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Show 2!3-! ISLAND LlFK [PAJl.'l' It. 1 are far easier to direct and simple, and even when more comp ex . . esides comprehend than those of continents; and the! exlnbi.t ~. · certain influences on the forms of life and cm·tam pecuhauties of distribution which continents do not present, and whose study offers many points of interest. . . . . In islands we have the fncts of d1stnbut10n ofte1: presented to us in their simplest forms, along with others wln?h become radually more and more complex; and we are therefore able to ~roceed step by step in tbe solution of tho problems th?y present. But as in studying these problems we have nec~ssanly to take into account the reUttions of the insular and contm?ntal ~annas, we also get some knowledge of the latter, and a,c.qmre bes1~es so much command over t'he general principles wlnch unclerhe all problems of distribution, that it is not too much to say tha~ ~h:n we have mastered the difficulties presented by the pecuhant1es of island life we shall find it comparatively easy to deal with the more complex and less clearly defined problems of continental distribution. Classijicat·ion of I slcmds with ?'efe?'ence to DislTib L~tion.-r-lslamls have had two distinct modes of origin; · they have either been separated from continents of which they are but detached fragments, or they have originated in tho ocean and have nev~r formed part of a continent or any large mass of land. Th1s difference of origin is fundamental, and loads to a most important difference in their animal inhabitants ; a~cl we may therefore first distinguish the two classes-oceamc and continental islands. Mr. Darwin appears to have been the first writer who called attention to the number and importance, both from a geological and biological point of view, of oceanic islands. He showed that with very few exceptions all the remoter islands of the great oceans were of volcanic or coralline formation, and that none of them contained indigenous mammalia or amphibia. He also showed the connection of these two phenomena, and maintained that none of the islands so characterised had ever formed part of a continent. This was quite opposed to the opinions of tho scientific men of the day, who almost all held the idea of continental extensions, and of oceanic islands being their CIIAI'. XI.] TilE CLASSIFICATION OF ISLANDS. 235 fragments, and it was long before Mr. Darwin's views obtained general acceptance. Even now the belief still linO'ers · ·and we cont.m ua1 1 y hear of old Atlantic or Pacific cont°i nen't s of "~tl.anti~" or "Lemuria," of which hypothetical lands m~ny ex1stmg Islands, although wholly volcanic, are thought to be the remnants. We have already seen that Darwin connected the peculiar geological structure of oceanic islands with the permanence of the great oceans which contain them, and we have shown that several distinct lines of evidence all point to the same conclusion. We may therefore define oceanic islands, as follows : -Islanrls of volcanic or coralline formation, usually far fro'in continents and always separated from them by very deep sea, entirely without indigenous land mammalia or amphibia, but with abundance of birds and insects, and usually with some reptiles. This definition will exclude only two islands which have been sometimes classed as oceanic-New Zealand and the Seychelles. Rodriguez, which was once thought to be another exception, has been shown by the explorations during the Transit of Venus Expedition to be essentially volcanic, with some upraised coralline limestone. Continental Islands.-Continental islands are always more varied in their geological formation, containing both ancient and recent stratified rocks. They are rarely very remote from a continent, and they always contain some land mammals and amphibia, as well as representatives of the other classes and orders in considerable variety. They may, however, be divided into two well-marked groups-ancient, and recent, continental islands-the characters of which may be easily defined. Recent continental islands are always situated on submerged banks connecting them with a continent, and the depth of the intervening sea rarely exceeds 100 fathoms. They resemble the continent in their geological structure, while their animal and vegetable productions are either almost identical with those of the continent, or if otherwise, the difference consists in the presence of closely allied species of the same types, with occasionally a very few peculiar genera. They possess in fact all the characteristics of a portion of the continent, separated from it at a recent geological period. |