GEOGRAPHICAL DISTRIBUTION 341 



subjected to great oscillations of level, I am strongly inclined to 

 extend the above view, and to infer that during some still earlier 

 and still warmer period, such as the older PHocene period, a large 

 number of the same plants and animals inhabited the almost 

 continuous circumpolar land; and that these plants and animals, 

 both in the Old and New Worlds, began slowly to migrate south- 

 ward as the climate became less warm, long before the commence- 

 ment of the Glacial period. We now see, as I believe, their de- 

 scendants mostly in a modified condition, in the central parts 

 of Europe and the United States. On this view we can under- 

 stand 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 during 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 inter-migration 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 productions are concerned, must have taken 

 place long ages ago. As the plants and animals migrated south- 

 ward, 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 mucb 

 modification — for tar 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 supposed), 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 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 



