531 DISPERSAL DURING THE GLACIAL PERIOD. [Chap. XII. 



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 hecome 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 favourable 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 N. 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 supposed), but we find in every great class many forms, 

 which some naturalists rank as geographical races, and others as dis- 

 tinct species; and a host of closely allied or representative forms 

 which are ranked by all naturalists as specifically distinct. 



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

 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 modifica- 

 tion, for many closely allied forms now living in marine areas com- 

 pletely 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 marine 

 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 theoiy 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 with parts of South Africa or Australia, we see countries 

 closely similar in all their physical conditions, with their inhab- 

 itants utterly dissimilar. 



