XV.] THE COTERMINOUS FLORAS. 271 



like the European or even like that of our own Paci- 

 fic coast. 



It is well known that the climate of the pliocene 

 epoch, preceding the glacial time, was much milder 

 than now. Over the Dakotas camels, horses, a masto- 

 don, a rhinoceros and an elephant roamed, and the 

 temperate floras extended much further north than they 

 do at the present time. The same conditions prevailed 

 in northern Asia, and the floras of the two continents 

 were coterminous and intermingled. Then came on the 

 glacial epoch, — "an extraordinary refrigeration of the 

 northern hemisphere, in the course of ages carrying 

 glacial ice and arctic climate down nearly to the lati- 

 tude of the Ohio. The change was evidently so grad- 

 ual that it did not destroy the temperate flora. * * * 

 These [the plants] and their fellows, or such as sur- 

 vive, must have been pushed on to lower latitudes as the 

 cold advanced, just as they now would be if the temper- 

 ature were to be again lowered ; and between them and 

 the ice there was a band of subarctic and arctic vegeta- 

 tion, — portions 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 vegetation during 

 the glacial era ; and that the change of climate at its 

 close was so gradual that it was not destructive to 

 vegetable species." So the plants were driven to the 

 southward, both down the Asian and American con- 

 tinents. Gradually the ice melted away, the climate 

 became milder and plants began to return northward. 

 After the glacial epoch had passed away, the arctic 



