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°-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 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 

 ISTew 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 num- 

 ber of the same plants and animals inhabited the al- 

 most 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 commencement 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 

 understand the relationship with very little identity, 

 between the productions of North America and Europe, 

 — a relationship which is highly remarkable, consider- 

 ing the distance of the two areas, and their separation 

 by the whole Atlantic Ocean. We can further under- 

 stand the singular fact remarked on by several observers 



