METHODS OF BREEDING 583 



breeding and a greatly lessened tendency toward the production of de- 

 fectives. There is a measure of truth in this belief for line-breeding, 

 by the mating of animals of slightly wider relationship than those used in 

 inbreeding, permits the introduction and intermingling of hereditary ele- 

 ments from slightly different lines of descent. It is in this that we must 

 seek the explanation for the greater success which line-breeding has had 

 among most practical breeders. That explanation is not far to seek, for if 

 the production of defectives depends upon factors which are distributed 

 in Mendelian fashion, and there is no reason to believe that it does not, 

 then any introduction of diverse hereditary elements is likely to result 

 in the neutralization of the defective elements in both hereditary systems, 

 for only under unusual conditions would such elements be identical 

 in the two systems. Along with this tendency toward decreased pro- 

 duction of defectives, however, there is the ever present possibility of 

 dissipating the elements characteristic of the ideal family type, of ming- 

 ling them with others not so productive or desirable. The tendency is 

 by no means so strong as it is in out-breeding, but it is stronger than 

 in inbreeding. It serves again to emphasize the fact that any system of 

 breeding must be based upon matings of superior individuals. 



Out-breeding. Out-breeding is merely a system of breeding best to 

 best, at the same time avoiding relationship in the animals which are 

 mated. While it may tend to avoid completely the disasters which often 

 attended inbreeding, it is subject to all the defects of the old system of 

 phenotypic selection. Chief among these is its tendency toward lack 

 of uniformity in the herd. The harm which it does, however, depends 

 largely upon the breed in which it is practised. Thus among Shorthorns 

 the extraordinary multiplication of individuals of certain families leaves 

 a wide field for the selection of superior individuals distantly related and 

 of the same type, so that in this breed, a form of out-bresding which is 

 really not out-breeding at all, but a very mild form of line-breeding, 

 may be adopted without much danger. Out-breeding, however, is in a 

 sense a harking-back to methods which have been discarded, and although 

 the new breeder may do well to start his operations by avoiding too close 

 affinities, he should steadily endeavor to master the problem of dealing 

 with consanguineous matings sanely and effectively. 



Other Systems of Breeding. Under the chapter on the utilization 

 of hybrids in animal breeding, we have discussed at some length grading 

 and cross-breeding. The former of these methods of breeding provides 

 a simple and practical method for improving livestock on a large scale, 

 and its practice is to be commended. Grading is not to be contrasted with 

 any of the systems of breeding which have been described, but it may be 

 compared on the one hand with pure breeding and on the other hand with 

 aimless scrub breeding. In grading, any of the systems of breeding 



