2l8 THE GENESIS AND MIGRATIONS OF PLANTS 



clays of similar origin in the basin of the great lakes and in 

 the West, and are not Arctic plants, but members of the North 

 Temperate flora.^ These have been called " interglacial," but 

 there is no evidence to prove that they are not truly glacial. 

 Thus, while the arctic flora must have continued to exist within 

 the Arctic circle in the Glacial age, we have evidence that those 

 of the cold temperate and subarctic zones continued to exist 

 pretty far north. At the same time the warm temperate flora 

 would be driven to the south, except where sustained in insular 

 spots warmed by the equatorial currents. It would return north- 

 ward on the re-elevation of the land and the return of warmth. 



If, however, our modern flora is thus one that has returned 

 from the south, this would account for its poverty in species 

 as compared with those of the early Tertiary. Groups of plants 

 descending from the north have been rich and varied. Re- 

 turning from the south they are like the shattered remains of 

 a beaten army. This, at least, has been the case with such re- 

 treating floras as those of the Lower Carboniferous, the Per- 

 mian, and the Jurassic, and possibly that of the Lower Eocene 

 of Europe. 



The question of the supply of light to an Arctic flora is 

 much less difficult than some have imagined. The long 

 summer day is in this respect a good substitute for a longer 

 season of growth, while a copious covering of winter snow not 

 only protects evergreen plants from those sudden alternations 

 of temperature which are more destructive than intense frost, 

 and prevents the frost from penetrating to their roots, but 

 by the ammonia which it absorbs preserves their greenness. 

 According to Dr. Brown, the Danish ladies of Disco long ago 

 solved this problem.^ He informs us that they cultivate in 



* Pleistocene Plants of Canada, Dawson and Penhallow, Bull. Geol. 

 Socy.y America, 1890. In Europe the Arctic flora extended, relatively to 

 present climate, farther south. 



* Florida Discoana, Botanical Society of Edinburgh, 1868. 



