OCR Text |
Show 402 GEOGRAPHICAL DISTRIBUTION. CHAP. XII. the several islands, and we may infer from certain facts that these have probably spread from some one island to the others. But we often take, I think, an erroneous view of the probability of closely allied species invading each other's territory, wh~n put in~o free intercommunication. Undoubtedly 1f one species has any advantage whatever ~ver another, it ~ill in ~ very brief time wholly or 1n part supplant It; but If both are equaUy well fitted for their own places in nature, both probably will hold their own places and keep separate fo! almost any length of tin1e. Being familiar with the fact that many species, naturalised through man's agency, have spread with astonishing rapidity over new countries, we are apt to infer that most species would thus spread; but we should remember that the forms which become naturalised in new countries are not generally closely allied to the aboriginal inhabitants, but are very distinct species, belonging in a large proportion of cases, as shown by Al ph. de Candolle, to distinct genera. In the Galapagos Archipelago, many even of the birds, though so well adapted for flying from island to island, are distinct on each; thus there are three closely-allied species of mocking-thrush, each confined to its own island. Now let us suppose the mocking-thrush of Chatham Island to be blown to Charles Island, which has its own mocking-thrush: why should it succeed in establishing itself there ? We may safely infer that Charles Island is well stocked with its own species, for annually more eggs are laid there than can possibly be reared ; and we may infer that the mocking-thrush peculiar to Charles Islan.d is at le.ast as well fitted for its home as is the speCies pecuhar to Chatham Island. Sir 0. Lyell and Mr. Wollaston hav.e communicated to me a remarkable fact bearing on tbJs subject; namely, that Madeira and the adjoining islet of CHAP. xu. OCEANIC ISLANDS. 403 Porto Santo possess many distinct but representative land-shells, some of which live in crevices of stone; and although large quantities of stone are annually transported from Porto Santo to Madeira, yet this latter island has not become colonised by the Porto Santo species : nevertheless both islands have been colonised by some European land-shells, which no doubt had some advantage over the indigenous species. From these considerations I think we need not gTeatly marvel at the endemic and representative species, which inhabit the several islands of the Galapagos Archipelago, not having universally spread from island to island. In many other instances, as in the several districts of the same continent, pre-occupation has probably played an important part in checking the commingling of species under the same conditions of life. Thus, the south-east and south-west corners of Australia have nearly the Saine physical conditions, and are united by continuous land, yet they are inhabited by a vast number of distinct mammals, birds, and plants. ~rhe principle which determines the general character of the fauna and flora of oceanic islands, namely, that the inhabitants, when not identically the same, yet are plainly related to the inhabitants of that region whence colonists could most readily have been derived,-the colonists having been subsequently modified and better fitted to their new homes,-is of the widest application throughout nature. We see this on every mountain, in every lake and n1arsh. For Alpine species, excepting in so far as the same forms, chiefly of plants, have spread widely throughout the world during the recent Glacial epoch, are related to those of the surrounding lowlands ;-thus we have in South America, Alpine humming-birds, Alpine rodents, Alpine plants, &c., all of strictly ..c\.merican forms, and it is obvious |