CHAP. XII.] THE GLACIAL PERIOD. 313 



the Polar Circle, in latitude 66-67 ; and that the present arctic 

 productions then lived on the broken land still nearer to the pole. 

 Now, if we look at a terrestrial globe, we see under the Polar 

 Circle that there is almost continuous land from western Europe, 

 through Siberia, to eastern America. And this continuity of the 

 circumpolar land, with the consequent freedom under a more 

 favourable climate for intermigration, will account for the sup- 

 posed uniformity of the sub-arctic and temperate productions of 

 the Old and New Worlds, at a period anterior to the Glacial 

 epoch. 



Believing, from reasons before alluded to, that our continents 

 have long remained in nearly the same relative position, though 

 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 Pliocene period, a large 

 number of the same plants and animals inhabited the almost con- 

 tinuous circumpolar land; and that these plants and animals, 

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

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

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

 descendants, 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 pro- 

 ductions 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 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 oft' 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 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 



