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Show 68 ISLAND LTFK [rAR'l' I. has been named Heliodus by Professor Newberry. Thus an enormous range in time is accompanied by a very wide and scattered distribution of the existing species. Whenever, therefore, we find two or more living genera be-longing to the same family or order but not very closely allied to each other, we may be sure that they are the remnants of a once extensive group of genera; and if we find them now isolated in remote parts of the globe, the natural inference is that the family of which they are fragments once had an area embracing the countries in which they are found. Yet this simple and very obvious explanation has rarely been adopted by naturalists, who have instead imagined changes of land and sea to afford a direct passage from the one fragment to the other. If there were no cosmopolitan or very wide-spread families still existing, or even if such cases were rare, there would be some justification for such a proceeding; but as about one-fourth of the existing families of land mammalia have a range extending to at least three or four continents, while many which are now represented by disconnected genera are known to have occupied intervening lands or to have had an almost continuous distribution in tertiary times, all the presumptions are in favour of the former continuity of the group. We have also in many cases direct evidence that this former continuity was effected by means of existing continents, while in no single case has it been shown that such a continuity was impossible, and that it either was or must have been effected by means of continents now sunk beneath the ocean. Concluding Remarks.-When writing on the subject of dis-tribution it usually seems to have been forgotten that the theory of evolution absolutely necessitates the former existence of a whole series of extinct genera filling up the gap between the isolated genera which in many cases now alone exist; while it is almost an axiom of " natural selection " that such numerous forms of one type could only have been developed in a wide area and under varied conditions, implying a great lapse of time. In our succeeding chapters we shall show that the known and probable changes of sea and land, the known changes of climate, and the actual powers of dispersal of the CHAP. IV.] EVOLUTION THE KEY TO DIS'flliDUTION. 69 different groups of animals, were such as would have enabled all t?e now dis?onnected groups to have once formed parts of a c?ntmuous. senes .. Proofs of such former continuity are con~ m~ally bei~g obtamed by the discovery of allied extinct forms In mt~rvemng lands, but the extreme imperfection of the g-eologi~al record .as regards l~nd animals renders it unlikely that this. proof wr~l be forthcoming in the majority of cases. The not10n that If such animals ever existed their remains wou]~ certainly be found, is a superstition which, notwithstandmg the e~orts of Lyell and Darwin, still largely prevails among naturalists; but until it is got rid of no true notions of the former distribution of life upon the earth can be attained. |