134 MOSTLY MAMMALS 



It is not easy to obtain information as to the physical 

 features of the Arizona Desert as compared with the 

 Sahara, and especially as to the amount of sand it contains 

 area for area ; but, judging from the comparatively slight 

 modifications which its mammals appear to have under- 

 gone as compared with those of the more humid regions 

 adjacent, it seems not unlikely that these deserts are of 

 more modern origin than the Sahara and the Gobi. 



Whether or no it be true in this particular case, it may 

 be laid down as a general rule that the greater the amount 

 of sand to be found in a desert, and the greater the 

 difference between the animals inhabiting that desert from 

 those dwelling in the adjacent districts, the greater will be 

 the antiquity of the desert itself. In the case of a desert 

 forming a complete barrier across a continent, like the 

 Sahara, if the animals on one side are quite different from 

 those on the other, its antiquity will be conclusively 

 demonstrated. If, on the other hand, they are more alike, 

 the age of the desert will be proportionately less. 



