334 THE FUTURE OF ARID LANDS 
conditions of high temperature with a substantial increase in sur- 
face area and heat dissipation. Wright introduced what he called 
Wilson’s rule. In describing the coats of mammals, Wilson dis- 
tinguished between the bristly external hair covering and the soft 
woolly fibers which are commonly hidden beneath these coarser 
hairs. He stressed the fact that in cold countries animals tend 
toward the woolly coat, whereas in warm regions the tendency 1s 
toward more development of hair and disappearance of wool. 
Animals inhabiting warm and humid regions show greater 
melanin pigmentation than the same species in cooler and drier 
regions. In arid desert regions the skin is characterized by the 
accumulation of yellow and reddish brown phaeomelanin pigment 
(Gloger’s rule). Wright considers that rainfall, through its effect 
on quality and seasonal growth of vegetation, acts indirectly upon 
size, conformation, and density of the animal population as well 
as on their grazing habits and structural formation. 
Knowledge of the fundamental physiology of farm animals re- 
garding heat tolerance and capability for withstanding unfavor- 
able conditions is not yet adequate to set up scales for comparing 
different breed adaptability (5). 
Rhoad (12) developed the Iberia heat tolerance test for calculating 
a heat tolerance coefficient for cattle. This simplified device, being 
a field test, has its advantages. Its results however, are liable to 
be affected by many external factors. Comparisons of tropical and 
half-tropical crossbreeds with the pure temperate breeds on the 
basis of this test show a high heat tolerance with concentration of 
the tropical blood. The fact that such coefficients do not evaluate 
other physiological factors in relation to productivity has always 
to be borne in mind. 
The Arabian camel (Camelus dromedarius) is a distinguished 
illustration of the effect of environmental conditions on a species 
(9). Its size and length of limbs and neck coincide with Berg- 
mann’s and Allen’s rules. Its hairy coat and yellowish brown 
coloration is a good proof for Wilson’s and Gloger’s rules. The 
flexible sole of the foot, the callous horny pads that bear most of 
the animal’s weight during the sitting position, the water sacs in 
the rumen, the cloven upper lip, the mouth which is almost un- 
