Chap. XII.] THE GLACIAL PERIOD. 157 



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 the intermigration 

 of their inhabitants. 



During the slowly decreasing warmth of the Pliocene 

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

 habited 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 mi- 

 grated 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. Con- 

 sequently 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 pro- 

 ductions 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. 



