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Show ISLAND LIFE. [I'AltT n. ------ become extinct in their native country ; and in the second place, insects have many more chances of reaching remote islat~ds than birds, for not only may they be carried by gales of WJ~d, but sometimes, in the egg or larva state or even as perfect 1~sec~s, they may be drifted safely for weeks over the ocean, b.uned. m the light stems of plants or in the solid wood of trees m whiCh many of them undergo their transformations. Thus we m.ay explain the presence of three common South American specws (two elaters and a longicorn), all wood-eaters, and therefore liable to be occasionally brought in floating timber by the Gu.lf Stream. But insects are also immensely more numerous ~n species than are land-birds, and their transmission would. be m most cases quite involuntary, and not dependent on th.e1r 4 own powers of fliaht as with birds; and thus the chances agamsv the same species 0 being frequently carried to the sa.me island would be considerable. If we add to this the dependence of so many insects on local conditions of climate and vegetation, and their liability to be destroyed by insectivorous birds, we shall see that, althourrh there may be a greater probability of insects as a whole reachir~g the islands, the chance against any particular insect arriving there, or against the same species arriving frequently, is much greater than in the case of birds. The result is, that (as compared with Britain for example) the birds ar~, proportionately, much more numerous than the beetles, wh1le the peculiar species of beetles are much more numerous than among birds, both facts being quite in accordance with what we know of the habits of the two groups. We may also remark, that the small size and obscure characters of many of the beetles renders it probable that species now supposed to be peculiar, really inhabit some parts of Europe or North Africa. It is interesting to note that the two families which are preeminently wood, root, or seed eaters, are those which present the greatest amount of speciality. The two Elateridre alone exhibit remote affinities, the one with a Brazilian the other with a Madagascar group; while the only peculiar genera belong to the Rhyncophora, but are allied to European forms. These last almost certainly form a portion of the more ancient fauna of the islands which migrated to them in pre-glacial times, while the ("liAP, Xll.) TIIE AZORES. 247 Brazilian elater appears to be the solitary example of a living insect brought by the Gulf Stream to these remote shores. The ela.ter, having its nearest living ally in Madagascar (Elastrus dolosus), cannot be held to indicate any independent communication between these distant islands; but is more probably a relic of a once more widespread type which has only been able to maintain itself in these localities. Mr. Crotch states that there are some species of beetles common to Madagascar and the Canary Islands, while there are several geneTa, common to Madagascar and South America, and some to Madagascar and Australia. The clue to these apparent anomalies is found in other genera being common to Madagascar, Africa, and South America, while others are Asiatic or Australian. Madagascar, in fact, has insect relations with every part of the globe, and the only rational explanation of such facts is, that they are indications of very ancient and once widespread groups, maintaining themselves only in a few widely separated portions of what was at one time or another the area of their distribution. Land-shells of the Azo?'es.-Like the insects and birds, the land-shells of these islands have a generally European aspect, but with a larger proportion of peculiar species. This was to be expected, because the means by which molluscs are carried over the sea are far less numerous and varied than in the case of insects ;1 and we may therefore conclude that their introduction is a very rare event, and that a species once arrived remains for long periods undisturbed by new arrivals, and is therefore more likely to become modified by the new conditions, and then fixed as a distinct type. Out of the sixty-nine known species, thirtyseven are common to Europe or the other Atlantic islands, while thirty-two are peculiar, though almost all are distinctly allied to European types. The majority of these shells, especially the peculiar forms, are very small, and many of them may date back to beyond the glacial epoch. The eggs of these would be exceedingly minute, and might occasionally be carried on leaves or other materials during gales of exceptional violence and duration, while others might be conveyed with the earth that often sticks to the feet of birds. There are also, probably, other 1 See Chap. V. p. 76. |
