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Show 350 GEOGRAPHICAL DISTRIBUTION. [OBAP. XII. both are equally well fitted for their own places in nature, both probably will hold their own place~ and k.e~p seJ?arate for almost any length of time. Bmng famihar With the fact that many species, na.tui:alised ~h;ough man's agency, have spread wi~h astomshing rapi~Ity over new countries, we are apt to Infer that most species would t~us spread· but we should rmnember ~hat the forms whiCh beeomd naturalised in new countries are not generally closely allied to the a~orig!nal inhabitants, ~ut are very distinct species belonging· In a large proportion of cases, as shown by Alph. de Candolle, to distinct genera. In the Galapagos Archipelago, many even of the birds, though so well adapted for flying from island to isla~d, 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 establishin~ 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 Inocking-thrush peculia~ to Charle.s Island is at least as well fitted for its home as IS the spemes peculiar to Chatham Island. Sir C. Lyell and Mr. W o~laston have communicated to me a re~arkable fact. b.e~rin~ on this subject· namely, that Madeira and the adJOining Islet of Porto S~nto 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 sorne advantage over the indigenou; species. From these cons~derations I think yre need not greatly n1arvel at the endeinic and representative species, which inhabi~ the s~veral islands of the G~lapagos Archipelago, not having universally .spread from Isl.an~ to island. In many other Instances, as In the several distncts of the same continent, pre-occupation has.pro?ably play.ed an important part in checking the comnnngling of specieS OBAP. XII.] OOEANIO ISLANDS. 351 under the same conditions of life. Thus, the south-east and ~outh-wes~ ?orners of Australia have nearly the same physical con~Itlon~, and are united by continuous land yet the:y are Inhabited by a vast nu1nber of distinct mam: mals, birds, and plants. The principle which determines the general character of the fauna and flora of oceanic islands namely th t th · h b't t l 'd · ' ' a e 1n a 1 an s, w.1en n?t I entiCally the same, yet are plainly related to the In?abltants of that region whence colonists could most readily have been derived -the colonists having been subse9.uently modi~ed and better fitted to their new homes,-Is of. the widest application throughout nature. We see tlus on every mountain in every lake and. marsh. For A~pine species, excepti~g in so far as the same forms, chiefly ?f plants, have spread widely throughout the world dunng the recent Glacial epoch are .relate~ to those of t~e surro~nding lowlands ;-thu~ we have In Sou.th Amenc-a, Alpine humming-birds, Alpine rode~t~, Alp:ne plants, &c., all of strictly American forms, and It IS obvious that a mountain, as it became slowly upheaved, would naturally be colonised from the surrounding lowlands. ~o i~ is with the inhabitants of lakes and marsh.es, excepting In so far as great facility of transport has given. the same general forms to the whole world ~ e see this samelrinciple in the blind animals inhabit~ mg the caves of mer~ca and of Europe. Other analogo? s facts could be given. And it will, I believe, be u~versally found to be true, that wherever in two regiOns, let t~em be ~ver so distant, many closely allied or repres~ntat~ve speCies occur, there will likewise be found some ~dentl.cal species, showing, in accordance with the foregmng VIew, that at some former period there has been Intercommunication or migration between the two regions. ~lfb wherever many closely:allied species occur, there .. e foun~ many forms whiCh some naturalists rank as fstmct sp~Cies, and some as varieties; these doubtful 0Th ~hoWin~ us the steps in the process of modification. t. Is rel~tlon between the power and extent of migra- £lO n of a speci·e s, m· t h ~r at the present time or at some ormer period under different physical conditions, and the |