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Show 106 DARWINISM CHAP. to whom wings were not absolutely essential escaped a serious danger by not using them, and the wings therefore became reduced or were completely lost. But when they were essential they were enlarged and strengthened, so that the insect could battle against the winds and save itself from destruction at sea. Many flying insects, not varying fast cuough, would be destroyed before they could establish themselves, and thus we may explain the total absence from Madeira of several whole fam_il~es of wing~d insec~s which must have had many opportumt~ e·s of rcachmg the Islands. Such arc the la,rge group!:> of t~e tiger-beetles (Cicindelidm), the chafers (Melolonthidro), tho chck-beetles (Elateridro), and many others. But the most curious and striking confirmation of this portion of Mr. Darwin's theory is afforded by the case of l~erguelen I~l~nd. Th.is island was visited by the 1'mnsit . of J enus expeditiOn. It Is one of the stormiest places on tho globe, being subject to almost perpetual gales, while th oro being no wood, it is almost entirely without shelte/ Tho Rev. A. E. Eaton, an experienced entomologist, was naturalist to the expedition, and he assiduously collected the few insects that were to be found. All were incapable of flight, and most of them entirely without wings. They included a moth several flies, and numerous beetles. As these insects coulcl hardly have reached the islands in a wingless state, even if there ~ere any other known land inhabited by them- which there IS not-we must assume that, like the Madeiran in sects, they were originally winged, and lost their power of fli aht because its possession was injurious to them. 0 It is no doubt due to the same cause that some butterflies on small and exposed islands have their winas reduced in size as is strikingly the case with the small tortgisc-shcll butted!; (Vanessa urticro) inhabiting the Isle of Man, which is only about half the size of the same species in Enghmd or Ircl:wcl; and Mr. V\T ollaston notes that Vanessa callirhoc-a closely allicu South European form of our red-admiral butterfly-is pcnnanently smaller in the small and bare island of Porto f 'anto than in the larger and more wooded adjacent island of Maucira. A very good example of comparatively recent divcr(rencc of character, in accordance with new conditions of life is afforded by our red grouse. This bird, the Lagopus scoticu~ of v NATURAL SELECTION 107 naturalists, is entirely confined to the British Isles. It is, however, very closely allied to the willow grouse (Lagop:ls albus), a bird which ranges all over Europe, Northern Asm, and North America, but which, unlike our species, chango. to white in winter. No difference in form or structure can be detected between the two birds, but as they differ so decidedly in colour- our species being usually rather darker in winter than in summer, while there are also slight differences in the call-note and in habits,-the two species are generally considered to be distinct. The differences, however, arc so clearly adaptations to changed conditions that we can hardly doubt that, during the early part of the glacial period, when our islands were united to the continent, our grou. c was identical with that of the rest of Europe. But when the cold passed away and onr islands became permanently epa,rated from the mainland, with a mild and equable climate and very little snow in winter, the change to white at that season became hurtful, rendering the birds more conspicuous instead of serving as a means of concca1mcnt. The colour was, therefore, gradually changed by the process of variation and natural selection; and as the birds obtained ample shelter :1mong the heather which clothes so many of our moorlands, it became useful for them to assimilate with its brown and dn ·ky stems and withered flowers rather than with the snow of the higher mountains. An interesting confirmation of this change having really occurred is afforded by the occasional occu:rc~co in Scotland of birds with a considerable amount of white m the winter plumage. This is considered to be a case of reversion to the ancestral type, just as the slaty colours and bandeu wings of the wild rock-pigeon sometimes reappear in our fancy breeds of domestic pigeons.1 The principle of "divergence of character " pervades all nature from the lowest groups to the highest, as may be well seen in the class of birds. Among our native species we sec it well marked in the different species of titmice, pipits, and chats. The great titmouse (Parus major) by its larger size and stronger bill is adapted to feed on larger i:1sccts, and is even said sometimes to kill small and weak bn·ds. The smaller and weaker coal titmouse (Parus ater) has adopted a 1 Yarrell's British Birds, fourth edition, vol. iii. p. 77. |