HEREDITY AND EVOLUTION 253 
Hybrid vigor. The crossing of two races which are not too 
distantly related frequently results in an increased vigor, known 
as hybrid vigor, in the offspring (Fig. 239). In wide crosses 
there is often an increased vigor accompanied by sterility, as in 
the well-known case of the mule; in very wide crosses steril- 
ity may be accompanied by a lessened vigor; while if the ger- 
minal composition of two races is too dissimilar, it is impossible 
to hybridize them. 
Within the range in which crossing results in an increased vigor 
without lessened fertility, the vigor increases with the degree of 



Fia. 239. Hybrid vigor in corn 
Right and left, two inbred strains; center, their F, progeny 
heterozygousness resulting from the cross. Self-fertilization of a 
heterozygous race results in a decrease in the heterozygous con- 
dition ; and so, as might be expected, self-fertilization of a vigor- 
ous hybrid race results in a decrease in vigor which continues 
until a homozygous condition is reached, after which there is no 
further deterioration. 
Hybrid vigor appears to be connected with the fact that the 
hybrid contains the dominant factors of both parents, and that, 
usually, dominant characters are desirable and promote vigor. 
Likewise, the deterioration which follows the self-fertilization 
of vigorous hybrids seems to be connected with a decrease in the 
number of dominant factors in the homozygous races. Moreover, 
when a large number of contrasting factors are involved, the 
