86 HEREDITY 



fulness of vigour appears to vary considerably in different 

 species, and in the same species under different con- 

 ditions. The earlier breeders of Shorthorns, "O'orking, 

 as they were, A\'ith foundation stock of very mixed 

 character, were accustomed to mate together very closely 

 related individuals — sire and daughter, half-brother and 

 sister, and the like, generation after generation. On 

 the other hand, it is safe to say that few turf winners 

 have been produced by matings of animals more closely 

 related than half-cousins. In any case, continued close 

 " inbreeding " invariably leads to greatly weakened con- 

 stitution, and often to partial or complete sterihty. 



We may assume with some degree of certainty that 

 this is due to the reduction of many characters to the 

 pure condition — to the formation of pure lines, in fact — 

 for the loss of vigour occurs in the same manner as in 

 plants. Whether these " pure hnes " can actually exist 

 we do not know, but in some animals at least, sterility 

 seems to result before this stage is reached. 



It is evident that where a certain amount of inbreed- 

 ing is practised, there will be less opportunity of varia- 

 tion within the strain. For the number of hybrid 

 characters is reduced, and the type is thus partially 

 fixed. Thus a certain amount of inbreeding introduces 

 an element of certainty into the work of the breeder. 

 The mating of almost entirely unrelated individuals 

 giv&g, on the average, the most vigorous type of off- 

 spring. But it is evident that if this method of breed- 

 ing is continued, the resulting offspring will tend to be 

 very in-egular, for there are chances of widely different 

 combinations of factors. 



" Breed ' in ' to fix type ; breed * out ' to secure vigour ; 

 in general, compromise " — this may be said to be a 

 summing uj) of the experience of animal breeders. 

 Theory may be said to have caught up with practice by 

 suggesting reasons for these rules, but it can offer httle 

 in addition. One might perhaps prophesy an extension 

 of the present practice of systematic crossing in order 

 to secure both vigour and uniformity, but there are 

 difficulties in the way of such systems of breeding. 



