No. 637] FIRST GENERATION HYBRIDS 117 



tions, showed that if dominance is correlated with vigor, 

 crossing would produce '^a mean vigor greater than the 

 collective mean vigor of the breeds." Only a few days 

 later appeared the paper of Keeble and Pellew (1910) 

 with a concrete illustration and the suggestion that the 

 "greater height and vigor which Fi generations com- 

 monly exhibit may be due to the meeting in the zygote of 

 dominant growth factors of more than one allelomorphic 

 pair." It appears to me unfortunate that in elaborating 

 this theory, Jones has retained the form of statement 

 used by Keeble and Pellew, and describes the phenome- 

 non of heterosis as due to the accumulation of dominant 

 growth factors instead of placing the emphasis on the 

 suppression of deleterious recessive characters. It may 

 seem that the difference is only verbal since a dominant 

 growth factor presupposes a recessive allelomorph. 

 There is, however, a difference in the point of view, espe- 

 cially if the evolutionary significance of the phenomenon 

 is considered. In speaking of dominant growth factors 

 we seem to assume as a starting point, strains of low 

 vigor subsequently improved by the appearance of domi- 

 nant mutations. It is known that advantageous varia- 

 tions, whether dominant or recessive, are of extremely 

 rare occurrence and while evolutionary progress as a 

 whole must be dependent on such rare progressive changes 

 the effect of these is negligible as a factor explaining 

 heterosis. 



Heterosis in Maize 



In all varieties of maize there are to be found plants 

 that are abnormal in some particular and these abnormal 

 individuals are almost always deficient in vigor and yield. 

 Wlien varieties are self -pollinated for a series of years at 

 least a large part of the degeneration that follows is 

 caused by these abnormalities. 



The bearing of abnormalities on heterosis will be more 

 easily understood if the behavior of two or three ex- 

 amples is described. 



