SPECIES HYBRIDIZATION 231 



some species hybrids show marked increases in vegetative vigor, whereas 

 others show just as marked decreases. 



Investigations since Gartner'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 F\ 

 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: 



