370 DISPERSAL DURING 



they will have been completely cut off from each other. 

 This separation, as far as the more temperate productions 

 are concerned, must have taken place long ages ago. As 

 the plants and animals migrated southward, they will have 

 become mingled in the one great region with the native 

 American productions, and would have had to compete with 

 them ; and in the other great region, with those of the Old 

 World. Consequently we have here everything favorable 

 for much modification — for far more modification than with 

 the alpine productions, left isolated, within a much more 

 recent period, on the several mountain ranges and on the 

 arctic lands of Europe and North America. Hence, it has 

 come, that when we compare the now living productions of 

 the temperate regions of the New and Old Worlds, we find 

 very few identical species (though Asa Gray has lately 

 shown that more plants are identical than was formerly sup- 

 posed), but we find in every great class many forms, which 

 some naturalists rank as geographical races, and others as 

 distinct species ; and a host of closely allied or representa- 

 tive forms which are ranked by all naturalists as specifically 

 distinct. 



As on the land, so in the waters of the sea, a slow south- 

 ern migration of a marine fauna, which, during the Pliocene 

 or even a somewhat earlier period, was nearly uniform along 

 the continuous shores of the Polar Circle, will account, on 

 the theory of modification, for many closely allied forms 

 now living in marine areas completely sundered. Thus, I 

 think, we can understand the presence of some closely allied, 

 still existing and extinct tertiary forms, on the eastern and 

 western shores of temperate North America; and the still 

 more striking fact of many closely allied crustaceans (as 

 described in Dana's admirable work), some fish and other 

 mariue animals, inhabiting the Mediterranean and the seas 

 of Japan — these two areas being now completely separated 

 by the breadth of a whole continent and by wide spaces of 

 ocean. 



These cases of close relationship in species either now or 

 formerly inhabiting the seas on the eastern and western 

 shores of North America, the Mediterranean and Japan, 

 and the temperate lands of North America and Europe, are 

 inexplicable on the theory of creation. We cannot maintain 

 that such species have been created alike, in correspondence 

 with the nearly similar physical conditions of the areas ; for 

 if we compare, for instance, certain parts of South America 



