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. 
