242 THE TREND OF THE RACE 
recessive factors, are prevented from becoming manifest owing to 
the cross pollination that usually occurs. When 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 corn 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. 
1 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, some 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 Outbreeding (Phila., 1919) which appeared after 
the above was written. 
