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, considering the distance of the two areas, and their separation by the whole 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 present 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 will have been completely cut off from each other.
This separation, as far as the more temperate productions are concerned, must have taken place long ages ago.