Chap. XI. 
DUEING THE GLACIAL PERIOD. 
371 
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 
most remarkable, considering the distance of the two 
areas, and their separation by the 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 pre¬ 
sent 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 must have been completely cut off from each 
other. This separation, as far as the more temperate pro¬ 
ductions are concerned, took place long ages ago. And 
as the plants and animals migrated southward, they will 
have become mingled in the one great region with the 
native American productions, and have had to compete 
with them; and in the other great region, with those 
of the Old World. Consequently we have here every¬ 
thing favourable for much modification,—for far more 
modification than with the Alpine productions, left 
isolated, within a much more recent period, on the 
several mountain-ranges and on the arctic lands of the 
two Worlds. Hence it has come, that when we compare 
