SPECIES HYBRIDIZATION 231 
some species hybrids show marked increases in vegetative vigor, whereas 
others show just as marked decreases. 
Investigations since Giirtner’s time have simply extended observa- 
tions on the comparative vigor of parents and hybrids in species hybrids 
as well as in the less violent variety hybrids. Thus Focke who inves- 
tigated large numbers of species hybrids found many that were abnor- 
mally weak, but these usually represented rather wide crosses. Crosses 
between more closely related species, however, generally showed an in- 
creased vegetative vigor. The increased vegetative vigor, he regards as 
merely an extension of the same condition which Darwin had investigated 
in variety crosses, namely that crossbreeding is advantageous from the 
standpoint of the general growth of the forms involved. The idea that 
sterility may be the cause of this increased vigor is refuted on the one 
hand by the fact that some of the most vigorous species hybrids are also 
highly fertile, and on the other hand by the fact that most of the weak 
hybrid forms are nearly or quite sterile. 
East and Hayes have attempted to offer an explanation for these 
phenomena on the basis of heterozygosis. They have reached this con- 
clusion from extensive investigations of the effect of self-fertilization in 
maize and of cross-fertilization in tobacco. In corn they have found, 
as we shall describe more in detail later, that continued self-fertilization 
results in the isolation of races which are very uniform as respects their 
character development, but which almost constantly show considerably 
decreased vigor of growth. This decrease in vigor is most rapid in the 
first generations and becomes less rapid as the races become more con- 
stant in their characters. Since the approach to constancy in char- 
acters may be regarded as evidence of approach to a homozygous condi- 
tion in this a normally highly heterozygous species, East and Hayes argue 
that the normal vigor of maize is largely an expression of its heterozygous 
condition and that the decrease in vigor is a consequence of reduction to 
a homozygous condition. This conclusion is in part confirmed by the 
evidence from crossing such homozygous strains of maize. The Fy 
of such crosses usually exhibits an immediate return to the vigor of the 
population from which the strains were isolated. However, it is not 
entirely clear why this behavior cannot be ascribed to the isolation of 
races possessing fewer dominant factors than most of the plants in the 
original population. When such races are crossed the original set of 
dominant factors would be reunited, and in consequence the normal 
vigor of the original population would be exhibited. 
Since the foregoing was written D. F. Jones has published an explana- 
tion of increased vegetative vigor of hybrids or “heterosis,” as it has 
been termed by Shull, which he has summarized as follows: 
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