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Show CAL DISTRIBUTION• CHAP. XII. 408 GEOGRAPHI . . . lt' be no t I. n superable in admitting If the d1fficu ws f t' e the individuals of the 1 urse o liD that in the ong co. . f allied species, have pro- . d likeWISe 0 d same species, an . then I think all the gran ceeded from som e one sohuirccael ' distribution are exp1 1' cab l e leading facts of geo~rapt. (generally of the more do-on the theory 0f .m i)g rat lOltl her with subsequent mo d'If i - • 1.' of hfe oge w minant lorms 1 '. 1' tion of new forms. e can cat1· on and the mu hti'p hIC 'am portance of barn•e rs, h tl w e 1er thus understand the .Igh l te our several zoological t w hlC separa of land or. wa er, . We can thus understand the and botaniCal provinces. and families . and how . . f ub-genera, genera, ' locahsatwn o s . t 1 t't des for instance in South · h t der d1fferen a l u ' . f it lS t . a uthne l. nh ab l' ta nt s of the plains and mountains., o America, d d ts are in so mystenous the forests, marshes, a~ ~ser rflnity and are likewise a manner linked tog~t . er y ~ch fo;merly inhabited the linked to the extinct e~ngs.w . d that the mutual rela- . t Beanng In 1n1n same continen. · to organi.s m are of the highest import-tions of organism two areas having nearly the same ance, we can s.e~ why ould often be inhabited by very physical conditiOn~ sh 1.' dl'ng to the length of time 1.' f life · 10r accor diff.e rent 1orms 0 .' · habitants entered one 1 1 sed s1nce new In . . whi.c h 1as . e adpi g to the nat ure of the communiCa.t iOn region ; accor n . £ nd not others to enter, mther which allowed certain orms a d'n or not as those in greater or lesser numh ers ·' acco.r Im go re or le' ss 'direct . d h pened to come 1n . · . . which e. n. tere 'tha p ch other and WI' th the aborigmes , competition WI ea . t were capable of vary-and according as the immihgran s ld ensue in different 1 · dly t ere wou infi ing more or ess rapi f their hysical conditions, - regions, independently.~ f lfre -there would be an nitely diversified conditions o . ' tion and reaction,- almost endless amount o~ ~g~~' :~me groups of beings and we should find, as w . 1 modified -some deve- {sreatly, and some only slight y ' CHAP. XII. SUMMARY. 409 loped in great force, some existing in scanty numbers:_ in the different great geographical provinces of the world. On these same principles, we can understand, as I have endeavoured to show, why oceanic islands should have few inhabitants: but of these a great number should be endemic or peculiar; and why, in relation to the means of migration, one group of beings, even within the same class, should have all its species endemic, and another group should have all its species common to other quarters of the world. We can see why whole groups of organisms, as batrachians and terrestrial mamnlals, should be absent from oceanic islands, whilst the most isolated islands possess their own peculiar species of aerial mammals or bats. We can see why there should be some relation between the presence of mammals, in a more or less modified condition, and the depth of the sea between an island and the mainland. We can clearly see why all the inhabitants of an archipelago, though specifically distinct on the several islets, should be closely related to each other, and likewise be related, but less closely, to those of the nearest continent or other source whence immigrants were probably derived. We can see why in two areas, however distant from each other, there should be a correlation, in the presence of identical species, of varieties, of doubtful species, and of distinct but representative species. As the late Edward Forbes often insisted, there is a striking parallelism in the laws of life throughout time and space: the laws governing the succession of forms in past times being nearly the same with those governing at the present time the differences in different areas. We see this in many facts. The endurance of each species and group of species is continuous in time; for the exceptions to the rule are so few, that they may T |