Darwinism (1889) Part 15

Many b.u.t.terflies, in all parts of the world, resemble dead leaves on their under side, but those in which this form of protection is carried to the greatest perfection are the species of the Eastern genus Kallima.

In India K. inachis, and in the larger Malay islands K. paralekta, are very common. They are rather large and showy b.u.t.terflies, orange and bluish on the upper side, with a very rapid flight, and frequenting dry forests. Their habit is to settle always where there is some dead or decaying foliage, and the shape and colour of the wings (on the under surface), together with the att.i.tude of the insect, is such as to produce an absolutely perfect imitation of a dead leaf. This is effected by the b.u.t.terfly always settling on a twig, with the short tail of the hind wings just touching it and forming the leaf-stalk. From this a dark curved line runs across to the elongated tip of the upper wings, imitating the midrib, on both sides of which are oblique lines, formed partly by the nervures and partly by markings, which give the effect of the usual veining of a leaf. The head and antennae fit exactly between the closed upper wings so as not to interfere with the outline, which has just that amount of irregular curvature that is seen in dry and withered leaves. The colour is very remarkable for its extreme amount of variability, from deep reddish-brown to olive or pale yellow, hardly two specimens being exactly alike, but all coming within the range of colour of leaves in various stages of decay. Still more curious is the fact that the paler wings, which imitate leaves most decayed, are usually covered with small black dots, often gathered into circular groups, and so exactly resembling the minute fungi on decaying leaves that it is hard at first to believe that the insects themselves are not attacked by some such fungus. The concealment produced by this wonderful imitation is most complete, and in Sumatra I have often seen one enter a bush and then disappear like magic. Once I was so fortunate as to see the exact spot on which the insect settled; but even then I lost sight of it for some time, and only after a persistent search discovered that it was close before my eyes.[75] Here we have a kind of imitation, which is very common in a less developed form, carried to extreme perfection, with the result that the species is very abundant over a considerable area of country.

_Protective Resemblance among Marine Animals._

Among marine animals this form of protection is very common. Professor Moseley tells us that all the inhabitants of the Gulf-weed are most remarkably coloured, for purposes of protection and concealment, exactly like the weed itself. "The shrimps and crabs which swarm in the weed are of exactly the same shade of yellow as the weed, and have white markings upon their bodies to represent the patches of Membranipora. The small fish, Antennarius, is in the same way weed-colour with white spots. Even a Planarian worm, which lives in the weed, is similarly yellow-coloured, and also a mollusc, Scyllaea pelagica." The same writer tells us that "a number of little crabs found clinging to the floats of the blue-sh.e.l.led mollusc, Ianthina, were all coloured of a corresponding blue for concealment."[76]

Professor E.S. Morse of Salem, Ma.s.s., found that most of the New England marine mollusca were protectively coloured; instancing among others a little red chiton on rocks clothed with red calcareous algae, and Crepidula plana, living within the apertures of the sh.e.l.ls of larger species of Gasteropods and of a pure white colour corresponding to its habitat, while allied species living on seaweed or on the outside of dark sh.e.l.ls were dark brown.[77] A still more interesting case has been recorded by Mr. George Brady. He says: "Amongst the Nullipore which matted together the laminaria roots in the Firth of Clyde were living numerous small starfishes (Ophiocoma bellis) which, except when their writhing movements betrayed them, were quite undistinguishable from the calcareous branches of the alga; their rigid angularly twisted rays had all the appearance of the coralline, and exactly a.s.similated to its dark purple colour, so that though I held in my hand a root in which were half a dozen of the starfishes, I was really unable to detect them until revealed by their movements."[78]

These few examples are sufficient to show that the principle of protective coloration extends to the ocean as well as over the earth; and if we consider how completely ignorant we are of the habits and surroundings of most marine animals, it may well happen that many of the colours of tropical fishes, which seem to us so strange and so conspicuous, are really protective, owing to the number of equally strange and brilliant forms of corals, sea-anemones, sponges, and seaweeds among which they live.

