HISTORY AND PROBLEMS 13 
ability to move about. Birds, insects, and large and fleet mammals 
can occupy arid lands if there is a water supply within their range. 
Smaller mammals and reptiles are more dependent on local con- 
ditions and have shown great adjustments to arid conditions. We 
may consider these adjustments under the same classification 
used in discussing the plants. 
1. Drought escaping animals are those that avoid drought areas 
and are not found permanently in or dependent on the arid lands. 
One may include here animals that enter the arid region only when 
an adequate supply of moisture is available either as water or lush 
vegetation, or that can move to the water supply. By far the 
greatest number of drought escaping animals found in arid areas 
belong to the insects and the lower groups of the invertebrates. 
Many of these animals are able to complete their life history or 
the active part of it during the short period when conditions are 
favorable. Here belong most of the insects that abound on the 
summer and winter annuals. These forms pass the drought period 
as drought enduring eggs or larvae and resume their active life 
only at the end of the drought period. One might include all 
animals of the humid zone and in water habitat as drought es- 
caping for they are not found in drought areas. 
2. Drought evading animals show many adjustments even more 
striking than those shown by plants. The ability of animals to 
move has enabled them to burrow into the soil and thus live in an 
environment much more favorable during the day and confine their 
activity to the night and early morning when the conditions are 
not extremely arid. Partly as a result of this ability some of 
the rodents do not need to provide water for temperature control 
except in extreme cases. K. and B. Schmidt-Nielsen in their excel- 
lent summary have listed about a dozen genera of rodents from 
widely separated deserts which do not expend water for heat regu- 
lation (15). This is a significant example of the development of the 
same adjustment in widely separated areas by animals not closely 
related phylogenetically. They show certain gross morphological 
similarities. They all are leaping animals with elongated hind legs 
and small front legs, with reduced number of toes, and they have 
cheek or gular pouches. 
