PALEOBOTANY BERRY. 407 



coasts yield their quota of forms, most of which are still existing 

 species, such as the bald cypress, loblolly pine, sycamore, Carolina 

 poplar, hickory, river birch, and various species of oaks. All of 

 these show that the Interglacial floras scarcely differed from those 

 of to-day except in the details of distribution of the various species. 



During the epochs of glaciation these temperate forests retired 

 southward and gave way along the ice front to arctic willows and 

 dwarf birches, which penetrated southward to about latitude 40°. 



The post-glacial amelioration of the climate, the opening of areas 

 that had been covered with glaciers to occupation, the mixing of 

 soils through ice action, all combined to stimulate evolutionary ac- 

 tivity in the plant kingdom, particularly among herbaceous forms, 

 many of which date from this time. It seems probable that the char- 

 acteristic Temperate Zone herbaceous families, already mentioned in 

 the description of the angiosperms (ante) date from about this 

 period. 



Possibly more potent than natural causes in modifying the existing 

 vegetation has been the action of humanity, with fire, ax, and do- 

 mesticated grazing animals. Forests are now waning. Human in- 

 tercourse results in untold feats of distribution, emphasized by the 

 familiar cosmopolitan weeds of these modern days. Insect and 

 fungal pests are similarly spread both rapidly and widely, and all 

 of these factors tend to increasingly restrict or even exterminate 

 the native vegetation. 



