516 GENETICS IN RELATION TO AGRICULTURE 
In this account species hybrids will be treated according to whether they 
themselves are of some value or are possible sources of breed foundation 
stock. 
The hybrids which are themselves of some practical value, so far as 
utilization in this country goes, are confined to the genus Equus and its 
allied genera, and the typical example of this class of hybrids is the mule. 
The mule has proved a very satisfactory draft animal, and at the present 
time it is largely used, particularly in warmer climates, for that purpose 
(Fig. 197). 
Fig. 197.—A choice draft mule. Height 18-2 hands, weight 1900 pounds. An unusually 
heavy mule of excellent type. (After Obrecht.) 
The mule is a hybrid between the mare and the jack; the reciprocal 
cross of the jennet and stallion is called a hinny (Fig. 198). The cross is 
an instance of strict species hybridization and both sexes of the hybrid 
are sterile. The importance of the mule breeding industry in the United 
States may be judged from the fact that according to statistics of 1915 the 
estimated number of mules was about 4,500,000 with a farm value exceed- 
ing $500,000,000. The average value of mules per head according to U.S. 
Department of Agriculture estimates was $112.36, while that of horses 
was placed at $103.33. These latter figures may be taken roughly as an 
indication of the relative esteem in which horses and mules are held for 
draft purposes, which is practically the only purpose for which mules are 
employed. 
Although the mule breeding industry is of such great magnitude, 
the relative merits of the horse and mule even for draft purposes are still 
in dispute. It is not the purpose of this account to enter into the debate, 
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