GEOGRAPHICAL DISTRIBUTION 333 



oscillations of level, I am strongly inclined to extend 

 the above view, and to infer that during some earlier 

 and still warmer period, such as the older Pliocene 

 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 southwards 

 as the climate became less warm, long before the com- 

 mencement of the Glacial period. We now see, as I 

 believe, their descendants, mostly in a modified con- 

 dition, in the central parts of Europe and the United 

 States. On this view we can understand the relation- 

 ship, with very little identity, between the productions 

 of North America and Europe, — a relationship which 

 is most remarkable, considering the distance of the two 

 areas, and their separation by the 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 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 must have been completely cut 

 off from each other. This separation, as far as the 

 more temperate productions are concerned, took place 

 long ages ago. And as the plants and animals migrated 

 southward, they will have become mingled in the one 

 great region with the native American productions, 

 and have had to compete with them ; and in the other 

 great region, with those of the Old World. Conse- 

 quently 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 



