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-I'reisian 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-I’reisian 
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 ve 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. 
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