HEREDITY AND EVOLUTION 258 
of approximately homozygous strains which differ greatly from 
each other. When, as a result of self-fertilization, the strains 
become homozygous, no further deleterious effects are produced 
by self-fertilization. When the deteriorated homozygous strains 
are crossed, the vigor of the original plants is restored (Fig. 239). 
This result shows that the deleterious effects following self- 
fertilization were the result of reducing the plants to a homo- 
zygous condition. Crossing the deteriorated strains restored the 
heterozygous condition, in which the unfavorable recessive 
characters were again masked by favorable ones. 
Deleterious effects of inbreeding. If a strain contained no un- 
favorable recessive characters, self-fertilization could not cause 
the appearance of any such characters. The valuable strains in 
such self-fertilized plants as wheat and beans appear to be homo- 
zygous, and self-fertilization cannot produce deterioration by 
permitting the appearance of unfavorable recessive characters. 
We may conclude, therefore, that self-fertilization can produce 
no harmful effect in homozygous strains, but that it can do so 
in strains which are heterozygous, as it decreases the number 
of dominant factors and allows unfavorable characters to appear. 
It is apparently for this reason that harmful results frequently 
follow the close mating of domestic animals and of man. These 
are usually complex hybrids and frequently carry unfavorable 
recessive factors which are masked by favorable dominant ones. 
The crossing of two separate individuals, which is characteris- 
tic of cross-pollinated plants and of all higher animals, has a 
tendency to promote a heterozygous condition, and an increase 
in heterozygousness is frequently associated with increased 
vigor. Self-fertilization, or close inbreeding, has a tendency to 
produce a homozygous condition, and so frequently results in 
decreased vigor in species that are normally cross-fertilized and 
heterozygous. 
The value of outbreeding. Outbreeding frequently affords a 
great advantage over inbreeding in that it results in increased 
vigor, and also because it permits the combination of characters 
of different varieties. The vigor of the /, generation of a cross 
