The Origin of the Desert Climax and Climate 127 



ber of similar species occur westward into central Arizona, but 

 stop short of the desert, namely, Andropogon contortus, nutans, 

 furcatus, and halli, Elionurus barbiculmis, Trachypogon secun- 

 dus, and Tripsaciim lemmoni. All these find their best develop- 

 ment in the southern part of the country and under a rainfall of 

 35 inches or more, though the first three range widely through 

 the river valleys of the Great Plains, where they persist as relicts 

 of an earlier warm-moist period. In their climatic relations in 

 the United States they are typically postclimax to true prairie 

 and subclimax to deciduous forest. This fact, taken with their 

 occurrence from the South and East to the Canadian border 

 and westward to Arizona or California, leaves little doubt that 

 they constituted the dominants of an earlier tall-grass prairie that 

 confronted the deciduous forests of the Eocene and Lower Oligo- 

 cene and gradually replaced them as they withdrew in the face 

 of cooler drier conditions in the later Oligocene and the Miocene. 

 Their presence in the Southwest, both as transads and as ad- 

 jacent dominants, justifies the assumption that they constituted 

 the grassland that preceded the mixed climax described above 

 and perhaps was still in occupation of much of the region before 

 the Upper Oligocene. 



The mid-grasses of the ancestral mixed prairie consisted of 

 more xeric dominants, as the name suggests. These are all more 

 northern in derivation and with lower water requirements than 

 the tall-grasses. Some of them, such as Stipa, Oryzopsis, and 

 Hilaria, were apparendy North American in origin, while others 

 were Eurasian. Stipa is the genus paramount of prairie and 

 steppe the world over, and has probably played the major role 

 not only in the evolution but also the intercontinental migra- 

 tion of grazing animals. Its indurated lemma, sharp callus, and 

 long twice-bent hygroscopic awn stamp it as the product of 



