MIMICRY. The word Mimicry is applied by naturalists to certain advantageous resemblances between animals, and in some instances between plants. The resemblance of the common dead nettle to the stinging nettle, with which it is commonly associated, is a good example of plant mimicry. These resemblances are in dependent of affinity, viz., they may exist between species of very different degrees of relationship, generally distant but sometimes near; they are such as appeal to the senses of enemies, especially to the sense of sight, not uncommonly to hearing, occasionally to smell and touch. They differ from the much larger class of Pro tective Resemblances in that these bring about concealment by a likeness to some object of no interest to enemies, whereas the true mimic resembles a conspicuous "model" feared or disliked by its enemies, and thus becomes conspicuous itself. Other differences have been suggested—that the mimic resembles an animal model and not the vegetable or mineral surroundings; that it resembles a moving and not a stationary object. These distinctions, although generally true, do not always hold. The resemblance of external parasites to the hair, feathers or skin of their host, and of many insects to the excrement of birds, snails or of other insects, and to empty snail-shells, is not mimicry but protective resemblance, which would also include those caterpillars whose curious swaying movements suggest a swinging fragment of twig, or butterflies and moths which float to the ground like a falling leaf or allow themselves to be driven like a leaf before the wind. Mimics are also commonly adapted to resemble their models in the position of rest as well as in flight.
Mimetic resemblances also differ from those which are an inci dental result of similar functions, such as the general likeness in form between the racehorse and the greyhound or between the mole-like species with mole-like habits of life in the insectivores, rodents and marsupials ; also from the incidental likeness between species which resemble the same part of their surroundings— sand, bark, leaves, etc. Mimetic resemblances differ from these
in that they have been developed for the sake of the resemblance because of some advantage conferred in the struggle for existence. The term "mimicry," implying in its ordinary use conscious imita tion, has been a source of confusion; it has now, however, a technical scientific meaning.
Other views as to the origin of these resemblances are :—that they are due ( ) to the direct influence of the environment acting similarly on different species; (2) to a physiological response to constant mental experience such as colour sensation; (3) to sexual selection, modified by the presence of other types of colour. This last suggestion, due to Fritz Muller, is not supported by the fact that female butterflies are far more commonly mimetic than males.
The suggestion that these resemblances are a mere coincidence fails because of the evident geographical relationship. Mimics are found in the same localities as their models and when the latter are modified in various parts of their range the mimics change with them. There are however exceptions when the mimics are greater wanderers than their models, and in one remarkable example two butterflies in western China appear to be undoubted mimics of a model very common south of the Himalayas; here it is suggested that selection by migratory insectivorous birds may be influenced by their remembered experiences of the tropics.
Although mimetic resemblance is believed to exist in mammals, birds and fishes the number of examples is small and their inter pretation often doubtful. Among reptiles poisonous snakes are mimicked by harmless species. Even the English grass-snake will often, when cornered, poise itself and strike like an adder. But for the study of the subject the insects are supreme, the small per centage of survival and swift succession of generations rendering natural selection peculiarly searching in its operation and rapid in its results.