Part 22

The object of the attack, provided it have no nest to defend, has everything to lose and nothing to gain by resisting the attack and giving battle. It matters little to a golden eagle on the look-out for quarry in which direction it flies; hence if, while it is sailing through the air, it is suddenly attacked by a couple of infuriated ravens, the obvious course is for it to change the direction of its flight. If it fail to do this it must either run the risk of being severely pecked by the ravens or fight them and thereby expose itself to injury. Under the circumstances it naturally chooses the line of least resistance.

It is absurd to speak of a bird that behaves in this manner as being defeated in battle. It does not suffer defeat. It merely declines to give battle.

The general rule in nature is, “Never fight when a fight can be avoided.”

This rule is unconsciously followed by all birds, except those that have nests.

The most familiar example of the rule in operation is the well-known habit of birds of surrendering their perches to new-comers. When individual A flies to a perch occupied by individual B the latter almost invariably gives way without demur. The particular perch is of no value to the occupier, but a whole body may be a matter of life or death.

XXXVIII BIRDS AND BUTTERFLIES

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 omit many facts which tend to show that their fetishes are shams. They regard the discussers of the awkward facts as enemies to be crushed. Hence the gulf between the two classes of biologists.

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 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.

“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 of caterpillars. It is the same in other parts of the world.

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.