Many other cases are known among insects in which the same species acquires a different tint according to its surroundings; this being particularly marked in some South African locusts, which correspond with the colour of the soil wherever they are found. There are also many caterpillars which feed on two or more plants, and which vary in colour accordingly. A number of such changes are quoted by Mr. R. Meldola, in a paper on "Variable Protective Colouring in Insects" (Proceedings of the Zoological Society of London, 1873, p. 153); and in some cases it has been shown that green chlorophyll remains unchanged in the tissues of leaf-eating insects, and being discernible through the transparent integument, produces the same colour as that of the food plant. In all these insects, as well as in the great majority of cases in which a change of colour occurs in other animals, the action is quite involuntary; but among some few of the higher animals the colour of the integument can be modified at the will of the individual, or at all events by a reflex action dependent on sensation. The most remarkable case of this kind occurs with the chameleon, which has the power of changing its colour from dull white to a variety of tints. This singular power has been traced to two layers of movable pigment-cells deeply seated in the skin, but capable of being brought near to the surface. The pigment-layers are bluish and yellowish, and by their contraction or concentration these can be forced upwards either together or separately. When the animal is passive the colour is dirty white, which changes to various tints of bluish, green, yellow, or brown, as more or less of either pigment is forced up and rendered visible. The animal is excessively sluggish and defenceless, and its power of changing its colour so as to harmonise with surrounding objects is essential to its safety. Here too, as with the pupa of Papilio Nireus, colours such as scarlet or blue, which do not occur in the natural environment of the animal, cannot be produced. Somewhat similar changes of colour occur in some prawns and flat-fish, according to the colour of the bottom on which they rest. This is very striking in the chameleon shrimp (Mysis chamæleon), which is gray when on sand, but brown or green when among sea so that weed of these two colours. Experiment shows, however, that when blinded the change does not occur; here too we probably have a voluntary or reflex senseaction. These peculiar powers of change of colour and adaptation are, however, rare and quite exceptional. As a rule there is no direct connection between the colours of organisms and the kind of light to which they are usually exposed. This is well seen in most fishes, and in such marine animals as porpoises, whose backs are always dark, although this part is exposed to the blue and white light of the sky and clouds, while their bellies are very generally white, although these are constantly subjected to the deep blue or dusky green light from the bottom. It is evident, however, that these two tints have been acquired for concealment and protection. Looking down on the dark back of a fish it is almost invisible, while, to an enemy looking up from below, the light undersurface would be equally invisible against the light of the clouds and sky. Again, the gorgeous colours of the butterflies which inhabit the depths of tropical forests bear no relation to the kind of light that falls upon them, coming as it does almost wholly from green foliage, dark brown soil, or blue sky; and the bright underwings of many moths, which are only exposed at night, contrast remarkably with the sombre tints of the upper wings, which are more or less exposed to the various colours of surrounding nature. Classification of Organic Colours We find, then, that neither the general influence of solar light and heat, nor the special action of variously tinted rays, are adequate causes for the wonderful variety, intensity, and complexity of the colours that everywhere meet us in the animal and vegetable worlds. Let us therefore take a wider view of these colours, grouping them into classes determined by what we know of their actual uses or special relations to the habits of their possessors. This, which may be termed the functional and biological classification of the colours of living organisms, seems to be best expressed by a division into five groups, as follows:— Animals 1. Protective colours. a. Of creatures specially protected. 2. Warning colours. {%. Of defenceless creatures, mimicking a. 3. Sexual colours. 4. Normal colours.1 Plants 5. Attractive colours. It is now proposed, firstly, to point out the nature of the phenomena presented under each of these heads; then to explain the general laws of the production of colour in nature; and, lastly, to show how far the varied phenomena of animal coloration can be explained by means of those laws, acting in conjunction with the laws of evolution and natural selection. Protective Colours The nature of the two first groups, protective and warning colours, has been so fully detailed and illustrated in my chapter on "Mimicry and other Protective Resemblances among Animals," that very little need be added here except a few words of general explanation. Protective colours are exceedingly prevalent in nature, comprising those of all the white arctic animals, the sandy-coloured desert forms, and It also comthe green birds and insects of tropical forests. prises thousands of cases of special resemblance-of birds to the