  Biological science is at present in a rather peculiar position. Biologists are divided into two parties. On the one side stand the theorists and their followers; on the other the practical men who think for themselves. At present, the theorists are the party in power (and they are quite Lloyd-Georgian in their methods), while the practical men, the breeders and the field naturalists, form the opposition. The reason of the division is that many facts, that have come to light lately, do not fit in with the theories that hold the field. Now, when facts are discovered which militate against a theory the proper course for the holder of the theory is to test carefully the alleged facts, and if they prove to be really facts to discard or modify his theory. Unfortunately the professional biologists of to-day do not usually follow this course. They have made fetishes of their theories, which they worship as the Israelites worshipped the golden calf. The consequence is that they feel in honour bound either to ignore or to gloss over the facts that are subversive of their fetishes. When they write books in honour of their fetishes, they One of the fetishes of the present day is the theory of protective mimicry. Butterflies and moths are the organisms which exemplify best this theory. 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 “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. “Moths include many major pests of varied habits—defoliators, miners, cut-worms, grain and fabric pests. The larvae form an inexhaustible supply of insect food to almost all species of insectivorous birds, and even many species of birds that when mature feed almost, if not quite, entirely on grain and seeds are when in the nest fed very largely on caterpillars by the parent birds.” Obviously, then, in India birds comparatively rarely attack butterflies; but they devour millions Mr. G. A. K. Marshall, in the course of five years’ observation in South Africa, recorded eight cases of birds capturing butterflies. Similarly Mr. Banta points out in various issues of Nature, in 1912, that all the evidence available shows that in North America birds very rarely capture butterflies. Field naturalists scarcely ever witness a butterfly chased by a bird. Of 40,000 stomachs of birds examined very few were found to contain remains of butterflies. In 1911 the butterflies of the species Eugonia californica were so numerous that “the ground was often blackened with them, and great swarms of them filled the air from morning to evening.” Yet of the birds in the locality where those butterflies were most numerous, only five out of forty-five species were found by direct observation and stomach examination to eat the eugonia, and the only bird that fed off them copiously was the brewer blackbird (Euphagus cyanocephalus) which is almost omnivorous, and eats insects of all kinds, even if they be what Darwinians call warningly coloured! Now, modern theorists, as a rule, ignore facts such as these, and this certainly is the wisest course they can pursue, unless they are ready to give up these theories or make themselves look foolish. However, I am glad to be able to record that Professor Poulton has, as regards the remarks of Mr. Banta, not followed the usual course of the modern theorist. He has had the courage to take up the cudgels and reply to Mr. Banta in Nature. The reason of this unusual course appears to be that Mr. C. F. M. Swynnerton has made some observations in South Africa which Professor Poulton considers are in favour of his pet theory. According to the Professor, Mr. Swynnerton, as the result of three and a half years’ investigation in South-East Rhodesia, “has obtained the records of nearly 800 attacks made by 35 species of birds belonging to 30 genera and 18 families, upon 79 species of butterflies belonging to 9 families or sub-families.” Professor Poulton does not seem to see that the researches of Mr. Swynnerton are altogether subversive of the theory of protective mimicry. In order that natural selection may totally change the colouring of a butterfly (as it does according to the theory of protective mimicry), that butterfly must be habitually preyed upon by large numbers of birds, which must be so vigilantly and unceasingly on the look-out for it, that its only chance of escaping from their attacks must be for it to assume a disguise. Compare with this the state of affairs revealed by Mr. Swynnerton’s observations. He worked for three and a half years, and, as his investigations extended to eighteen families of birds, they must have been very extensive. Exactly how extensive they have been we do not know, because he has not yet published them. Nevertheless, as the result of three and a half years’ watching and stomach examination he has evidence of only “nearly 800” attacks made Watch a bee-eater feeding and you will see it take twenty or thirty insects in less than an hour. If you were to watch it one whole day you might see it capture 300 insects, but certainly not more than one of its victims, on an average, would be a butterfly. Yet, the theory of mimicry is based upon the assumption that butterflies are so greatly preyed upon by birds that they require special means of protection! I ask all who are interested in the subject to be ever on the look-out for birds chasing butterflies or moths. These are so large and so easy to identify that there can be no chance of mistaking them. Even a casual observer, when watching a bird, cannot fail to notice the capture of a butterfly by it. And when a bird has captured a butterfly it cannot dispose of it very quickly. According to Mr. Swynnerton, “some (birds) swallow the insect (butterfly or moth) whole, but usually after masticating or beating it; some remove inconvenient portions by ‘worrying’ like a dog or beating against perch or ground; some grasp the prey in one foot and tear off the rejected portions with the bill, eating the rest piecemeal.” The fact that the average bird has to go through all the above performances before devouring a creature containing so little nourishment as a butterfly, is sufficient to show that it does not pay birds to chase butterflies. But it is best not to rely on arguments to refute the theories of persons who have no logic in them. |
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