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Show 346 DAR\VINISM CIIAl'. above the surface could occur without an equivalent depress ion elsewhere. The fact that the waters of the ocean arc suffieient to cover the whole globe to a depth of two miles, is almw sufficient to indicate that the great ocean basins are perman ent fea-tures of the earth's surface, since any process of altema tion of these with the land areas would have been alm o~t certain to result again and again in the total disappoaranee of large portions, if not of all, of the dry land of the globe. Bnt the continuity of terrestrial life since the Devonian and ( ~ar boniferous periods, and the existence of very similar form. in the corresponding deposits d every continent-as well a:· t Jw occurrence of sedimentary rocks, indicating the proximi ty of land at the time of their deposit, over a large portion of the surface of all the continents, and in every geological period--assure us that no such disappearance has ever occurred. Oceanic a?td Continental Areas. When we speak of the permanence of oceanic and continental areas as one of the established facts of modern re::;e:m;h, we do not mean that existing continents and ocean:s have always maintained the exact areas and outlines that they now present, but merely, that while all of them have been nnd<'l'going changes in outline and extent from age to age, they have yet maintained substantially the same positions, a11tl have never actually changed places with each other. There are, moreover, certain physical and biological fa cts which enable us to mark out these areas with some confidence. We have seen that there are a large number of i:'il an< ls which ma.y be classed as oceanic, because they have lll'' <'l' formed parts of continents, but have originated in mitl-nn'a ll, and have derived their forms of life by migration acros;-; t lll~ sea. Their peculiarities are seen to be very ma.rkecl in ('omparison with those islands which there is crood rea~on to believe are really fragments of more extensive land arear-;, aJHl are hence termed "continental." The e continental i:-dand~ consist in every case of a variety of stratified rocks of various ages, thus corresponding closely with the usual structure of continents; although many of the islands are small like Jersey or the Shetland Islands, or far from continrnta 1 land like the Falkland Islands or New Zealand. They all XII GEOGRAPHICAL DISTRIBUTION OF ORG-ANISMS 347 contain indigenous mammalia or batrachia, and generally a much greater variety of birds, reptiles, insect , and plants, than do tho oceanic islands. From these various characteristics we conclude that they have all once formed parts of continents, or at all events of much larger land areas, and have become isolated, either by subsidence of the intervenino- land or by the effects of long-continued marine denudation. 0 Now, if we trace the thousand-fathom line around all onr existin~ continez:ts we find that, with only two exceptions, every 1sland whiCh can be classed as "continental" falls within this line, while all that lie beyond it have the undoubted characteri tics of "oceanic" islands. We, therefore, conclude that the thousand-fathom line marks out approximately, the "continental area,"-that is the limits within which continental development and chang~ throuahout known geological time h~ve gone on. There may, of 0 course, have been some extensiOns of land beyond this limit while some areas within it may always have been ocean; but so far as we have any direct evidence, this line may be taken to mark out, approximately, •the most probable boundary between the "continental area," which has always consisted of land and shallow sea in varying proportions, and the areat oceanic ba~in~, within the limits of which volcanic acti~ity has been bmldmg up numerous island , but whose profound depths have apparently undergone little change. Madagascar and New Zealand. Th~ two exceptions just referred to are Madaaascar and New Zealand, and all the evidence goes to show tb~t in these cases the land connection with the nearest continental area was ver;y remote in time. The extraordinary isolation of the productiOns of Madagascar-almost all the most characteristic forms of mammalia, birds, and reptiles of Africa beirw absent from it-renders it certain that it' must have bee~ separated from that continent very early in the Tertiary, if not as f.ar back as the ~att~r p~rt. of. the Secondary period ; and . th1s extreme ant1qmty IS md1eated by a depth of c~ns1derably more than a thousand fathoms in the Mozamblque Cha?nel, though this deep portion is less than a hundred miles wide between the Cornaro Islands and the main- |
