These cases of chance resemblance should make us unwilling to talk about "mimicry," unless there is actual proof that one or other of the similar species benefits by the resemblance.
These cases, further, throw light on the origin of protective mimicry where it does exist.
Protective mimicry is usually said to have been brought about by the action of natural selection. This is not strictly accurate. Natural selection cannot cause two showy, dissimilar species to resemble one another; all it can do is to seize upon and perfect a resemblance that has been caused by the numerous factors that have co-operated to bring about all the diversity of organic life upon this earth.
THE GREAT HIMALAYAN BARBET
Barbets may be described as woodpeckers that are trying to become toucans. The most toucan-like of them all is the great Himalayan barbet (Megalæma marshallorum). Barbets are heavily-built birds of medium size, armed with formidable beaks, which they do not hesitate to use for aggressive purposes. As regards the nests they excavate, the eggs they lay, the pad that grows on the hocks of young birds, and their flight, they resemble their cousins the woodpeckers. But they are fruit-eating birds, and not insectivorous; it is this that constitutes the chief difference between them and the woodpeckers. Barbets are found throughout the tropical world. A number of species occur in India. The best known of these is the coppersmith, or crimson-breasted barbet (Xantholæma hæmatocephala), the little green fiend, gaudily painted about the head, which makes the hot weather in India seem worse than it really is by filling the welkin with the eternal monotone that resembles the sound of a hammer on a brazen vessel. Nearly as widely distributed are the various species of green barbet (Thereiceryx), whose call is scarcely less exasperating than that of the coppersmith, and may be described as the word kutur shouted many times and usually preceded by a harsh laugh or cackle.
The finest of all the barbets are the Megalæmas. The great Himalayan barbet attains a length of 13 inches. There is no lack of colour in its plumage. The head and neck are a rich violet blue. The upper back is brownish olive with pale green longitudinal streaks. The lower back and the tail are bright green. The wings are green washed with blue, brown, and yellow. The upper breast is brown, and the remainder of the lower plumage, with the exception of a scarlet patch of feathers under the tail, is yellow with a blue band running along the middle line. This bright red patch under the tail is not uncommon in the bird world, and, curiously enough, it occurs in birds in no way related to one another and having little or nothing in common as regards habits. It is seen in many bulbuls, robins, and woodpeckers, and in the pitta. The existence of these red under tail-coverts in such diverse species can, I think, be explained only on the hypothesis that there is an inherent tendency to variation in this direction in many species.
A striking feature of the great Himalayan barbet is its massive yellow bill, which is as large as that of some species of toucan. Although the bird displays a number of brilliant colours, it is not at all easy to distinguish from its leafy surroundings. It is one of those birds which are heard more often than seen.