HYBRIDIZATION IN ANIMAL BREEDING 515 



In certain cases, however, crossbreeding has been used for the avowed 

 purpose of employing a given breed for a double purpose. An example 

 of this is the practice in some herds of grade Holstein-Freisian cattle of 

 using Aberdeen-Angus bulls in order to obtain calves which may be 

 fattened for the baby-beef market. In this case the Holstein-Freisian 

 calves are themselves not unsuited to the purpose, and the Aberdeen- 

 Angus cross simply gives them increased excellence in quality and early 

 maturity. The use of the Dorset ram on Merino ewes for the production 

 of grade ewes for hot-house lamb production is another instance of cross- 

 breeding for a definite purpose. There is room for purposeful cross- 

 breeding such as this, but for unsystematic crossbreeding without definite 

 purpose, condemnation cannot be too severe. 



The reasons for the condemnation of crossbreeding as a systematic 

 breeding program are not far to seek. The threatened extinction of 

 the Aberdeen-Angus breed in Scotland in the early 19th century is 

 only one phase of the problem. The first and primary reason for such 

 disfavor, however, is the neglect of pure-bred stock which follows such a 

 practice. The success of crossbreeding depends largely upon the excel- 

 lence of the breeding stock which is utilized, but it is probably true that 

 minor defects in the foundation stock are often totally obliterated by in- 

 creased vigor and excellence in the cross-bred progeny. The temptation 

 to lower the high standards of excellence in the pure-bred stock which is 

 being used in crossing and to retain all animals which give any promise 

 whatever of producing good cross-bred offspring is, therefore, very strong. 

 Moreover, for continuation of the practice it is necessary to maintain 

 two lines of breeding, one to supply the pure-bred foundation stock for 

 crossing and the other to supply the cross-bred animals themselves. 

 When only a small part of the herd is set aside for continuing the pure- 

 bred lines, the number of individuals from which selection may be made 

 is so much smaller that the chances of producing superior individuals is 

 considerably less. It is also extremely difficult to enforce the rule that 

 the cross-bred stock must not be used for breeding purposes. The 

 tendency to breed from particularly excellent individuals which are 

 sometimes obtained by crossbreeding is very great ; but, if yielded to it 

 will surely result in loss of the uniformity of type and excellence which 

 characterized the original cross-bred animals, a natural consequence of 

 the operation of the Mendelian law of segregation and recombination. 

 These are facts the full gravity of which must be realized before embark- 

 ing on crossbreeding operations. 



Species Hybridization Among Domestic Animals. Species hybrids 

 among domestic animals are by no means uncommon, although in large 

 part they have been regarded as curiosities rather than as foundation 

 sources of breed improvement, or as themselves of practical value. 



