SPECIES HYBRIDIZATION 233 
between parents that normal cell division is impossible. When on the 
other hand the differences are not great enough to obstruct normal cell 
division, the degree of stimulation is held to increase directly with the 
amount or kind of heterozygosis present. 
These conclusions, however, do not appear to be very firmly estab- 
lished on the experimental side for by no means the only explanation 
has been offered. When we consider the recent work with Drosophila 
it is clear that many factor differences are known which in addition to 
resulting in some definite character distinction display a rather ill-defined 
effect in decreasing vigor. Thus the factor for white eye color in addi- 
tion to determining white eyes has such an effect on the viability of 
the white-eyed phenotype that in segregation this class never comes 
up to Mendelian expectations. Similarly other factors have definitely a 
weakening effect in vigor, in sterility, and in other characteristics. This 
effect, also, is apparently cumulative, so that in Drosophila strains 
containing many recessive factors almost invariably must be carried 
on in a heterozygous condition on account of their low viability. Here 
very evidently the increased vigor of the heterozygous strains is to be 
attributed to a recombination of the dominant factors normal to the 
wild type, for the heterozygous forms display the characters of the 
normal wild type and a size and vigor approximating that of the homo- 
zygous wild type. Similarly in corn the occurrence of open pollination 
makes it possible for a relatively large number of such factors which 
lower vigor to exist in a variety. These show their effects in marked 
degree only on self-fertilization for such self-fertilization automatically 
results in a rather rapid reduction of strains to a homozygous condition. 
If the number of recessive factors affecting vigor is fairly large then it is 
evident that the mathematical probability of isolating some of them in 
continued self-fertilization is relatively great, but the chances that the 
same ones will be isolated in different pure strains is relatively slight. 
It follows that usually pure strains resulting from continued self-fer- 
tilization will display lessened vigor and productiveness, and that dif- 
ferent strains isolated in this fashion will give hybrids approximating 
the normal condition of fertility and productiveness. The increased or 
even decreased vigor of species hybrids of the wider type appears, there- 
fore, to belong to a distinctly different category for which we are not 
‘yet fully prepared to provide an explanation. To suggest that the 
increased stimulation depends on the specific interactions which occur 
between two different contrasted hereditary systems is, confessedly, 
falling back on a less definite explanation, but one which does not appear 
improbable when viewed in the light of our knowledge of the unexpected 
relations which certain factor combinations display when brought 
together. 
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