138 ESSA YS. 



came slowly on, — an extraordinary refrigeration of the 

 northern hemisphere, in the course of ages carrying glacial 

 ice and arctic climate down nearly to the latitude of the Ohio. 

 The change was evidently so gradual that it did not destroy 

 the temperate flora, at least not those enumerated above as 

 existing species. These and their fellows, or such as survive, 

 must have been pushed on to lower latitudes as the cold ad- 

 vanced, just as they now would be if the temperature were to 

 be again lowered ; and between them and the ice there was 

 doubtless a band of subarctic and arctic vegetation, — por- 

 tions of which, retreating up the mountains as the climate 

 ameliorated and the ice receded, still scantily survive upon 

 our highest Alleghanies, and more abundantly upon the colder 

 summits of the mountains of New York and New England ; — 

 demonstrating the existence of the present arctic-alpine vege- 

 tation during the glacial era ; and that the change of climate 

 at its close was so gradual that it was not destructive to vege- 

 table species. 



As the temperature rose, and the ice gradually retreated, the 

 surviving temperate flora must have returned northward pain 

 passu, and — which is an important point — must have ad- 

 vanced much farther northward, and especially northwest- 

 ward, than it now does ; so far, indeed, that the temperate 

 floras of North America and of eastern Asia, after having been 

 for long ages most widely separated, must have become a sec- 

 ond time conterminous. Whatever doubts may be entertained 

 respecting the existence of our present vegetation generally 

 before the glacial era, its existence immediately after that 

 period will hardly be questioned. Here, therefore, may be 

 adduced the direct evidence recently brought to light by Mr. 

 Lesquereux, who has identified our Live Oak ( Quercus virens), 

 Pecan (Carya olivceformis), Chinquapin (Castanea pumila^), 

 Planer-tree (Planer a aquatica), Honey-Locust (Gleditschia 

 triacanthos), Prinos coriaceus, and Acorns Calamus, — be- 

 sides an Elm and a Ceanothus doubtfully referable to existing 

 species, — on the Mississippi, near Columbus, Kentucky, in 

 beds which Mr. Lesquereux regards as anterior to the drift. 

 Professor D. D. Owen has indicated their position " as about 



