Chap. xii. Dispersal during the Glacial Period. 333 



same species could then or previously have entered the two con- 

 tinents. The explanation, I believe, lies in the nature of the 

 climate before the commencement of the Glacial period. At this, 

 the newer Pliocene period, the majority of the inhabitants of the 

 world were specifically the same as now, and we have good reason 

 to believe that the climate was warmer than at the present day. 

 Hence we may suppose that the organisms which now live under 

 latitude 60°, lived during the Pliocene period farther north under the 

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

 ductions 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 circum- 

 polar land, with the consequent freedom under a more favourable 

 climate for intermigration, will account for the supposed 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 

 continuous 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 tiie 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 



