CREATION BY EVOLUTION 



have Keen almost neglected, is that found in the Don Valley, 

 near Toronto, Canada. Here are found impressions of 

 leaves and other parts of the sycamore, maple, osage orange, 

 and other types that do not to-day quite reach that latitude. 

 Other traces of Pleistocene floras are found in cave deposits, 

 associated with fossil remains of animals, some of which are 

 now extinct. Swamp deposits that were overwhelmed by 

 sand during changes along the coasts yield many species of 

 plants, most of which still exist, such as the bald cypress, 

 loblolly pine, sycamore, poplar, hickory, river birch, and 

 several species of oaks. All these fossils show that the inter- 

 glacial floras scarcely differed from those of to-day except 

 in the details of distribution of the species. During the 

 periods of glaciation these temperate forests retired south- 

 ward and gave way along the ice front in this country to 

 arctic willows and dwarf birches, which reached southward 

 to about latitude 40°. 



The post-glacial amelioration of the climate, the opening 

 to occupation by plants of areas that had been covered with 

 glaciers, the mixing of soils through the action of the ice, 

 all combined to stimulate the evolutionary activity of plants, 

 particularly the herbaceous forms. It seems probable that 

 the herbaceous families that are characteristic of the Tem- 

 perate Zone originated at this time. 



Possibly more potent than natural causes in modifying the 

 character and distribution of the existing vegetation has 

 been the work of man, which includes the action of flre, the 

 ax, and domesticated grazing animals. The forests are now 

 waning. Human intercourse results in surprising feats of 

 plant distribution, such as are shown in our familiar cosmo- 

 politan weeds. Insect and fungal pests are similarly spread, 

 both rapidly and widely, and tend increasingly to restrict or 

 even to exterminate the native vegetation. 



[172] 



