56 ANIMAL LIFE IN DESERTS 



contrasts this with the fact that " many dozens of 

 species are known from places in American deserts 

 no larger than an ordinary farm, in single canons, 

 or on one slope of unique environic combinations." 

 He suggests that the reason for this lies in the 

 comparative newness of the American deserts, which 

 consist of relatively small plains and steep mountains. 

 The environment is varied and the plants vary with 

 it. In the course of ages these deserts will undergo 

 a process of base-levelling, and will perhaps come to 

 resemble the immense desert plains of the Sahara, 

 Mesopotamia, and Transcaspia. By that time the 

 conditions will be so uniform, and have altered so 

 profoundly from those now prevailing in America, 

 that very many species wiU have ceased to exist. 

 The relatively unspecialized types, which have not 

 adapted themselves too closely to their environment, 

 will have spread widely and come to occupy vast 

 areas. 



The plants which are found in deserts belong to 

 special genera and species which are so modified in 

 structxu*e, and in their manner of performing their 

 vital functions, that they can support life under very 

 unfavourable conditions. These modifications are 

 so great and so fundamental that the transfer of a 

 desert plant to a less severe environment, or of a 

 seaboard plant to the Arizona Desert, was always 

 followed by the death of the plant, in certain of 

 MacDougal's experiments ; on the other hand, the 

 transfer of plants to environments which were 

 strange to them, but were not desert, resulted in 

 quite a number of survivals. 



