The Origin of the Desert Climax and Climate 91 



An effort has been made to marshal all the facts that have a 

 bearing, direct or otherwise, upon the modification of grassland 

 into desert under compulsion of a major climatic change. The 

 primary cause of the latter is to be sought in the elevation of 

 neighboring mountain systems, just as its effects are disclosed 

 in the fossil record of the plants and animals that passed across 

 the changing scene. In order to permit a sharp focus, the exist- 

 ence of a grassland climax as widespread as that of today must 

 be taken for granted. This assumption is well justified by the 

 transmontane occurrence of the great genera of grasses, such as 

 Stipa,Bout€loua,Sporobolus,Poa,Andropogon,ttc, and attested 

 by an ungulate fauna far more varied than that of the present 

 epoch. While the origin of the prairie climax has a direct bearing 

 upon the present problem, this is a matter of such scope and 

 complexity as to demand separate treatment in the future. 



In assembling the evidence from all sources, it is necessary to 

 employ the essential method of paleo-ecology by proceeding 

 from the existing biome to the past, and in this task the relict 

 provides the missing key. These witnesses to past events are of 

 several kinds. Chief among them are the relicts still present in 

 the desert, which are few in the climax itself, but more frequent 

 in the protection of mountains in or bordering upon the region. 

 Much more numerous and also of signal value are what may be 

 termed "transads" plants and animals either present or absent 

 today, but at home in the desert when it was grassland as shown 

 by their presence on both sides of the climatic barrier. Next in 

 significance are the species that occur in the desert and on one 

 side of it, either to the east or to the west. Of similar meaning 

 are the changes that have taken place farther away in the grass- 

 land without transforming it into desert. A correlated body of 

 evidence is to be derived from the migration of the desert climax 



