408 GEOGEAPHICAL DISTRIBUTION. Chap. XII. 



If the difficulties be not insuperable in admitting 

 that in the long course of time the individuals of the 

 same species, and likewise of allied species, have pro- 

 ceeded from some one source ; then I think all the grand 

 leading facts of geographical distribution are explicable 

 on the theory of migration (generally of the more do- 

 minant forms of life), together with subsequent modifi- 

 cation and the multiplication of new forms. We can 

 thus understand the high importance of barriers, whether 

 of land or water, which separate our several zoological 

 and botanical provinces. We can thus understand the 

 localisation of sub-genera, genera, and families ; and how 

 it is that under different latitudes, for instance in South 

 America, the inhabitants of the plains and mountains, of 

 the forests, marshes, and deserts, are in so mysterious 

 a manner linked together by affinity, and are likewise 

 linked to the extinct beings which formerly inhabited the 

 same continent. Bearing in mind that the mutual rela- 

 tions of organism to organism are of the highest import- 

 ance, we can see why two areas having nearly the same 

 physical conditions should often be inhabited by very 

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

 which has elapsed since new inhabitants entered one 

 region ; 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 

 winch entered happened to come in more or less direct 

 competition with each other and with the aborigines ; 

 and according as the immigrants were capable of vary- 

 ing more or less rapidly, there would ensue in different 

 regions, independently of their physical conditions, infi- 

 nitely diversified conditions of life, — there would be an 

 almost endless amount of organic action and reaction, — 

 and we should find, as we do find, some groups of beings 

 greatly, and some only slightly modified, — some deve- 



