The Origin of the Desert Climax and Climate 129 



chief dominants of the desert plains of today and hence the 

 short-grasses of the Great Plains must have found their way into 

 the mid-grass prairies of the Tertiary in a later dry period, per- 

 haps as a result of Pliocene deformation. The short-grasses profit 

 by overgrazing in competition with the taller species, and hence 

 have spread irresistibly to the east during the historical period. 

 They must have behaved similarly in some degree under the 

 pressure of the great ungulate populations of the later Tertiary, 

 in the evolution of which they apparently played a part. They 

 "cure on the ground" and provide forage during the resting sea- 

 son that is far superior to that of the taller forms, their high 

 nutritive value being associated with the unique starch-sheaths 

 of the leaves. In contrast to the mid-grasses, they prospered under 

 warmer drier phases of the climatic cycles, and at such times 

 probably constituted the larger part of the forage. 



The presence of a well-defined sagebrush climax lying above 

 the grassland on slopes and plateaus, especially toward the north, 

 is attested by the list of transads on page 98, which are domi- 

 nants in this climax today. The most important of these are 

 members of the main association typical of the Great Basin, but 

 the coastal sagebrush of central and southern California is rep- 

 resented by several of the lesser dominants such as Eriodictyutn, 

 Pentstemon, and Viguiera. Two of the most important species, 

 Artemisia californica and Salvia apiana, extend into the western 

 edge of the Colorado Desert, while Eriogonum fasciculatum 

 with its variety polifolium ranges across the desert and well 

 to the east. No fossil remains of this climax have been found 

 in or about the region, a fact readily explained by the general 

 nature of the leaves and the arid habitat. As with the grassland, 

 this community must have moved north and south and up and 

 down the lower mountain slopes in its proper zonal relation in 



