336 THE FUTURE OF ARID LANDS 
slow in establishing new combinations of characteristics through 
mutation and recombination of genes. Haldane has estimated 
that it takes 10,000 years for a favorable combination of fifteen 
genes to become established, whereas under artificial selection this 
could be accomplished in relatively few years. This emphasizes 
the importance of artificial selection for improvement of animals 
and plants in arid zones. 
There is, however, a fourth method of improvement that can 
be used in arid zones, that 1s the z#troduction of preadapted species. 
Certain strains of goats, fat-tailed sheep, and zebu cattle that 
have originated in arid environments are likely to be successfully 
adjusted for introduction to similar arid regions. 
In planning for such trials, Wright (13) recommends two meth- 
ods for comparing homoclimatic areas, Le., climatographs and 
hitherographs. The former is based on the sdaiion of air tempera- 
ture to relative humidity, and the latter, on the relation of tem- 
perature to rainfall. 
Examples of preadaptation are more prominent in the plant 
kingdom. Crested wheatgrass (4gropyron cristatum) is a native 
of the wide plains of Siberia and Central Asia, yet its merits in 
reseeding depleted areas was first proved in the northern Great 
Plains of the United States and not in the eastern hemisphere 
where it originated. 
Harding grass (Phalaris tuberosa) and subterranean clover 
(Trifolium subterraneum), both Mediterranean species, are now 
considered the best adapted to many areas in southern Australia. 
Recently, Kochia indica (Wight), a native of India and Pakis- 
tan, has been successfully established in the coastal belt of the 
Egyptian western desert, under less than 6 inches annual rainfall. 
The species, although originally a salt soil plant, proved to have 
enough drought resistancy to withstand the semi-arid conditions 
of the new habitat. Although useless in its original country, its 
high palatability and easy establishment made the species of some 
use under Egyptian conditions. 
In comparing improved breeds and local breeds, Hagedoorn (7) 
expressed the fallacy of looking down upon local “unimproved” 
breeds of domestic animals and plants just because each individual 
