370 
GEOGKAPHICAL DISTRIBUTION, 
Chap. XI. 
Atlantic Ocean and by the extreme northern part of 
the Pacific. During the Glacial period, when the in¬ 
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 difl&culty 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°-67^; 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 
