44 



BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 



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 uniformity, 

 depends on the amount 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 Tam worth 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 



