Xo. 5G2] 



THE PROBLEM OF INBREEDING 



605 



But is the thing measured worth measuring? Does any 

 significance attach to knowing how much an animal is 

 inbred in the kinship sense? I think there is no doubt 

 that every breeder of the larger domestic animals would 

 answer this question in the affirmative. The question, 

 however, demands careful consideration, because of the 

 suggestion recently advanced that the effect of inbreed- 

 ing, if there be any, depends entirely upon the nature of 

 the combinations of hereditary units (genes) formed. 

 Thus, for example, we have the suggestion of Bruce 11 to 

 the effect that the vigor of the individual increases as the 

 number of dominant elements in its hereditary make-up 

 increases, while an increase of recessive elements con- 

 notes a decrease in physiological vigor. A thorough and 

 far-reaching discussion of the problem of the relation 

 between the gametic constitution of the individual and its 

 physiological characteristics is to be found in the very 

 valuable paper by East and Hayes 12 on heterozygosis. 

 The most significant conclusions of that paper in the 

 present connection are these: 13 



Stimulus to development is greater when certain, or possibly all, 

 characters are in the heterozygous condition than when they are in a 

 homozygous condition. 



This stimulus to development is cumulative up to a limiting point and 

 varies directly with the number of heterozygous factors in the organism, 

 although it is recognized that some of the factors may have a more 

 powerful action than others. 



These conclusions appear to be supported beyond any 

 chance of doubt or question, for certain characters of 

 plants subjected to self-fertilization, by the experimental 

 evidence set forth in the paper. Inbreeding tends, accord- 

 ing to these authors, to isolate homozygous strains 

 "which lack the physiological vigor due to heterozygosity. 

 Decrease in vigor due to inbreeding lessens with decrease 

 of heterozygosity and vanishes with the isolation of a 

 completely homozygous strain. Moreover, these homo- 



