HISTORY AND PROBLEMS 15 
is not lower than 10%. They do not store water when abundantly 
supplied and do not lose weight on dry feed. They do not lose 
water to control temperature under normal conditions. When fed 
only on dry protein, dehydration resulted in a reduction to 
67.2% of normal weight, or the animals lost nearly a third of their 
weight and died shortly afterward. But they are not forced to 
endure desiccation when feeding on a desert dry food supply of 
grasses and seeds. The kangaroo rat, pocket mice, and jarobas 
can maintain balance on a dry diet and thus resist dehydration. 
Babcock in his important work (1) on metabolic water in I912 
discussed its role in insects that feed on dry plant and animal 
material, for many are able to oxidize the hydrogen of their foods 
to obtain enough water for their normal life processes. 
4. In the field of drought enduring we have the least physiological 
details, and yet we know that many rodents that do not de- 
velop metabolic water pass into an inactive state called estivation 
in summer from which they can recover when the hot dry period 
ends. There are many invertebrates that can be desiccated 
and recover quickly when water is again available. Many cases 
could be cited. Among the mammals the round-tailed ground 
squirrels and pocket gophers estivate during the hot dry periods, 
but apparently they do not suffer a very marked desiccation. They 
reduce the rate of respiration and heart beat and assume the 
temperature of the air around them and are inactive. This summer 
sleep or estivation still offers a most promising field of physio- 
logical investigation. 
Primitive Man 
Primitive man made remarkable progress in the use of the arid 
lands. In the field of animal husbandry he domesticated all of our 
animals with the possible exception of the turkey (13). His use 
of arid lands was made possible partly by grazing these lands with 
cattle, camels, goats, sheep, asses, and horses. Sauer traced the 
origins and dispersals of not only the domesticated animals but 
also many of the cultivated crops. Excepting the grazing animals 
and seed crops, most of the crops and domestic animals have de- 
veloped in the humid rather than the arid zone. However, the 
