It often happens that two species of butterfly occur in the same locality which resemble one another in outward appearance. In such cases zoologists assert that one species mimics the other. They maintain that this mimicry has been brought about by natural selection, because the one species profits by aping its neighbour. The species that is copied is said to be unpalatable. The copy-cat, if I may use the expression, may be either palatable or unpalatable. In either case it is believed to profit by the resemblance. If it is edible the birds that are supposed to prey upon butterflies are said to leave it alone, because they mistake it for its unpalatable neighbour. This resemblance of an edible form to an unpalatable one is called Batesian mimicry.
If the copy-cat be unpalatable it is nevertheless said to profit by the likeness, because young birds are supposed to feed on every kind of butterfly and only to learn by experience which are unpalatable. The theory is that if they attack a red-coloured butterfly and find it nasty to the taste, they leave all red-coloured butterflies alone henceforth. Thus, the imitating species may benefit by the sacrifice of the other red-coloured species. This is known as Mullerian mimicry.
The mimicry theory is very enticing; indeed, it is so enticing that those who hold it, as, for example, Professor Poulton, of Oxford, seem to think that there must be something wrong with the evidence opposed to it.
I assert that it is not the evidence against the theory, but the theory itself that is wrong.
The objections to the hypothesis are many and weighty. Finn and I summarised most of them in The Making of Species.
Two of the objections appear to be insuperable.
The likeness cannot be of much use until it is fairly strong. How, then, is the beginning of the resemblance to be explained?
In order that natural selection should have produced these astounding resemblances, it is necessary that butterflies should be preyed on very largely by birds; but all the evidence goes to show that birds very rarely eat butterflies. In the course of some ten years spent in India I have not seen butterflies chased by birds on more than a dozen occasions. Similarly, Colonel Yerbury, during six years’ observation in India and Ceylon, can record only about six cases of birds capturing, or attempting to capture, butterflies. Colonel C. T. Bingham, in Burma, states that between 1878 and 1891 he on two occasions witnessed the systematic hawking of butterflies by birds, although he observed on other occasions some isolated cases.
Nor is the evidence, as regards India, confined to the experience of the casual observer. Mr. C. W. Mason, when supernumerary entomologist to the Imperial Department of Agriculture for India, conducted a careful enquiry into the food of birds. The enquiry was made at Pusa in Bengal, in the years 1907, 1908, 1909. The results arrived at by Mr. Mason are published in the Memoirs of the Department of Agriculture for India (Entomological Series, Vol. III, January, 1912). As the result of this enquiry, in the course of which the contents of the stomachs of hundreds of Indian birds were examined, Mr. Mason writes (page 338, loc. cit.): “Butterflies do not form any appreciable proportion of the food of any one species of bird, though a good many birds take these insects at times. . . .
“The butterflies include a number of minor pests, of which Melanitis ismene was taken by Merops viridis and Papilio pammon by Acridotheres tristis. Other well-known pests are Pieris brassicae, Virachola isocrates and Papilio demoleus. Belenois mesentina, a Pierid, was seen to be taken on one occasion by the king-crow, and Ilerda sena by Passer domesticus, both of which insects are neutral.