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Show 10 ISLAND LIFE. [rAnT r. dispersal and existing distribution of organisms. Tho other important theory, or rather corollary from the preceding theory -that of the permanence of oceans and tho general stability of continents throughout all geological time, is as yet very imperfectly understood, and seems, in fact, to many persons in the nature of a paradox. The evidence for it, however, appears to me to ·be conclusive; and it is certainly the most fundamental question in regard to the subject we have to deal with: since, if we once admit that continents and oceans may have changed places over and over again (as many writers maintain), we lose all power of reasoning on the migrations of ancestral forms of life, and are at the mercy of every wild theorist who chooses to imagine the former existence of a now-submerged continent to explain the existing distribution of a group of frogs or a genus of beetles. As already shown by the illustrative examples adduced in this chapter, some of the most remarkable and interesting facts in the distribution and affinities of organic forms are presented by islands in relation to each other and to the surrounding continents. The study of the productions of the Galapagosso peculiar, and yet so ·decidedly related to the American continent- appear to have had a powerful influence in determining the direction of Mr. Darwin's researches into the origin of species; and every naturalist who studies them has always been struck by the unexpected relations or singular anomalies which arc so often found to characterize the fauna and flora of islands . . Yet their full importance in connection with the history of tho earth and its inhabitants has hardly yet been recognised; and it is in order to direct the attention of n~turalists to this most promising field of research, that I restrict myself in this volume to an elucidation of some of the problems they preseut to us. By fur the larger part of the islands of the globe arc but portions of continents undergoing some of the various changes to which they are ever subject; and the correlative statement, that every part of our continents have again and again passed through insular conditions, has not been sufficiently considered, but is, I believe, the statement of a great and most suggestive truth, and one which lies at the foundation of all CHAP. 1.] INTRODUCTORY. 11 accurate conception of the physical and organic changes which have resulted in the present state of the earth. The indications now given of tho scope and purpose of the present volume renders it evident that, before we can proceed to the discussion of the remarkable phenomena presented by insular faunas and floras, and the complex causes which have produced them, we must go through a series of preliminary studies, adapted to give us a command of the more important facts and principles on which the solution of such problems depends. The succeeding eight chapters will therefore be devoted to the explanation of the mode of distribution, variation, modification, and dispersal, of species and groups, illustrated by facts and examples ; of the true nature of geological change as affecting continents and islands; of changes of climate, their nature, causes, and effects; of the duration of geological time and the rate of organic development. |