Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/361

Rh I M I C B Y 343 and only slowly discovered his mistake. The cases here mentioned form but a small part of all those that have hitherto been observed and described in the insect world. They amount altogether to many hundreds. Among plants, though included in the above definition for the sake of formal completeness, instances of true mimicry are rare or almost unknown. Perhaps the nearest approach to this phenomenon in the vegetal world is found in the resemblance borne by the dead-nettle, Lamium album, and a few other labiates, to the stinging nettle, Urtica dioica and U. urens. The true nettles are strik ingly protected from animal foes by their stinging hairs ; and the general appearance of the dead-nettle is sufficiently like them to prevent human beings from plucking it, and therefore probably to deter herbivorous mammals from eating it down. Mr Mansel Weale mentions another labiate, Ajuya ophrydis, of South Africa, which closely resembles an orchid, and may thus induce insects to fertilize its flowers. Mr Worthington Smith has found three rare British fungi, each accompanying common species which they closely resembled ; and one of the common species possesses a bitter and nauseous taste ; so that this would seem to be a case of true mimicry. Many diverse instances alleged by Mr A. W. Bennett, Dr Cooke, and others cannot be considered as genuine mimetic resem blances in the sense here laid down. They are mere coincidences or similar adaptations to similar needs ; and the word ought to be applied strictly to such likenesses alone as benefit the organism in which they occur by caus ing it to be mistaken for another possessing some special advantage of its own. The theoretical explanation of mimicry on evolutionary principles may best be considered in connexion with the general subject of protective coloration and variation in form, of which it is a very special case. There are two ways in which imitative colouring may benefit a species. It may help the members of the species to escape the notice of enemies, or it may help them to deceive prey. In the first case imitative hues enable the animal or plant 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 such individuals as did not possess the deceptive colouring have been discovered and destroyed by enemies with highly developed sight, while those which possessed it have survived. In the latter instance we must suppose that the individuals which have no protective colouring have failed to secure sufficient prey, through too readily betraying their presence, and that only those which possessed such colouring have become the parents of future generations. It is difficult, however, to separate these two cases, and in many instances the same colouring may aid a species both in escaping its peculiar enemies and in deceiving its peculiar prey. They may therefore most conveniently be considered together. Colour is always liable to vary from individual to individual, as we see in the case of domesticated fowls, rabbits, dogs, and other animals, as well as in most cultivated flowers, wherever natural selection cannot act to keep the typical specific hues pure and true. But iu a wild state certain conspicuous colours are sure to prove disadvantageous by betraying the individual, and these will sooner or later get weeded out, under certain circumstances, either through the action of enemies or by starvation resulting from the inability to escape the notice of prey. On the other hand, certain other colours are sure to benefit the individual by harmonizing with the tints of the environment and these will be spared by natural selection, so that the individuals possessing them will pair with one another, and will hand down their peculiarities to their de scendants. In this way many species will acquire and retain a i-oloration that harmonizes with their environment as a whole or with some special part of it. The degree to which the protective coloration will be carried, however, must depend upon the sharp ness of the senses in those other organisms which it is desirable to deceive. Large dominant herbivorous or frugivorous mammals or birds, with relatively few enemies, would not be benefited by protective coloration, and so they seldom exhibit it. The grasses or fruits on which they feed cannot make any attempt to escape them. But carnivores generally require to deceive their prey, and therefore a large number of them exhibit marked deceptive colouring Still more especially do small defenceless birds or mammals need to escape the notice of the carnivores, and they accordingly very generally possess dull colours, because any variation in the direction of conspieuousness is certain to be promptly cut off. Above all, among insects, which are so largely the prey of birds, of reptiles, and of other animals possessing highly developed vision, protective coloration in one form or other is almost universal, except where a nauseous taste, hairy skin, or hard external coverings afford a different kind of protection. In every case the weeding out of ill-protected forms must depend upon the