370 GEOGRAPHICAL DISTRIBUTION, Chap. XI. 



habitants of the Old and New Worlds lived further 

 southwards than at present, they must have been still 

 more completely separated by wider spaces of ocean. 

 I believe the above difficulty may be surmounted by 

 looking to still earlier changes of climate of an opposite 

 nature. We have good reason to believe that during 

 the newer Pliocene period, before the Glacial epoch, 

 and whilst the majority of the inhabitants of the world 

 were specifically the same as now, the climate was 

 warmer than at the present day. Hence we may sup- 

 pose that the organisms now living under the climate 

 of latitude 60°, during the Pliocene period lived further 

 north under the Polar Circle, in latitude 66°-b'7° ; and 

 that the strictly arctic productions then lived on the 

 broken land still nearer to the pole. Now if we look at 

 a globe, we shall see that under the Polar Circle there 

 is almost continuous land from western Europe, through 

 Siberia, to eastern America. And to this continuity of 

 the circumpolar land, and to the consequent freedom 

 for intermigration under a more favourable climate, I 

 attribute the necessary amount of uniformity in the 

 sub-arctic and northern 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 rela- 

 tive position, though subjected to large, but partial 

 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- 



