298 EDMUND W. SINNOTT AND IRVING W. BAILEY 



suffered less by extinction than have the woody plants, and conse- 

 quently that even more than 50 per cent of the original boreal 

 dicotyledonous flora consisted of woody forms. This analysis 

 would seem to indicate that previous to the glacial invasion the 

 climate of the north temperate zone had, for a long time at least, 

 been devoid of extremes of cold. We know from fossil evidence 

 that in several regions during the Tertiary temperate genera, such 

 as oaks and poplars, grew side by side with such tropical or sub- 

 tropical types as the palm and fig; and it therefore seems reasonable 

 to infer that the climate was a very equable one and devoid of 

 extremes of heat as well as of cold. 



More conclusive evidence, however, is at hand as to climatic 

 conditions near the ice front during the actual period of the glacial 

 invasion. The vegetation of the whole temperate zone at that 

 time of course lost heavily by extermination, but this extermination 

 must have been much more pronounced among woody plants than 

 among herbs, owing to the greater ability of the latter to withstand 

 cold and other adverse conditions. It is a significant fact that 

 the present flora of Europe north of the Alps is decidedly more 

 impoverished than is that of corresponding temperate North 

 America, and that although in the latter region approximately 25 

 per cent of the dicotyledons are woody, in the former only from 10 

 to 15 per cent are so. This paucity of indigenous trees and shrubs 

 in northern Europe is especially noteworthy since experiment has 

 shown that many species of delicate and warmth-loving trees and 

 shrubs will grow in England, France, and Germany which cannot 

 stand the droughts and winters of the northern United States. In 

 America, too, there are representatives of a considerable number 

 of woody famihes which are now absent in northern Europe, 

 many of which occur there as fossils. These facts are evidently 

 to be explained by the much greater adversities suffered by the 

 European flora during the Glacial epoch. In North America, 

 especially in its eastern portion, the vegetation could easily migrate 

 southward at the advance of the ice and return northward at its 

 retreat. In northern Europe, on the other hand, the southward, 

 escape of the vegetation was blocked, and it was crowded against 

 the Alps, the Pyrenees, and the Mediterranean, thus suffering 



