GEOGRAPHICAL DISTRIBUTION 165 



the United States. On this view we can understand the 

 relationship, with very little identity, between the pro- 

 ductions 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 re- 

 marked 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 north- 

 ern 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 inhab- 

 ited 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. As the plants and animals mi- 

 grated southward, they will have become mingled in the 

 one great region with the native American productions, 

 and would have had to compete with them; and in the 

 other great region, with those of the Old World. Con- 

 sequently we have here everything favorable 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 Europe and North America. Hence 

 it has come, that when we compare the now living pro- 

 ductions of the temperate regions of the New and Old 



