THE EVOLUTION OF HERBACEOUS PLANTS 297 



probable, but it seems equally probable that their great develop- 

 ment did not take place until after the close of that epoch. 



Such, in brief, is the evidence for beheving that herbaceous 

 angiosperms are comparatively recent in origin; that they have 

 been developed most abundantly in the north temperate zone, 

 mainly since the Cretaceous, as an adaptation to the advent of a 

 winter season; and that they have spread thence over most of the 

 other regions of the earth. Let us now see what conclusions of 

 importance to geologists and climatologists may be drawn from 

 a study of the origin and dispersal of these herbs. 



In the first place the distribution of herbs in the north temper- 

 ate zone provides us with evidence as to the climate of this area 

 at the time of the last Glacial epoch. 



As to what was the composition of the northern flora before 

 the advance of the ice sheet we cannot be very certain, but there is 

 reason to believe that a much higher proportion of woody plants 

 flourished there than at present, indicating the existence of a climate 

 devoid of extreme cold. Such an inference is based on the composi- 

 tion of that considerable body of related species and genera which 

 are common to eastern North America and eastern Asia and 

 which are nearly or quite absent elsewhere. Gray^ was the first 

 to suggest that these plants were the remnant of the widespread 

 pre-Glacial boreal flora which had been forced south by the advance 

 of the ice and had been exterminated almost everywhere save in 

 these two regions, which were the only ones in warmer latitudes 

 into which it had been able to escape. If this hypothesis is correct, 

 as subsequent investigations seem to indicate, and if this group of 

 plants is truly a fair sample of the pre-Glacial flora of the north, a 

 study of its composition is of considerable interest. Gray has pub- 

 lished a list of the genera and species of eastern North America 

 which are absent in Europe but which are represented by identical 

 or closely related forms in eastern Asia. This list comprises 142 

 genera of dicotyledons, of which 70 are woody or predominantly so, 

 and 240 species, of which 128 are woody. The flora is thus just 

 about equally divided between herbs and woody plants. From the 

 greater hardiness of most herbs it is probable that they have 



^ A. Gray, Scientific Papers (1879). 