relative keenness of vision in the various enemies or of the prey, be they mammals, birds, reptiles, insects, or spiders. Hence the existence of protective coloring and of mimicry incidentally affords us valuable hints as to the perceptive faculties of the various classes against which each organism is thus unconsciously guarded. Where the general aspect of the environment is most uniform, and where little but a vague impression of colour without individual form can be conveyed, the hues of animals arc also usually uniform, to match their surroundings, and no special imitative adaptations of form occur. Thus, among the Arctic snows, a brown or black animal would immediately be perceived, and if defenceless at once devoured, -while if a carnivore it would seldom or never approach unperceived near enough to its prey to effect a capture. Hence all such variations are at once repressed, and almost all Arctic animals, like the American polar hare, are pure white. Elsewhere bears are black or brown ; in the polar region the native species is nearly indistinguishable from the snow in which it lives. Where the environment undergoes a regular change from season to season the colour of the fauna varies with it. The Arctic fox, the ermine, the alpine hare, the ptarmigan and many other birds, are all more or less brown among the brown hill-sides of autumn, and snow-whito among the winter snows. Almost equally general is the sandy colour of deserts, though this, instead of being uniform, is slightly varied from grain to grain ; and nearly all the birds, reptiles, and insects of Sahara exactly copy the sandy grey hue of the desert around them. Soles and other flat-fish (Pleuroncctidse) closely imitate the colour and speckled appearance of the sand on which they lie. The fishes and crustaceans which inhabit the sargasso weed are coloured the same yellow as the masses of algai to which they cling. Aphides and many small leaf-eating caterpillars are bright green like the neighbouring foliage. Where the environment is somewhat more diverse, the resemblance begins to show more specialized features. The lion, a large ground- cat of desert or rocky districts, is uniformly brown ; but the tiger and other jungle-cats have perpendicular stripes which harmonize with the bamboos and brown grass of their native haunts ; while the leopards, jaguars, and other tree-cats have ocellated spots which conceal them among the mingled light and shade of the forests. Large marine animals have the back black, because the water looks dark when seen from above, but their bellies are white, so as to harmonize with the colour of the surface when seen from below. Dr Weismann has shown that most edible unprotected caterpillars imitate the stripes and shades of the leaves among which they feed. Those which live upon grasses are longitudinally striped like the blades, those which live among small leaves are spotted and variid so as to resemble the distribution of light and shade in the bushes, and those which live upon large veined leaves with oblique ribs have oblique lines to harmonize with them. In some cases even the unripe berries are represented on the caterpillar by small reddish spots. A specialized form of this particular protective device is found in the chameleon, the chameleon-shrimp, many flat-fish, and some amphibians, all of which can vary their coloration to suit that of the surface on which they rest. The action is reflex, and 1 ceases if the animal is blinded. Where the environment is very varied, as in tropical forests, we of particular forms ; and the protective resemblances become at once closer and more common. Birds, reptiles, spiders, monkeys, and other active predaceous creatures are constantly hunting for insects and similar small prey amongst the fallen sticks or leaves ; and among the most powerless classes of insects only those which very closely resemble specific objects in the environment can easily escape them. A gradual passage can be traced from the most general to the most special resemblances under such circumstances. Many forestine birds have a ground-tone of green in their plumage, which occurs nowhere but in the tropics. Some tree-lizards are green like the leaves on which they sit, others are marbled to resemble the bark where they lie in wait for their prey. Arboreal snakes often hang like lianas or other creepers. Insects which cling to the trunks of trees can seldom be distinguished from the bark. A Sumatran butterfly, Kallima paralcda, always settles on dry bushes among dead leaves, and can then hardly be perceived among the brown foliage, which it imitates even in the apparent blotches and mildew with which its wings are covered. The family of Phasmulss, including the leaf and stick insects, carries such forms of imitation very far indeed. Most of them are large, soft, defenceless creatures ; but some, like Phyllium, closely reseinble green leaves, so as to be almost indistinguishable while feeding ; and others exactly imitate short broken twigs of bamboo. Mr Wallace found one such insect, Ceroxylus laceratus, in Borneo,
 * find the greatest variety of colouring as well as actual imitation