34 THE WANDERINGS OF ANIMALS [CH. 



The making of deserts is still going on in various 

 parts of the world. It is quite likely that many of 

 the Central Asian desert regions were more habit- 

 able even within historic times, and that, when their 

 conditions changed for the worse great migrations of 

 the people were the result. (See Sven Hedin's and 

 Aurel Stein's descriptions of the sand-buried cities 

 of Tibet.) The same applies to the extensive ruins 

 in Arizona, New Mexico and North Mexico, affording 

 incontestable evidence of former thriving populations. 



Now, what applies to people, applies also to other 

 animals. The change of a country from bushland or 

 a fertile prairie into a semi-desert may sweep off all 

 the original inhabitants if that change is quick ; but 

 if it is slow and gradual, many plants and creatures 

 will have a chance of adapting themselves to the new 

 conditions. 



Such conditions are : (1) scarcity of water, 

 especially rain. (2) Abundance of sand and dust. 

 (3) Great variations of temperature, not only with 

 the seasons, but often daily. For instance in the 

 Mojave desert it may be insufferably hot in the 

 daytime, under a broiling clear sky with shade tem- 

 perature far above 100 F. ; shortly before sunrise, 

 owing to the unchecked radiation the water freezes 

 in an open pan. Before sunset you do not know 

 where to hide from the heat : amidst an ocean of 

 glaring sand, with sandspouts swirling over the 