_Protection by Terrifying Enemies._

A considerable number of quite defenceless insects obtain protection from some of their enemies by having acquired a resemblance to dangerous animals, or by some threatening or unusual appearance. This is obtained either by a modification of shape, of habits, of colour, or of all combined. The simplest form of this protection is the aggressive att.i.tude of the caterpillars of the Sphingidae, the forepart of the body being erected so as to produce a rude resemblance to the figure of a sphinx, hence the name of the family. The protection is carried further by those species which retract the first three segments and have large ocelli on each side of the fourth segment, thus giving to the caterpillar, when the forepart of its body is elevated, the appearance of a snake in a threatening att.i.tude.

The blood-red forked tentacle, thrown out of the neck of the larvae of the genus Papilio when alarmed, is, no doubt, a protection against the attacks of ichneumons, and may, perhaps, also frighten small birds; and the habit of turning up the tail possessed by the harmless rove-beetles (Staphylinidae), giving the idea that they can sting, has, probably, a similar use. Even an unusual angular form, like a crooked twig or inorganic substance, may be protective; as Mr. Poulton thinks is the case with the curious caterpillar of Notodonta ziczac, which, by means of a few slight protuberances on its body, is able to a.s.sume an angular and very unorganic-looking appearance. But perhaps the most perfect example of this kind of protection is exhibited by the large caterpillar of the Royal Persimmon moth (Bombyx regia), a native of the southern states of North America, and known there as the "Hickory-horned devil."

It is a large green caterpillar, often six inches long, ornamented with an immense crown of orange-red tubercles, which, if disturbed, it erects and shakes from side to side in a very alarming manner. In its native country the negroes believe it to be as deadly as a rattlesnake, whereas it is perfectly innocuous. The green colour of the body suggests that its ancestors were once protectively coloured; but, growing too large to be effectually concealed, it acquired the habit of shaking its head about in order to frighten away its enemies, and ultimately developed the crown of tentacles as an addition to its terrifying powers. This species is beautifully figured in Abbott and Smith's _Lepidopterous Insects of Georgia_.

_Alluring Coloration._

Besides those numerous insects which obtain protection through their resemblance to the natural objects among which they live, there are some whose disguise is not used for concealment, but as a direct means of securing their prey by attracting them within the enemy's reach. Only a few cases of this kind of coloration have yet been observed, chiefly among spiders and mantidae; but, no doubt, if attention were given to the subject in tropical countries, many more would be discovered. Mr.

H.O. Forbes has described a most interesting example of this kind of simulation in Java. While pursuing a large b.u.t.terfly through the jungle, he was stopped by a dense bush, on a leaf of which he observed one of the skipper b.u.t.terflies sitting on a bird's dropping. "I had often," he says, "observed small Blues at rest on similar spots on the ground, and have wondered what such a refined and beautiful family as the Lycaenidae could find to enjoy, in food apparently so incongruous for a b.u.t.terfly.

I approached with gentle steps, but ready net, to see if possible how the present species was engaged. It permitted me to get quite close, and even to seize it between my fingers; to my surprise, however, part of the body remained behind, adhering as I thought to the excreta. I looked closely, and finally touched with my finger the excreta to find if it were glutinous. To my delighted astonishment I found that my eyes had been most perfectly deceived, and that what seemed to be the excreta was a most artfully coloured spider, lying on its back with its feet crossed over and closely adpressed to the body." Mr. Forbes then goes on to describe the exact appearance of such excreta, and how the various parts of the spider are coloured to produce the imitation, even to the liquid portion which usually runs a little down the leaf. This is exactly imitated by a portion of the thin web which the spider first spins to secure himself firmly to the leaf; thus producing, as Mr. Forbes remarks, a living bait for b.u.t.terflies and other insects so artfully contrived as to deceive a pair of human eyes, even when intently examining it.[79]

A native species of spider (Thomisus citreus) exhibits a somewhat similar alluring protection by its close resemblance to buds of the wayfaring tree, Viburnum lantana. It is pure creamy-white, the abdomen exactly resembling in shape and colour the unopened buds of the flowers among which it takes its station; and it has been seen to capture flies which came to the flowers.

