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Show 340 DARWINISl\1 CHAP. the Asiatic deserts, whose nearest allies .arc the llamas and alpacas of the Andes ; and ~h~ n:arsupmls, only fom:d m· Australia and on the opposite side . of the b globe.,_, m America. Yet, again, although mammaha .may e mu to he universally distributed over the globe, bemg found abnndantly on all the continents and on .a g~'e<tt many of the hro·?r Hands, yet they are entirely w~ntmg m ~ ew Zealand, and 111 <t considerable number of other Islands whiCh arc, nevcrtheles~:;, perfectly able to support .t~em :vhen introduced. . Now most of these difficulties can be solved by means ot well-known geographical and geological facts. When the pr<~ductions of remote countries resemble each other, there m almost always continuity of land with .simil~rity of clima ~c between them. 'Vhcn adjacent countnes d1ffer gre~ttly Ill their productions, we find th~m ~eparat~d by a. se.a or straiL whose great depth is an indicatlm: of It.s ant.Iqmty or permanence. " Then a group of ammals mh.abi.ts two conn tries or regions separated by wide oceans, It 1s found that in past aeoloaical times the same group was much more widely dfstrib~ted, and may ha.ve .reach~d _th~ countri~s it inhabits from an intermediate region m whiCh 1t IS now extmct. vVe know, also, that countries now united by land w~re divided by arms of the sea at a not very remote epoch ; '~'h1le there is aood reason to believe that others now entirely isolated b; a broad expanse of sea ':ere formerly u~itcd and formed a sinale land area. There IS also another Important factor to be taken account of in considering how animals and plants have acquired their present peculia~·ities of distributi~n, -changes of climate. vVe know that qmte recently a gbe1al epoch extended over much of what are now the temperate regions of the northern hemisphere, and that consequently the organisms which inhabit those parts must be, comparatively speaking, recent immigrants from more southcm lands. But it is a yet more important fact that, down to middle Tertiary times at all events, an equable temp~ra~e climate, with a luxuriant vegetation, extended to far w1thm the arctic circle over what are now barren wastes, coYerecl for ten months' of the year with snow and ice. The arctic zone has, therefore, been in past times capable of supporting almost all the forms of life of our temperate regions; and we XU GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 341 must take account of this condition of things whenever we have to speculate on the possible migrations of organisms between the old and new continents. The Condit,ions which have determined Di::;t?-ibution. 'Vhen we endeavour to explain in detail the facts of the existing distribution of organic beings, we are confronted by several preliminary questions, upon the solution of which ·will depend our treatment of the phenomena presented to us. Upon the theory of descent which we have adopted, all the different species of a genus, as well as all the genera which compose a family or higher group, have descended from some -common ancestor, and must therefore, at some remote epoch, have occupied the s:tme area, from which their descendants have spread to the regions they now inhabit. In the numerous -c:tses in which the same group now occupies countries separated by oceans or seas, by lofty mountain-chains, by wide deserts, or by inhospitable climates, we have to consider how the migration which must certainly have taken place has been effected. It is possible that during some portion of the time which has elapsed since the origin of the group the interposing barriers have not been in existence; or, on the other hand, the particular organisms we are dealing with may have the power of overpassing the barriers, and thus reaching their present remote dwelling-places. As this is really the fundamental question of distribution on which the solution of all its more difficult problems depends, we have to inquire, in the first place, what is the nature of, and what are the limits to, the changes of the earth's surface, especially during the Tertiary and latter part of the Secondary periods, as it was during those periods that most of the existing types of the higher animals and pl:tnts came into existence; and, in the next place, what are the extreme limits of the powers of dispersal possessed by the chief groups of anim:tls and plants. 'Ve will first consider the question of barriers, more especially those formed by seas and oceans. The Pm·manence of Oceans. It was formerly a very general belief, even amongst geologists, that the great features of the earth's surface, no less than the smaller ones, were subject to continual mutations, |