124 E E. Clements 



An invaluable corollary of these is found in inference from 

 distribution in space and in time. The latter enables us to close 

 many a gap in the fossil record, and the former permits us to fill 

 in the fragmented area of a dominant or community. This at- 

 tains its most striking success in the form of the transad, and 

 especially in that of the relict, where the community relation 

 finds greater expression (Clements, 1934). Finally, an epitome 

 of all other methods is that of cycles, in which it is recognized 

 that all forces and effects exhibit cycles, not merely of deforma- 

 tion, erosion, and climate, but also of mass migration and evolu- 

 tion in the biome. Thus, evidence of elevation carries with it 

 the necessary assumption of climatic shift as well as accelerated 

 erosion, with inescapable effects in the climax series. Conversely, 

 the displacement of a climax presupposes the chain of physical 

 processes of which it is the final consequence. Out of all these 

 ^ . principles springs the axiom that the present provides the surest 

 route into the past and that reconstruction proceeds most re- 

 liably in that direction. 



The former grassland climax. — The preliminary endeavor to 

 outline the earlier grassland of the desert region was made a 

 decade ago upon the basis of the relict grasses in the Colorado, 

 Mohave, and Death Valley (Clements, 1922, 1923). As indicated 

 previously, almost no perennial grasses survive today on the 

 typical climax level and soil, except transient individuals ascrib- 

 able to wet phases. Even though a product of the desert climate, 

 the shrubby Hilaria rigida becomes dominant only in sand- 

 plains or washes, as is true likewise of the other most xeric 

 species, Triodia pulchella, Stipa speciosa, and Oryzopsis hyme- 

 noides. In the driest part of the desert, where there is a rainfall 

 of about 2 inches, such habitats supply compensation sufficient at 

 least to double the rainfall in effect and to convert the climax of 