But the most curious and beautiful case of alluring protection is that of a wingless Mantis in India, which is so formed and coloured as to resemble a pink orchis or some other fantastic flower. The whole insect is of a bright pink colour, the large and oval abdomen looking like the labellum of an orchid. On each side, the two posterior legs have immensely dilated and flattened thighs which represent the petals of a flower, while the neck and forelegs imitate the upper sepal and column of an orchid. The insect rests motionless, in this symmetrical att.i.tude, among bright green foliage, being of course very conspicuous, but so exactly resembling a flower that b.u.t.terflies and other insects settle upon it and are instantly captured. It is a living trap, baited in the most alluring manner to catch the unwary flower-haunting insects.[80]

_The Coloration of Birds' Eggs._

The colours of birds' eggs have long been a difficulty on the theory of adaptive coloration, because, in so many cases it has not been easy to see what can be the use of the particular colours, which are often so bright and conspicuous that they seem intended to attract attention rather than to be concealed. A more careful consideration of the subject in all its bearings shows, however, that here too, in a great number of cases, we have examples of protective coloration. When, therefore, we cannot see the meaning of the colour, we may suppose that it has been protective in some ancestral form, and, not being hurtful, has persisted under changed conditions which rendered the protection needless.

We may divide all eggs, for our present purpose, into two great divisions; those which are white or nearly so, and those which are distinctly coloured or spotted. Egg-sh.e.l.ls being composed mainly of carbonate of lime, we may a.s.sume that the primitive colour of birds'

eggs was white, a colour that prevails now among the other egg-bearing vertebrates--lizards, crocodiles, turtles, and snakes; and we might, therefore, expect that this colour would continue where its presence had no disadvantages. Now, as a matter of fact, we find that in all the groups of birds which lay their eggs in concealed places, whether in holes of trees or in the ground, or in domed or covered nests, the eggs are either pure white or of very pale uniform coloration. Such is the case with kingfishers, bee-eaters, penguins, and puffins, which nest in holes in the ground; with the great parrot family, the woodp.e.c.k.e.rs, the rollers, hoopoes, trogons, owls, and some others, which build in holes in trees or other concealed places; while martins, wrens, willow-warblers, and Australian finches, build domed or covered nests, and usually have white eggs.

There are, however, many other birds which lay their white eggs in open nests; and these afford some very interesting examples of the varied modes by which concealment may be obtained. All the duck tribe, the grebes, and the pheasants belong to this cla.s.s; but these birds all have the habit of covering their eggs with dead leaves or other material whenever they leave the nest, so as effectually to conceal them. Other birds, as the short-eared owl, the goatsucker, the partridge, and some of the Australian ground pigeons, lay their white or pale eggs on the bare soil; but in these cases the birds themselves are protectively coloured, so that, when sitting, they are almost invisible; and they have the habit of sitting close and almost continuously, thus effectually concealing their eggs.

Pigeons and doves offer a very curious case of the protection of exposed eggs. They usually build very slight and loose nests of sticks and twigs, so open that light can be seen through them from below, while they are generally well concealed by foliage above. Their eggs are white and s.h.i.+ning; yet it is a difficult matter to discover, from beneath, whether there are eggs in the nest or not, while they are well hidden by the thick foliage above. The Australian podargihuge goatsuckers--build very similar nests, and their white eggs are protected in the same manner. Some large and powerful birds, as the swans, herons, pelicans, cormorants, and storks, lay white eggs in open nests; but they keep careful watch over them, and are able to drive away intruders. On the whole, then, we see that, while white eggs are conspicuous, and therefore especially liable to attack by egg-eating animals, they are concealed from observation in many and various ways. We may, therefore, a.s.sume that, in cases where there seems to be no such concealment, we are too ignorant of the whole of the conditions to form a correct judgment.

