156 DISPERSAL DURING [Chap. XII. 



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°- 6 7°; 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 wes- 

 tern Europe, through Siberia, to eastern America. And 

 this continuity of the circumpolar land, with the con- 

 sequent freedom under a more favourable climate for in- 

 termigration, 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 in- 

 habited 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 

 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 



