VEGETATION OF DESERT MOUNTAINS 307 



range the total length of all the constant streams above 8000 feet 

 is only 1.5 miles. The very moist soil along these streams is 

 the only habitat for over 25 species of plants. Making a gen- 

 erous allowance of 5 feet as the average width of the moist soil 

 along the streams, we have a total of 39, GOO square feet, or slightly 

 less than one acre. Even if we hypothecate that most of the dis- 

 persal from mountain to mountain has been by the agency of 

 birds, and that the plants growing in moist situations above 8000 

 feet have been distributed by birds which chiefly frequent that 

 habitat, we have nevertheless a case in which the chances for 

 invasion are rendered extremely poor. This case is scarcely 

 more extreme than others that might be mentioned, and it is 

 cited as bearing on the rate at which invasion by new species 

 has probably been taking place. 



The vegetational differences between the several desert moun- 

 tain ranges are relatively simple, and may be interpreted in 

 terms of the differences in physical conditions which are brought 

 about by differences in altitude, in basal elevation, in character 

 of rock and other features. The floristic differences between 

 them indicate a secular movement of species from the larger 

 and more elevated mountain masses toward the smaller and 

 more isolated ones, with a rapid impoverishment of the flora 

 as the latter are approached. 



