Geographical Distribution. 215 



natural affinity among inhabitants of the same areas ; 

 second, another of a more detailed character between 

 degrees of geographical restriction and degrees of 

 natural affinity. The more distant the affinity, the 

 more general is the extension. This, of course, is 

 what we should expect on the theory of descent 

 with modification, because the more distant the 

 affinity, and therefore, ex hypothesi, the larger and 

 the older the original group of organisms, the greater 

 must be the chance of dispersal. The 400 species 

 of humming-birds may. well be unable to migrate 

 from their native continent ; but it would indeed 

 have been an unaccountable fact if no other species 

 of all the class of birds had ever been able to have 

 crossed the Atlantic Ocean. Thus, on the theory of 

 evolution, we can well understand the second corre- 

 lation now before us namely, between remoteness 

 of affinity and generality of dispersal, so that 

 there is no considerable portion of the habitable 

 globe without representatives of all the classes of 

 animals, few portions without representatives of all 

 the orders, but many portions without many of the 

 families, innumerable portions without innumerable 

 genera, and, of course, all portions without the great 

 majority of species. Now, while this general correla- 

 tion thus obviously supports the theory of natural 

 descent with progressive modification, it makes di- 

 rectly against the opposite theory of special creation. 

 For we have recently seen that when we restrict our 

 view to the case of species and genera, the theory of 

 special creation is obliged to suppose that for some 

 inscrutable reason the Deily had regard to systematic 

 affinity while determining on what large areas to 



