Chap. XII. AND TKESENT CHAPTER. 3(J9 



north and soutli, allowed the productions of ojipositc hemi- 

 spheres to mingle, and left some of them stranded in all parts 

 of the world on the mountain-summits. As showing how diver- 

 sified are the means of occasional transport, I have discussed 

 at some little length the means of dispersal of fresh-water pro- 

 ductions. 



If the difliculties be not insuperable in admitting that in 

 the long course of time all the individuals of the same species, 

 and likewise of the several species belonging to the same ge- 

 nus, have proceeded from some one source ; then all the grand 

 leading facts of geographical distribution are explicable on the 

 theory of mioTation, together with subsetiuent modification and 

 the multiplication of new forms. We can thus understand the 

 high imixtrtancc of barriers, whether of land or water, in not 

 only s{'j)arating, but in apparently forming the several zoologi- 

 cal and botanical provinces. We can thus understand the 

 concentration of related species within the same areas; and 

 how it is that under different latitudes, for instance in South 

 America, the inliabitants of the i>lains and mountains, of the 

 forests, marshes, and deserts, are linked together in so mysteri- 

 ous a manner, and are likewise linked to the extinct beings 

 which fonnerly inhabited the same continent. Bearing in mind 

 that the mutual relation of organism to organism is of the high- 

 est importance, we can see why two areas liaving nearly the 

 same phj'sical conditions sliould often be inhabited by very 

 different forms of life ; for, according to the length of time 

 which has elapsed since the colonists entered one of the regions, 

 or both; according to the nature of the communication which 

 allowed certain forms and not others to enter, either in greater 

 or lesser numbers; according or not as those which entered 

 happened to come into more or less direct coinpetiti(m with 

 each other and with the aborigines ; and according as the im- 

 migrants were capable of varying more or less rapidly, there 

 would ensue in the two or more regions, indejiendently of their 

 phj-sical conditions, infinitcly-diver.sified conditions of life — 

 there would be an almost endless amount of organic action and 

 reaction — and we should lind, as we. do find, some groups of 

 beings greatly, and some only slightly, modified — some devel- 

 oped in great force, some existing in scanty numbers — in the 

 several great geographical provinces of the world. 



On these same principles we can undc^rstand, as I have en- 

 deavored to show, why oceanic islands should have few in- 

 habitants, but of these a great number should be endemic or 



