The Origin of the Desert Climax and Climate 105 



gains exceeds all others, with Aster and Sphaeralcea next. 

 Opnntia, Dalea, and Salvia lead in the number of shrubby 

 species, followed by Eriogoniim, Brickellia, Atriplex, and Echi- 

 nocactus. 



Differentiation of the Colorado and Mohave Floras 



Though both deserts belong to the Larrea-Franseria climax, they 

 exhibit a noteworthy difference in genera and species. This is 

 evidently caused by the San Bernardino Range and its continua- 

 tion to the Colorado River. This serves as a definite physical 

 barrier, though it is not insuperable, and its eastern flank is 

 readily turned. It is also a climatic obstacle to some degree, 

 as was especially true during wet phases, and is in part a causa- 

 tive factor in the not inconsiderable climatic differences between 

 the two regions. Of endemics as defined above, the Mohave con- 

 tains 90, the Colorado 57, and 89 are common to both ; most of 

 the common element may antedate the separation of the deserts, 

 but part of it probably has been caused by climatic pulsations and 

 the resulting migrations. In contrast. Death Valley with two 

 wide entrances has a mere half-dozen strict endemics and only a 

 few more that it shares with the regions round about. 



With respect to the total number of species according to life 

 form, 5 grasses are found in the Mohave that do not occur in the 

 Colorado, 4 show the reverse relation, and 17 are present in both. 

 There are 138 forbs in the Mohave, 69 in the Colorado, and 157 

 are common to both; in the same order, the shrubs number 51, 

 43, and 59. As would be expected, neither the few grasses nor 

 the many forbs produce much visible difference. This is like- 

 wise true of the shrubs and low trees over the general climax 

 area, but these do afford conspicuous contrasts through valleys 

 and washes and along mountain slopes and alluvial fans. In spite 



