THE EVOLUTION OF HERBACEOUS PLANTS 299 



heavily by extinction. These same natural barriers have also 

 prevented any considerable northward migration since the retreat 

 of the ice. The vegetation of northern Europe today seems, 

 therefore, to be descended directly from that remnant which was 

 able to survive on the unglaciated portions of France, Germany, 

 and England. We have already noted the fact that the per- 

 centage of woody plants in the dicotyledonous flora of northern 

 Europe is amazingly low, being only from 10 to 15 per cent of the 

 whole. This is really a percentage typical of alpine or northern 

 regions. Indeed, the proportion of herbs in Switzerland today is 

 but little higher than that in the adjacent lowlands. The alpine 

 character of the northern European flora is further emphasized 

 by the strong resemblance which it bears to that of the flora of the 

 Rocky Mountains, for the two floras are composed of almost 

 exactly the same families and include a host of identical genera 

 and even a large number of identical species. In fact, the 

 flora of the Rockies presents a much closer resemblance to 

 that of Europe than does the flora of the eastern part of the 

 continent. 



If the flora of northern Europe is indeed typically representa- 

 tive of that which flourished near the ice front during glacial times, 

 the proportion of woody forms within it affords us a valuable index 

 as to climatic conditions during the height of the ice age. The 

 facts seem to indicate that when the ice sheeet had reached its 

 greatest extent the country in its immediate front was neither a 

 barren arctic tundra, as has sometimes been supposed, nor covered 

 with a luxuriant temperate vegetation; but that the climate in 

 general resembled that of the lower portions of the Alps or the 

 Rockies today, being cold enough in winter to kill off all but the 

 hardiest trees and shrubs but not sufficiently cold to reduce the 

 whole vegetation to the few perennial herbs and stunted shrubs 

 which are characteristic of arctic regions today. Of course this 

 evidence is of value only as indicating the climate during the coldest 

 period of the ice invasion, just as the percentage of herbs in a flora 

 is indicative of the minimum winter temperature of the region. As 

 to the climate of the presumably warmer interglacial periods it tells 

 us nothing. 



