Chap. XI. DUKING THE GLACIAL PERIOD. 371 



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 inter-migration 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 

 the now living productions of the temperate regions of 

 the New and Old Worlds, we find very few identical 