We now come to the large cla.s.s of coloured or richly spotted eggs, and here we have a more difficult task, though many of them decidedly exhibit protective tints or markings. There are two birds which nest on sandy sh.o.r.es--the lesser tern and the ringed plover,--and both lay sand-coloured eggs, the former spotted so as to harmonise with coa.r.s.e s.h.i.+ngle, the latter minutely speckled like fine sand, which are the kinds of ground the two birds choose respectively for their nests. "The common sandpipers' eggs a.s.similate so closely with the tints around them as to make their discovery a matter of no small difficulty, as every oologist can testify who has searched for them. The pewits' eggs, dark in ground colour and boldly marked, are in strict harmony with the sober tints of moor and fallow, and on this circ.u.mstance alone their concealment and safety depend. The divers' eggs furnish another example of protective colour; they are generally laid close to the water's edge, amongst drift and s.h.i.+ngle, where their dark tints and black spots conceal them by harmonising closely with surrounding objects. The snipes and the great army of sandpipers furnish innumerable instances of protectively coloured eggs. In all the instances given the sitting-bird invariably leaves the eggs uncovered when it quits them, and consequently their safety depends solely on the colours which adorn them."[81] The wonderful range of colour and marking in the eggs of the guillemot may be imputed to the inaccessible rocks on which it breeds, giving it complete protection from enemies. Thus the pale or bluish ground colour of the eggs of its allies, the auks and puffins, has become intensified and blotched and spotted in the most marvellous variety of patterns, owing to there being no selective agency to prevent individual variation having full sway.

The common black coot (Fulica atra) has eggs which are coloured in a specially protective manner. Dr. William Marshall writes, that it only breeds in certain localities where a large water reed (Phragmites arundinacea) abounds. The eggs of the coot are stained and spotted with black on a yellowish-gray ground, and the dead leaves of the reed are of the same colour, and are stained black by small parasitic fungi of the Uredo family; and these leaves form the bed on which the eggs are laid.

The eggs and the leaves agree so closely in colour and markings that it is a difficult thing to distinguish the eggs at any distance. It is to be noted that the coot never covers up its eggs, as its ally the moor-hen usually does.

The beautiful blue or greenish eggs of the hedge-sparrow, the song-thrush, and sometimes those of the blackbird, seem at first sight especially calculated to attract attention, but it is very doubtful whether they are really so conspicuous when seen at a little distance among their usual surroundings. For the nests of these birds are either in evergreens, as holly or ivy, or surrounded by the delicate green tints of our early spring vegetation, and may thus harmonise very well with the colours around them. The great majority of the eggs of our smaller birds are so spotted or streaked with brown or black on variously tinted grounds that, when lying in the shadow of the nest and surrounded by the many colours and tints of bark and moss, of purple buds and tender green or yellow foliage, with all the complex glittering lights and mottled shades produced among these by the spring suns.h.i.+ne and by sparkling raindrops, they must have a quite different aspect from that which they possess when we observe them torn from their natural surroundings. We have here, probably, a similar case of general protective harmony to that of the green caterpillars with beautiful white or purple bands and spots, which, though gaudily conspicuous when seen alone, become practically invisible among the complex lights and shadows of the foliage they feed upon.

In the case of the cuckoo, which lays its eggs in the nests of a variety of other birds, the eggs themselves are subject to considerable variations of colour, the most common type, however, resembling those of the pipits, wagtails, or warblers, in whose nests they are most frequently laid. It also often lays in the nest of the hedge-sparrow, whose bright blue eggs are usually not at all nearly matched, although they are sometimes said to be so on the Continent. It is the opinion of many ornithologists that each female cuckoo lays the same coloured eggs, and that it usually chooses a nest the owners of which lay somewhat similar eggs, though this is by no means universally the case. Although birds which have cuckoos' eggs imposed upon them do not seem to neglect them on account of any difference of colour, yet they probably do so occasionally; and if, as seems probable, each bird's eggs are to some extent protected by their harmony of colour with their surroundings, the presence of a larger and very differently coloured egg in the nest might be dangerous, and lead to the destruction of the whole set. Those cuckoos, therefore, which most frequently placed their eggs among the kinds which they resembled, would in the long run leave most progeny, and thus the very frequent accord in colour might have been brought about.

