242 THE TREND OF THE RACE 



recessive factors, are prevented from becoming manifest owing to 

 the cross pollination that usually occurs. WTien self-fertilization 

 takes place these recessive factors have an opportunity to find 

 expression. With continued self-fertilization the strain becomes 

 homozygous for more and more factors, until finally a condition is 

 reached with complete homozygosis in which no further deterio- 

 ration results. In com much more deterioration occurs in some 

 varieties than in others. This is what one would expect according 

 to the Mendelian interpretation, inasmuch as the characters for 

 which the strain comes to be homozygous would vary in different 

 cases. ^ 



Inbreeding in forms containing no recessive factors that make 

 for reduced vigor would, according to this interpretation, produce 

 no ill effects. Inbreeding does not cause defect; it simply brings 

 out latent defect when it occurs in both parents. Whether or 

 not inbreeding is followed by inferior progeny depends, therefore, 

 upon the composition of the germ plasm of the inbred stock. 

 If the stock is good it not only produces no degeneracy, but 

 affords a means of perpetuating valuable qualities, and it becomes 

 especially useful when the desired qualities are recessive. 



The usual Mendelian interpretation of the results of inbreeding 

 and cross breeding which has been briefly outlined affords a 

 plausible explanation, so far as it goes, of the diverse results 

 obtained, and is supported by other lines of evidence which we 

 shall not here attempt to discuss. 



' Keeble and Pellew have attempted to explain the fact that heterozygosis is 

 commonly associated with increased vigor, by assuming that there are more dom- 

 inant factors present in the heterozygous state. The results of heterozygosis are 

 doubtless dependent not merely on the number of different factors present, but 

 upon their quality and the nature of their interactions. If recent investigations 

 throw doubt on the doctrine of seneseence and the theory of rejuvenescence, 

 several problems in regard to inbreeding and cross breeding still remain obscure. 

 From the standpoint of vigor and fertility we can only say that some crosses are 

 good, soma are bad and others indifferent. While Mendel's law may have brought 

 us nearer the explanation of why these diverse results occur, the final solution of 

 the problem must await further research. See also the discussions of this topic 

 in East and Jones' Inbreeding and OuLbrceding (Phila., 1919) which appeared after 

 the above was written. 



