416 ORIGIN OF SPECIES 



mostly in a modified condition, in the central parts of Europe 

 and the United States. On this view we can understand the 

 relationship with very little identity, between the productions 

 of North America and Europe, — a relationship which is 

 highly remarkable, considering the distance of the two areas, 

 and their separation by the whole Atlantic Ocean. We can 

 further understand the singular fact remarked on by several 

 observers that the productions of Europe and America dur- 

 ing the later tertiary stages were more closely related to 

 each other than they are at the present time; for during 

 these warmer periods the northern parts of the Old and New 

 Worlds will have been almost continuously united by land, 

 serving as a bridge, since rendered impassable by cold, for 

 the intermigration of their inhabitants. 



During the slowly decreasing warmth of the Pliocene 

 period, as soon as the species in common, which inhabited 

 the New and Old Worlds, migrated south of the Polar 

 Circle, they will have been completely cut off from each 

 other. This separation, as far as the more temperate produc- 

 tions 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 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 tem- 

 perate 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 nat- 

 uralists rank as geographical races, and others as distinct 

 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 south- 

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

 or even a somewhat earlier period, was nearly uniform along 