Some writers have suggested that the varied colours of birds' eggs are primarily due to the effect of surrounding coloured objects on the female bird during the period preceding incubation; and have expended much ingenuity in suggesting the objects that may have caused the eggs of one bird to be blue, another brown, and another pink.[82] But no evidence has been presented to prove that any effects whatever are produced by this cause, while there seems no difficulty in accounting for the facts by individual variability and the action of natural selection. The changes that occur in the conditions of existence of birds must sometimes render the concealment less perfect than it may once have been; and when any danger arises from this cause, it may be met either by some change in the colour of the eggs, or in the structure or position of the nest, or by the increased care which the parents bestow upon the eggs. In this way the various divergences which now so often puzzle us may have arisen.

_Colour as a Means of Recognition._

If we consider the habits and life-histories of those animals which are more or less gregarious, comprising a large proportion of the herbivora, some carnivora, and a considerable number of all orders of birds, we shall see that a means of ready recognition of its own kind, at a distance or during rapid motion, in the dusk of twilight or in partial cover, must be of the greatest advantage and often lead to the preservation of life. Animals of this kind will not usually receive a stranger into their midst. While they keep together they are generally safe from attack, but a solitary straggler becomes an easy prey to the enemy; it is, therefore, of the highest importance that, in such a case, the wanderer should have every facility for discovering its companions with certainty at any distance within the range of vision.

Some means of easy recognition must be of vital importance to the young and inexperienced of each flock, and it also enables the s.e.xes to recognise their kind and thus avoid the evils of infertile crosses; and I am inclined to believe that its necessity has had a more widespread influence in determining the diversities of animal coloration than any other cause whatever. To it may probably be imputed the singular fact that, whereas bilateral symmetry of coloration is very frequently lost among domesticated animals, it almost universally prevails in a state of nature; for if the two sides of an animal were unlike, and the diversity of coloration among domestic animals occurred in a wild state, easy recognition would be impossible among numerous closely allied forms.[83]

The wonderful diversity of colour and of marking that prevails, especially in birds and insects, may be due to the fact that one of the first needs of a new species would be, to keep separate from its nearest allies, and this could be most readily done by some easily seen external mark of difference. A few ill.u.s.trations will serve to show how this principle acts in nature.

My attention was first called to the subject by a remark of Mr. Darwin's that, though, "the hare on her form is a familiar instance of concealment through colour, yet the principle partly fails in a closely allied species, the rabbit; for when running to its burrow it is made conspicuous to the sportsman, and no doubt to all beasts of prey, by its upturned white tail."[84] But a little consideration of the habits of the animal will show that the white upturned tail is of the greatest value, and is really, as it has been termed by a writer in _The Field_, a "signal flag of danger." For the rabbit is usually a crepuscular animal, feeding soon after sunset or on moonlight nights. When disturbed or alarmed it makes for its burrow, and the white upturned tails of those in front serve as guides and signals to those more remote from home, to the young and the feeble; and thus each following the one or two before it, all are able with the least possible delay to regain a place of comparative safety. The apparent danger, therefore, becomes a most important means of security.

The same general principle enables us to understand the singular, and often conspicuous, markings on so many gregarious herbivora which are yet, on the whole, protectively coloured. Thus, the American p.r.o.ng-buck has a white patch behind and a black muzzle. The Tartarian antelope, the Ovis poli of High Asia, the Java wild ox, several species of deer, and a large number of antelopes have a similar conspicuous white patch behind, which, in contrast to the dusky body, must enable them to be seen and followed from a distance by their fellows. Where there are many species of nearly the same general size and form inhabiting the same region--as with the antelopes of Africa--we find many distinctive markings of a similar kind. The gazelles have variously striped and banded faces, besides white patches behind and on the flanks, as shown in the woodcut.

