58 H. A. GLEASON VEGETATIONAL HISTORY OF MIDDLE WEST 



character of the vegetation. The result was the grassland type which 

 still prevails in that region. 



This result naturally required thousands of years for its accomplish- 

 ment, but the modus operandi may be summarized as involving five 

 processes: (1) the disappearance of the arborescent flora; (2) the 

 great increase in the number of individuals of the herbaceous species ; 

 (3) competition for space among the herbs, leading to the eventual 

 dominance of the grasses, as the group best suited by growth-form and 

 ecological requirements to the new conditions; (4) the development by 

 evolution of new and characteristic species and genera, and (5) the 

 immigration, usually accompanied by specific or generic evolution, of 

 other species from the Sonoran deserts at the south and the Great Basin 

 deserts at the west. 



How far toward the east the prairie vegetation may have extended 

 in Tertiary time and how many advances and retreats it may have 

 made in response to climatic variation are unknown. We may believe, 

 however, that the present climatic center of the Prairie Province in 

 western Kansas and Nebraska and eastern Colorado has been occupied 

 by this vegetation continually since its origin, and that amoeba-like 

 arms have been pushed out many times in many directions and with- 

 drawn again. It is also probable that the arctotertiary forests have 

 continuously occupied the Ozark uplift, since it still harbors many 

 old species, although not so many as the Appalachian uplift of the 

 same latitude, from which it was isolated during the Tertiary by the 

 oceanic waters of the Mississippi Embayment. This isolation and 

 the proximity of the Ozarkian region to prairie and Sonoran floras on 

 the west have led to a considerable differentiation between the Ozarkian 

 and Appalachian forest centers, as has already been noted in Part III. 



Fossil evidence indicates that the arctotertiary flora included both 

 angiosperms and gymnosperms and had little or no latitudinal differ- 

 entiation, at least as far north as 70°. The second great floristic devel- 

 opment affecting our region was the general segregation of these two 

 groups into a northern flora, with gymnosperms predominating, and a 

 southern flora, in which angiosperms were dominant. This probably 

 began only with the approach of the first glacial period and may not 

 have been completed until the ice age was well under way. The 

 processes were essentially the same as those outlined above for the 

 prairie vegetation, but in this case involved also the permanent dis- 

 appearance from both floras of a number of species belonging to genera 

 whose modern representatives are tropical. The former presence of 

 Ficus, Artocarpus, Sabal, and other genera of similar climatic require- 

 ments in northern latitudes has frequently been taken to indicate a 

 tropical or subtropical climate in those regions. The writer sees no 



