174 PROCEEDINGS OF THE AMERICAN ACADEMY 



mountains as the climate gradually grew milder, still exist 

 scantily upon the highest peaks of the Alleghanies, and in 

 greater numbers upon the cooler mountain-summits of New 

 England and New York. 



As the ice receded northward at the close of the glacial 

 period, the temperate flora would naturally follow it ; and 

 Professor Gray insisted, as a most important point in the 

 present discussion, that the temperate vegetation must have 

 again advanced, after the glacial epoch, much farther north, 

 and especially northwest, than it now does ; so far north, 

 indeed, that the temperate floras of North America and of 

 Eastern Asia — before conterminous, and then most widely 

 separated — must have again become conterminous. How- 

 ever it may have been in the ante-glacial period, — although 

 it appears certain that some, and probable that many, of our 

 species of plants then existed, — Professor Gray thought it 

 could not be doubted that most of our present species were 

 in existence immediately after the glacial period, and there- 

 fore liable to interchange with Eastern Asia at a time when 

 the temperate floras of the two regions were contiguous. 



The evidence of such contiguity during what Professor 

 Dana terms the fluvial epoch, which succeeded the glacial. 

 Professor Gray remarked, was that a milder climate than the 

 present then supervened, — perhaps not so much higher in 

 the mean temperature of the year at the North, as more equa- 

 ble, — a more oceanic climate, such as would naturally result 

 from the extensive submergence of northern, or at least of 

 northeastern land, when the sea stood five hundred feet above 

 its present level in the basin of the St. LawTcnce, and our 

 great alluvial plains, from fifty to three hundred feet above 

 the present bed of the rivers, were flooded. Professor Gray 

 alluded to the character of the herbivorous animals of that 

 period, and their high northern range, as demonstrating 

 that our temperate flora then reached northward far beyond 

 the arctic circle ; for that was the era of the Megatherium, 

 Megalonyx, Mylodon, Mastodon, a Dicotyles, a wild horse, 



