OCR Text |
Show 254 DARWINISM CHAP. merged in another much more abundant species, by similarity of external appearance, would be an advantage. The question of fact remains. Do young birds pursue and capture these distasteful butterflies till they have learned by bitter experience what species to avoid 1 On this point Dr. Mi.iller has fortunately been able to obtain some direct cYidence, by capturing several Aeneas and Heliconidre which h:ul evidently been seized by birds but had afterwards escaped, as they had pieces torn out of the wing, sometimes symmetrically out of both wings, showing that the insect had been seized when at rest and with the two pairs of wings in contaet. There is, however, a general impression that this knowledge is hereditary, and does not need to be acquired by young binls; in support of which view Mr. Jenner W cir states that his birds always disregarded inedible caterpillars. \Vhen, day by day, he threw into his aviary various larvre, those which W('l'e edible were eaten immediately, those which were inedible were no more noticed than if a pebble had been thrown before the birds. The cases, however, are not strictly comparable. The birds were not young birds of the first year ; and, what is more important, edible larvre have a comparatively simple coloration, being always brown or green and smooth. Uneatable larvre, on the other hand, comprise all that are of conspicuous colours and are hairy or spiny. But with butterflies there is no such simplicity of contrast. The eatable butterflies compri e not only brown or white species, but hundreds of N ymphalidre, Papilionidre, Lycrenidre, etc., which arc gaily coloured and of an immense variety of patterns. The colours and patterns of the inedible kinds arc also greatly varied, while they are often equally gay; and it is quite impossible to suppose that any amount of in tinct or inherited habit (if uch a thing exists) could enable young in cctivorons birds to distinguish all the species of one kind from all those of the other. There is also some evidence to show that animals do learn by experience what to eat and what to avoid. Mr. Poulton was assured by Rev. G. J. Bursch that very young chickens peck at insects which they afterwards avoid. Lizards, too, often seized larvre which they were unable to eat and ultimately rejected. IX WARN-ING COLORATION AND MIMICRY 255 Although the Heliconidre present, on the whole, many varieties of coloration and pattern, yet, in proportion to the number of distinct species in each district, the types of color;ttion are few and very well marked, and thus it becomes easier for a bird or other animal to learn that all belonging to such types are uneatable. This must he a decided advantage to the family in question, because, not only do fewer individuals of ca.ch species need to be sacrificed in order that their enemies may learn the lesson of their inedibility, hut they are more easily recognised at a distance, and thus escape even pursuit. There is thus a kind of mimicry between closely allied species as ·well as between species of distinct genera, all tending to the same beneficial end. This may be seen in the four or five distinct species of the genus Heliconius which all have the same peculi<tr type of coloration-a yellow band across the upper wings and radiating red stripes on the lower,-::md are all found in the same forests of the Lower Amazon ; in the numerous very similar species of Ithomia with transparent wings, found in every locality of the same region; and in the very numerous species of Papilio of the "JEncas" group, all having a similar style of marking, the resemblance being especially close in the females. The very uniform type of colouring of the blue-black Euplrea and of the fulvous Acrrea. is of the same character.1 In all these case the similarity of the allied species is so great, that, when they are on the wing at some distance off, it is difficult to distinguish one species from another. But this close external resemblance is not always a sian of very near affinity ; for minute examination detect. diff~rences in the form and scalloping of the wings, in the markino·s on the body, and in those on the under surface of the wing~, which do not usually characterise the closest allies. It is to be further noted, that the pre ence of groups of very similar species of the same genus, in one locality, is not at all a common phenomenon among unprotected groups. Usually the species of a genus found in one locality are each well marked ::md belong to somewhat distinct types, while the 1 This extension of the theory of mimicry was pointed out by Professor Meldola in the paper already referred to ; and he has answered the objections to Dr. F. Miiller's theory with great force in the Annals ctnd .Mag. of Nat. ]Jist., 1882, p. 417. |