In considering the phenomena under review, it may be well to give first the chief observed facts, which are quite independent of any particular explanation, and then the theory which has been started to account for them by Mr Bates and Mr Wallace. Before doing so, however, true mimicry should be carefully discriminated from one or two superficially similar modes of resemblance among organic beings, whose real implications are very different.

It must not be confused with mere accidental or adaptive resemblance, due either to simple chance or to similarity of external conditions. As a case of the first sort, we may adduce the real or fancied resemblance between certain orchids and flies or spiders; as a case of the second sort, we may take certain African Euphorbiaceae, which, growing in dry deserts, have acquired a very close likeness to the cactuses that cover the equally dry deserts of Mexico; or again the sub-Antarctic gallinaceous bird, Chionis alba, which, living on the sea-shore, has acquired a coloration like that of the gulls, together with the legs of a wader. These resemblances, however, do not as such subserve any function. The species apparently mimicking and the species apparently mimicked either do not inhabit the same district or do not come into any definite relation with one another. The likeness is either accidental, or else it is due to similar adaptation to similar circumstances. In cases of true mimicry, on the other hand, the mimicking species derives a direct advantage from its likeness to the species mimicked; the resemblance is deceptive; and this is equally true whether we suppose the mimicry to be produced by creative design or by natural selection. On either hypothesis, however it came by its likeness, the mimicking species escapes certain enemies or obtains certain sorts of food by virtue of its resemblance to some other kind.

It should also be added that the word mimicry, as applied to such cases, is used only in a metaphorical sense. It is not intended to imply any conscious or voluntary imitation by one species of the appearance or habits of another. All that is meant is the fact of an advantageous resemblance, a delusive similarity, which gives the mimicking animal or plant some extra protection or some special means of acquiring food which it would not otherwise have possessed but for its likeness to the creature mimicked.

Taking animals first, mimicry does not occur very frequently among the higher classes. In the vertebrates it is comparatively rare, and among mammals probably only one good case has yet been adduced. This is that of Cladobates, an insectivorous genus of the Malayan region, many species of which closely resemble squirrels in size, in colour, and in the bushiness and posture of the tail. It has been suggested by Mr Wallace (from whom most of the following examples have been borrowed) that Cladobates may thus be enabled to approach the insects and small birds which form its prey under the disguise of the harmless fruit-eating squirrel. In this case, as in some others, the resemblance is not protective, but is apparently useful to the animal in the quest for food.

Among birds, Mr Wallace has pointed out that the general likeness of the cuckoo, a weak and defenceless group, to the hawks and gallinaceous tribe makes some approach to real mimicry. But besides such vague resemblances there are one or two very distinct cases of true mimicry in this class of vertebrates. In Australia and the Moluccas lives a genus of dull-hued honey-suckers, Tropidorhynclιus, consisting of large, strong, active birds, with powerful claws and sharp beaks. They gather together in noisy flocks, and are very pugnacious, driving away crows and even hawks. In the same countries lives a group of orioles, forming the genus Mimeta; and these, which are much weaker birds, have not the usual brilliant colouring of their allies the golden orioles, but are usually olive-green or brown. In many cases species of Mimeta closely resemble the Tropidoτhynclιi inhabiting the same island. For example, on the island of Bouru are found the Tτopidorhynchus bouruensis and Mimeta bouruensis, the latter of which mimics the former in the particulars thus noted by Mr Wallace :—“The upper and under surfaces of the two birds are exactly of the same tints of dark and light brown. The Tτopidorhynchus ha⅛ a large bare black patch round the eyes; this is copied in the Mimeta by a patch of black feathers. The top of the head of the Tropidorhynchus has a scaly appearance from the narrow scale-formed feathers, which are imitated by the broader feathers of the Mimeta having a dusky line down each. The Tropidorhynchus has a pale ruff formed of curious recurved feathers on the nape (which has given the whole genus the name of friar-birds); this is represented in the Mimeta by a pale band in the same position. Lastly, the bill of the Tropidorhynchus is raised into a protuberant keel at the base, and the Mimeta has the same character, although it is not a common one in the genus. The result is that on a superficial examination the birds are identical, although they have important structural differences, and cannot be placed near each other in any natural arrangement.” Allied species of Tropidorhynchus in Ceram and Timor are similarly mimicked by the local Mimeta of each island. Mr Osbert Salvin has likewise noticed a case of mimicry among the birds of prey near Rio Janeiro. An insect-eating hawk, Harpagus diodon, is closely resembled by a bird-eating haλvk, Accipiter pileatus. Here the advantage seems to be that the small birds have learned not to fear the Harpagus, and the Accipiter is able to trade upon the resemblance by catching them unawares, both birds being reddish-brown when seen from beneath. But the Accipiter has the wider range of the two; and where the insect-eating species is not found it no longer resembles it, but varies in the under wing-coverts to white. Here again the resemblance, though advantageous, is not protective.

Among reptiles, Mr Wallace has instanced some curious cases where a venomous tropical American genus of snakes, Elaps, with brightly-banded colours, is closely mimicked by several genera of harmless snakes, having no affinity with it, but inhabiting the same region. Thus the poisonous Elaps fulvus of Guatemala has black bands on a coral-red ground; the harmless Pliocerus aequalis of the same district is coloured and banded precisely like it. The likeness affords the unarmed snakes a great protection, because other animals probably will not touch them, mistaking them for the venomous kinds.

