Q2 E E. Clements 



from the Mexican homeland, and the evolution of endemics at 

 this or a later time. The indispensable background for this is 

 the evaluation of fossil horizons of plants from Miocene to 

 Pleistocene, with which must be associated the fossil animals 

 and especially the vertebrates. Finally, behind all these lies the 

 record of great orogenic movements expressed in the slow rise 

 of mountain blocks, the reduced access of rain-laden winds, and 

 the "spiral" desiccation of the regions within the lee of the 

 rising chains. The word "spiral" is employed to characterize the 

 resultant of progressive desiccation and of recurring climatic 

 cycles of varying intensity, of which those of the Pleistocene 

 were among the most pronounced. 



Relicts Within the Desert Borders 



Of this group the perennial grasses, as dominants of the original 

 climax, are obviously the most significant, while the annuals 

 are of little importance apart from their serai relations. These 

 climax dominants have persisted only as a result of local pro- 

 tection, maintaining themselves in some degree in shallow 

 washes, in sand, or on rocky or northerly slopes, wherever 

 edaphic compensation for climatic drouth was to be found. A 

 number have taken refuge in mountain ranges by virtue of 

 higher rainfall and reduced evaporation. Naturally, the grass 

 genera of southern origin have supplied most of these relicts, 

 though the circumpolar Stipa, Elymus, and Poa are numbered 

 among them. In the following list, the species are arranged 

 under the associations in which they play major roles today, 

 accompanied by those of subclimax or serai character. As is to 

 be expected from their nature and proximity, the desert plains 

 exhibit the largest number. 



