138 ESSAYS. 
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 pari 
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 oliveformis), Chinquapin ( Castanea pumila), 
Planer-tree (Planera aquatica), Honey-Locust ( Gleditschia 
triacanthos), Prinos coriaceus, and Acorus 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 
