ADAPTATION OF PLANTS AND ANIMALS 333 
conditions animal life is characterized by extreme ecologic and 
physiologic adaptation to feed scarcity and to absence of free 
water. Since climatologically most of the arid zones are extremely 
hot at least for a certain season of the year, animals to be adapted 
to such conditions should have a high degree of heat tolerance. 
Although better range management practices might partially 
improve the environment by providing better feed, shade, wind- 
breaks, etc., this usually takes a long time. 
Selection of Animals 
As possibilities of modifying environment conditions are 
limited, the best adapted animals for the use of the limited avail- 
able feed and water reserves must be selected. 
Fluctuation in weather conditions does not materially change 
body temperature of warm-blooded animals. It activates thermo- 
regulatory apparatus to counteract the external change. Accord- 
ing to hereditary capacity, animals differ in ability to dissipate 
body heat. Heat dissipation can be increased through the effect 
of several factors such as increase of surface area, change in color 
of body coverings, vasodilatation of subcutaneous vessels and/or 
increased vaporization through sweating, and acceleration of 
respiration rate. 
Species capacity to counteract any external change rests upon 
a genetic basis, but the environment may stimulate or limit its 
expression (1). 
Wright (13), in discussing general principles governing form 
and function in different classes of domesticated animals, has 
shown that the environmental temperature and the rainfall are 
the most important factors which influence the distribution of 
domesticated animals. He also brought together certain broad 
principles formulated by the earlier zoologists, which were found 
to affect form and function. 
As to direct climatic effects he referred to Bergmann’s rule, 
which implies that animals tend to be larger in size in cold cli- 
mates than do comparable species in warmer regions. This rule 
was amplified by Allen’s rule which states that in general the 
peripheral parts of the same species tend to be enlarged under 
