44 BULLETIN 905, U. S. DEPARTMENT OF AGKICTJETUEE. 
with equal ability, some will degenerate rapidly, the majority, perhaps, 
will show some unfortunate characteristic, and in only a few will 
the desired type become fixed in association with high vigor and 
fertility. Once obtained, however, the type can' be kept for a long 
time at its high level by close breeding, and can supply prepotent 
males for the improvement of inferior stock, 
CROSSBREEDING FOR THE MARKET. 
As already noted, control over heredity, and hence imiformity, 
depends on the amoimt of close breeding back of the parents and 
not at all on their relationship to each other. There is no doubt, for 
example, that mules can be produced as true to type as any pure 
breed, by using closely bred jacks and mares Experience is, of 
Fig. 10.— A Poland China boar, illustrating the lard type of hog. 
course, necessary to determine just what lines of jacks and mares 
will produce a mule of a particular size and type. It is probable that 
systematic crossing of breeds could be practiced to a larger extent 
than at present in cases in which the offspring are not to be used for 
breeding. An increase in vigor is to be expected. In some cases 
advantage can be taken of the good qualities of two breeds. The 
highest development of the meat type in animals is generally cor- 
related with reduced fertility. By choosing females from a breed 
distinguished for its vigor and fertility, and the male from a breed 
of the most extreme meat type, it is possible to produce progeny of 
the best market type without the losses due to defective fertility. 
Among hogs, Yorkshire or Tamworth sows can be used to advan- 
tage with an extreme type of Poland China or Duroc-Jersey boar. 
Purebred or grade Dorset ewes are useful in crosses where it is desired 
