ADAPTATION OF PLANTS AND ANIMALS 335 
affected by injury of thorny bushes or trees, as well as many struc- 
tural modifications, all operate to make the camel best suited to 
arid conditions and food scarcity. The camel’s hump was devel- 
oped to store a food reserve in the form of fat. Babcock (2) has 
pointed out that fat metabolism produces not only energy but also 
free water, which exceeds in volume the volume of the original fat. 
This has a profound significance in metabolism, more especially 
in the camel, which has to exist for long periods in waterless re- 
gions. 
Similar combinations of exo- and endo-adaptations could be 
traced in other domesticated animals that were developed under 
arid conditions such as the Arabian horse, certain strains of fat- 
tailed sheep and zebu cattle. 
Conditions in the deserts and semi-deserts usually result in a 
certain amount of topographic isolation of animals. This provides 
enormous potential variability through occasional crosses between 
isolated populations. Natural selection under such conditions as 
Allee (1) has stated ‘‘will act as a sieve that eliminates the unfit 
and allows the fit to pass through. In another sense, natural selec- 
tion is a causative force that determines the pattern of hereditary 
units through selective sorting after recombination.” 
It is most probable that the Arabian horse, the horse of the 
desert with its noble qualities of endurance and speed, as well as 
its specific anatomical constitution, has been developed through 
such a pattern of inheritance. 
Breeding of Animals 
Phillips (11), in discussing methods of breeding that can be 
applied in underdeveloped areas, recommended the use of the fol- 
lowing methods: (1) selection within the native types, (2) grading 
up with already improved types or breeds from other countries, 
(3) development of new types out of animals that are graded up 
only a part of the way to the improved type. 
A detailed discussion of these methods has appeared in a recent 
Food and Agricultural Organization publication, showing the use 
and limitations of each method (11). 
Compared with artificial selection, natural selection is very 