surroundings of their nests, and especially of insects to the bark, leaves, flowers, or soil, on or amid which they dwell. Mammalia, fishes, and reptiles, as well as mollusca and other marine invertebrates, present similar phenomena; and the more the habits of animals are investigated, the more numerous are found to be the cases in which their colours tend to conceal them, either from their enemies or from the creatures they prey upon. One of the last-observed and most curious of these protective resemblances has been communicated to me by Sir Charles Dilke. He was shown in Java a pink-coloured Mantis which, when at rest, exactly resembled a pink orchis-flower. The mantis is a carnivorous insect which lies in wait for its prey; and, by its resemblance to a flower, the insects it feeds on would be actually attracted towards it. This one is said to feed especially on butter 1 Many, or perhaps all, of these are now believed to be diversely coloured for purposes of recognition. See Darwinism, p. 217. flies, so that it is really a living trap, and forms its own bait ! 1 All who have observed animals, and especially insects, in their native haunts and attitudes, can understand how it is that an insect which in a cabinet looks exceedingly conspicuous, may yet when alive, in its peculiar attitude of repose and with its habitual surroundings, be perfectly well concealed. We can hardly ever tell by the mere inspection of an animal whether its colours are protective or not. No one would imagine the exquisitely beautiful caterpillar of the emperor-moth, which is green with pink star-like spots, to be protectively coloured; yet, when feeding on the heather, it so harmonises with the foliage and flowers as to be almost invisible. Every day fresh cases of protective colouring are being discovered, even in our own country; and it is becoming more and more evident that the need of protection has played a very important part in determining the actual coloration of animals. Warning Colours The second class—the warning colours-are exceedingly interesting, because the object and effect of these is, not to conceal the object, but to make it conspicuous. To these creatures it is useful to be seen and recognised; the reason being that they have a means of defence which, if known, will prevent their enemies from attacking them, though it is generally not sufficient to save their lives if they are actually attacked. The best examples of these specially protected creatures consist of two extensive families of butterflies, the Danaidæ and Acræidæ, comprising many hundreds of species inhabiting the tropics of all parts of the world. insects are generally large, are all conspicuously and often most gorgeously coloured, presenting almost every conceivable tint and pattern; they all fly slowly, and they never attempt to conceal themselves; yet no bird, spider, lizard, or monkey (all of which eat other butterflies) ever devours them. The reason simply is that they are not fit to eat, their juices having a powerful odour and taste that is absolutely disgusting to all these animals. Now we see the reason of their 1 These cases form a distinct sub-group of "alluring coloration." Darwinism, p. 210. These See showy colours and slow flight. It is good for them to be seen and recognised, for then they are never molested; but if they did not differ in form and colouring from other butterflies, or if they flew so quickly that their peculiarities could not be easily noticed, they would be captured, and though not eaten would be maimed or killed. As soon as the cause of the peculiarities of these butterflies was clearly recognised, it was seen that the same explanation applied to many other groups of animals. Thus, bees and wasps and other stinging insects are showily and distinctively coloured; many soft and apparently defenceless beetles, and many gay-coloured moths, were found to be as nauseous as the above-named butterflies; other beetles, whose hard and glossy coats of mail render them unpalatable to insect-eating birds, are also sometimes showily coloured; and the same rule was found to apply to caterpillars, all the brown and green (or protectively coloured species) being greedily eaten by birds, while showy kinds which never hide themselves-like those of the magpie-, mullein-, and burnet-moths were utterly refused by insectivorous birds, lizards, frogs, and spiders (p. 84). Some few analogous examples are found among vertebrate animals. I will only mention here a very interesting case not given in my former work. In his delightful book, entitled The Naturalist in Nicaragua, Mr. Belt tells us that there is in that country a frog which is very abundant, which hops about in the day-time, which never hides himself, and which is gorgeously coloured with red and blue. Now frogs are usually green, brown, or earth-coloured, feed mostly at night, and are all eaten by snakes and birds. Having full faith in the theory of protective and warning colours, to which he had himself contributed some valuable facts and observations, Mr. Belt felt convinced that this frog must be uneatable. He therefore took one home, and threw it to his ducks and fowls; but all refused to touch it except one young duck, which took the frog in its mouth, but dropped it directly, and went about jerking its head as if trying to get rid of something nasty. Here the uneatableness of the frog was predicted from its colours and habits, and we can have no more convincing proof of the truth of a theory than such previsions. |