the colour-sense. For eyes, as we observed in Chapter iii., are most developed in the most locomotive races; and the colour-sense, the highest mode of sight, seems to be most highly developed only in exceptionally locomotive races. It would appear natural that only very mobile animals could derive any special advantage from the indications afforded by colour, and hence we may account for the special share which flying insects and birds have borne in the production of bright-hued flowers and fruits, as well as for the frequent brilliancy of their own colouration.
Once more, it is worth noting that the hair of mammals seems very little adapted for the display of brilliant and pure pigments. The best that can be said of the hues produced is that they are bluish, rufous, or white, never that they are scarlet, purple, golden, or bright blue. The only cases in which mammals present really brilliant colouring are those like the mandrill and certain other monkeys, where the pigment is displayed beneath patches of bare skin, not in the hair or other epidermal modification.
After these remarks it is hardly necessary for me to add that I attach full weight to Mr. Wallace’s general principles with regard to the importance of activity and of extended tegumentary surfaces as a groundwork for the production of colour or ornamental adjuncts. But I regard sexual selection, in the modified sense already noted, as the agency by which the particular colours and ornaments have been chosen from the whole possible number, and fixed in the typical specific mould.
Finally, it may be well to point out that a certain analogy seems to exist between the commonest animal pigments, and those of fruits and flowers. Some of the colours of animals fade after death, whence we may conclude that the substances of which they are composed remain in a state of comparative chemical instability. But most of the colours here treated of remain permanently after death, as may be seen in the case of stuffed birds and preserved insects. Accordingly, these bodies may be considered with great probability as comparatively stable in chemical composition, and as little affected by the danger of oxidisat
ion. So we may perhaps guess that they are themselves oxidation products whose affinities are nearly saturated. If so, they might fall ultimately under the same category with the colouring matter of flowers, fruits, fading leaves, and other bright-hued vegetal products.
The reader will doubtless object that this chapter is far from being conclusive. I am well aware of its deficiencies in this respect, and shall feel fully satisfied if it only prove suggestive.
CHAPTER X.
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.