It is among the invertebrates, however, and especially among insects, that cases of mimicry are most frequent and were first observed. In the order Lepidoptera, besides the classical instance of Leptalis and the Heliconidae, a genus of another family, the Erycinidee, also mimics the same group. The flocks of one species of Lthomia, an uneatable butterfly, often have flying with them a few individuals of three other widely different genera, quite indistinguishable from them when on the wing. In the tropics of the Old World, the Danaidae and A craeidae possess a similar protective odour, and are equally abundant in individuals; they are closely mimicked by various species of Papilio and Diadema. Mr Trimen, in a paper on “Mimetic Analogies among African Butterflies,” gives a list of sixteen species or varieties of Diadema or its allies, and ten species of Papilio, each of which mimics a Danais or Acraea of the same region in the minutest particulars of form and colour. The Danais tytia of India has semi-transparent bluish wings, and a border of reddish-brown; this coloration is exactly reproduced in Papilio agestor and Diadema nama, all three insects frequently coming together in collections from Darjiling. In the Malay Archipelago the common and beautiful Euplaea midamus is so exactly mimicked by two rare species of Papilio that Mr Wallace generally mistook the latter at first for the ordinary insect. An immense number of other instances among the Lepidoptera have been quoted from other parts of the world.

Occasionally species of Lepidoptera also imitate insects of other orders. Many of them take on the appearance of bees or wasps, which are of course protected by their stings. Thus the Sesiidae and Aegeriidae, two families of diurnal moths, have species so like hymenopterous insects that they are known by such names as apiformis, vespiforme, ichneumoniforme, sphegiforme, and so forth. The British sesia bombiliformis closely resembles the humble bee; the Sphecia craboniformis is coloured like a hornet, and carries its wings in the same fashion. Some Indian Lepidoptera have the hind legs broad and densely hairy, so as exactly to imitate the brush-legged bees of the same country. Mi-Belt mentions a Nicaraguan moth, Pionia lycoides, which closely mimics a distasteful coleopterous genus, Calopteron∙, and Professor Westwood pointed out that the resemblance to the beetle is still further increased in the moth by raised lines of scales running lengthwise down the thorax.

Among the Coleoptera, or beetles, and other orders, similar disguises are not uncommon. Mr Belt noticed species of Hemiptera and Coleoptera, as well as spiders, in Nicaragua, which exactly resemble stinging ants, and thus no doubt escape the attacks of birds. The genus Caloρteron is mimicked by other beetles, as well as by the moth Pionia. In the same country, one of the Hemiptera, Spiniger luteicomis, has every part coloured like the hornet, Priocnemis, which it mimics; “in its vibrating coloured wing-cases it departs greatly from the normal character of the Hemiptera, and assumes that of the hornets.” Mr Wallace mentions the longicorn beetle, Cyclopeplus batesii, which “differs totally in outward appearance from every one of its allies, having taken upon itself the exact shape and colouring of a globular Corynomalus, a little stinking beetle, with clubbed antennae.” Erythroplatis corallifer, another longicorn, almost exactly resembles Cephalodonta spinipes, one of the common South-American Hispidx, which possesses a disagreeable secretion; and Mr Bates also found a totally different longicorn, Streptolabis hispoides, which resembles the same insect with equal minuteness. Some of the large tropical weevils have the elytra so hard that they cannot be pierced by a bird’s beak; and these are mimicked by many other comparatively soft and eatable insects. In southern Brazil, Acanthotritus dorsalis closely resembles a Curculio of the hard genus Heiliρlus; and Mr Bates found Gymnocerus cratosomoides, a longicorn, on the same tree with the hard weevil, Cratosomus, which it mimics. Other beetles resemble bees, wasps, and shielded bugs. Hairy caterpillars are well known to be distasteful to birds, and comparatively free from attack; and Mr Belt found a longicorn, Desmiphora fasciculata, covered with long brown and black hairs, and exactly mimicking some of the short, thick, woolly caterpillars common on the bushes around.

Amongst other orders, one of the most interesting cases is that of certain Diptera or two-winged flies which mimic wasps and bees. Sometimes this likeness only serves to protect the insect from attack, by inspiring fear of a sting. But there are also a number of parasitic flies whose larvae feed upon the larvae of bees, as in the British genus Volucella; and these exactly mimic the bees, so that they can enter the nests or hives to deposit their eggs without being detected even by the bees themselves. In every country where such flies occur they resemble the native bees of the district. Similarly, Mr Bates found a species of Mantis on the Amazons which exactly mimicked the white ants on which it fed. On the other hand, the defenceless species itself may mimic its persecutor, as in the case of several crickets, Scaphura, that exactly resemble various sand-wasps, and so escape the depredations of those cricket-killing enemies. Another cricket from the Philippine Islands, Condylodera tricondyloides, so closely copies a tiger-beetle, Tricondyla, that even Professor Westwood long retained it among that group in his cabinet, 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 strikingly 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, Ajuga 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 resemblances 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 causing 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 in 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 descendants. In this way many species will acquire and retain a coloration 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 sharpness 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 conspicuousness 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 are 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...