The spring-bok has a white patch on the face and one on the sides, with a curiously distinctive white stripe above the tail, which is nearly concealed when the animal is at rest by a fold of skin but comes into full view when it is in motion, being thus quite a.n.a.logous to the upturned white tail of the rabbit. In the pallah the white rump-mark is bordered with black, and the peculiar shape of the horns distinguishes it when seen from the front. The sable-antelope, the gems-bok, the oryx, the hart-beest, the bonte-bok, and the addax have each peculiar white markings; and they are besides characterised by horns so remarkably different in each species and so conspicuous, that it seems probable that the peculiarities in length, twist, and curvature have been differentiated for the purpose of recognition, rather than for any speciality of defence in species whose general habits are so similar.

[Ill.u.s.tration: FIG. 18.--Gazella soemmerringi.]

It is interesting to note that these markings for recognition are very slightly developed in the antelopes of the woods and marshes. Thus, the grys-bok is nearly uniform in colour, except the long black-tipped ears; and it frequents the wooded mountains. The duyker-bok and the rhoode-bok are wary bush-haunters, and have no marks but the small white patch behind. The wood-haunting bosch-bok goes in pairs, and has hardly any distinctive marks on its dusky chestnut coat, but the male alone is horned. The large and handsome koodoo frequents brushwood, and its vertical white stripes are no doubt protective, while its magnificent spiral horns afford easy recognition. The eland, which is an inhabitant of the open country, is uniformly coloured, being sufficiently recognisable by its large size and distinctive form; but the Derbyan eland is a forest animal, and has a protectively striped coat. In like manner, the fine Speke's antelope, which lives entirely in the swamps and among reeds, has pale vertical stripes on the sides (protective), with white markings on face and breast for recognition. An inspection of the figures of antelopes and other animals in Wood's _Natural History_, or in other ill.u.s.trated works, will give a better idea of the peculiarities of recognition markings than any amount of description.

Other examples of such coloration are to be seen in the dusky tints of the musk-sheep and the reindeer, to whom recognition at a distance on the snowy plains is of more importance than concealment from their few enemies. The conspicuous stripes and bands of the zebra and the quagga are probably due to the same cause, as may be the singular crests and face-marks of several of the monkeys and lemurs.[85]

[Ill.u.s.tration: FIG. 19--Recognition marks of three African plovers.]

Among birds, these recognition marks are especially numerous and suggestive. Species which inhabit open districts are usually protectively coloured; but they generally possess some distinctive markings for the purpose of being easily recognised by their kind, both when at rest and during flight. Such are, the white bands or patches on the breast or belly of many birds, but more especially the head and neck markings in the form of white or black caps, collars, eye-marks or frontal patches, examples of which are seen in the three species of African plovers figured on page 221.

Recognition marks during flight are very important for all birds which congregate in flocks or which migrate together; and it is essential that, while being as conspicuous as possible, the marks shall not interfere with the general protective tints of the species when at rest.

Hence they usually consist of well-contrasted markings on the wings and tail, which are concealed during repose but become fully visible when the bird takes flight. Such markings are well seen in our four British species of shrikes, each having quite different white marks on the expanded wings and on the tail feathers; and the same is the case with our three species of Saxicola--the stone-chat, whin-chat, and wheat-ear--which are thus easily recognisable on the wing, especially when seen from above, as they would be by stragglers looking out for their companions. The figures opposite, of the wings of two African species of stone-curlew which are sometimes found in the same districts, well ill.u.s.trates these specific recognition marks. Though not very greatly different to our eyes, they are no doubt amply so to the sharp vision of the birds themselves.

Besides the white patches on the primaries here shown, the secondary feathers are, in some cases, so coloured as to afford very distinctive markings during flight, as seen in the central secondary quills of two African coursers (Fig. 21).

