26 EDWARD W. BERRY 



CajQada to Alaska and the mouth of the Mackenzie River, and 

 encircling the globe in high latitudes. They have been recorded 

 from Greenland, Grimiell Land, Spitzbergen, SachaHn, Siberia 

 and Manchuria. A few are found in central Europe, but the 

 great bulk are American and Arctic, and the climate of more 

 southern lands appears to have been too warm for their presence 

 in any great numbers, for in the abundant Eocene floras of 

 southeastern North America we find no traces of poplars but 

 in their place a subtropical flora of figs, laurels, bread fruit, 

 rain trees and their allies, thatch and date pahns, nutmegs, 

 pond applies, and similar types unfamiliar to dwellers in the 

 Temperate zone. This subtropical flora extends as far north 

 as the mouth of the Ohio in America and a similar warm flora 

 extends to southern England in Europe. 



During the Ohgocene, which succeeded the Eocene, the scanty 

 records have yielded few poplars. Three species have been 

 described from deposits of this age in the \¥est and four or five 

 are known from central and southern Europe. Southeastern 

 North America was still too tropical in its climate to permit the 

 existence of poplars and although we lack the proof it may be 

 assumed that the numerous Eocene forms lived on in Arctic 

 lands until they were gradually exterminated or driven south- 

 ward by the more severe climate that commenced to prevail in 

 high latitudes before the close of the Oligocene. 



The poplars are represented during the Miocene period by 

 about thirty species, which are found from Greece westward to 

 Spain in Europe and throughout the western United States and 

 Canada. The Miocene lake of Florissant in the heart of the 

 Colorado Rockies has furnished seven forms of poplar — one a 

 splendidly preserved cottonwood that may well have been the 

 ancestor of the existing forms that are found at the present time 

 in Colorado. Poplars are found at this time along the Pacific 

 Coast, but none are known from the Atlantic or Gulf Coasts. 



The Pliocene period, which succeeded the Miocene and im- 

 mediately preceded the Glacial period, has furnished about 16 

 species of poplars, several of which are very close to, if not identi- 

 cal with, still existing European forms such as the European 