[Ill.u.s.tration: FIG. 20.--Oedicnemus vermiculatus (above). Oe.

senegalensis (below).]

Most characteristic of all, however, are the varied markings of the outer tail-feathers, whose purpose is so well shown by their being almost always covered during repose by the two middle feathers, which are themselves quite unmarked and protectively tinted like the rest of the upper surface of the body. The figures of the expanded tails of two species of East Asiatic snipe, whose geographical ranges overlap each other, will serve to ill.u.s.trate this difference; which is frequently much greater and modified in an endless variety of ways (Fig. 22).

Numbers of species of pigeons, hawks, finches, warblers, ducks, and innumerable other birds possess this cla.s.s of markings; and they correspond so exactly in general character with those of the mammalia, already described, that we cannot doubt they serve a similar purpose.[86]

[Ill.u.s.tration: FIG. 21.--Secondary quills.]

[Ill.u.s.tration: FIG. 22.--Scolopax megala (upper). S. stenura (lower).]

Those birds which are inhabitants of tropical forests, and which need recognition marks that shall be at all times visible among the dense foliage, and not solely or chiefly during flight, have usually small but brilliant patches of colour on the head or neck, often not interfering with the generally protective character of their plumage. Such are the bright patches of blue, red, or yellow, by which the usually green Eastern barbets are distinguished; and similar bright patches of colour characterise the separate species of small green fruit-doves. To this necessity for specialisation in colour, by which each bird may easily recognise its kind, is probably due that marvellous variety in the peculiar beauties of some groups of birds. The Duke of Argyll, speaking of the humming birds, made the objection that "A crest of topaz is no better in the struggle for existence than a crest of sapphire. A frill ending in spangles of the emerald is no better in the battle of life than a frill ending in spangles of the ruby. A tail is not affected for the purposes of flight, whether its marginal or its central feathers are decorated with white;" and he goes on to urge that mere beauty and variety for their own sake are the only causes of these differences.

But, on the principles here suggested, the divergence itself is useful, and must have been produced _pari pa.s.su_ with the structural differences on which the differentiation of species depends; and thus we have explained the curious fact that prominent differences of colour often distinguish species otherwise very closely allied to each other.

Among insects, the principle of distinctive coloration for recognition has probably been at work in the production of the wonderful diversity of colour and marking we find everywhere, more especially among the b.u.t.terflies and moths; and here its chief function may have been to secure the pairing together of individuals of the same species. In some of the moths this has been secured by a peculiar odour, which attracts the males to the females from a distance; but there is no evidence that this is universal or even general, and among b.u.t.terflies, especially, the characteristic colour and marking, aided by size and form, afford the most probable means of recognition. That this is so is shown by the fact that "the common white b.u.t.terfly often flies down to a bit of paper on the ground, no doubt mistaking it for one of its own species;" while, according to Mr. Collingwood, in the Malay Archipelago, "a dead b.u.t.terfly pinned upon a conspicuous twig will often arrest an insect of the same species in its headlong flight, and bring it down within easy reach of the net, especially if it be of the opposite s.e.x."[87] In a great number of insects, no doubt, form, motions, stridulating sounds, or peculiar odours, serve to distinguish allied species from each other, and this must be especially the case with nocturnal insects, or with those whose colours are nearly uniform and are determined by the need of protection; but by far the larger number of day-flying and active insects exhibit varieties of colour and marking, forming the most obvious distinction between allied species, and which have, therefore, in all probability been acquired in the process of differentiation for the purpose of checking the intercrossing of closely allied forms.[88]

Whether this principle extends to any of the less highly organised animals is doubtful, though it may perhaps have affected the higher mollusca. But in marine animals it seems probable that the colours, however beautiful, varied, and brilliant they may often be, are in most cases protective, a.s.similating them to the various bright-coloured seaweeds, or to some other animals which it is advantageous for them to imitate.[89]