Hybrid Vigor 169 



course were hybrid, since the yellow factor was intro- 

 duced by the foreign pollen, while the white endosperm 

 grains must have resulted from "own" pollen and were 

 homozygous. In this way JONES obtained side by side 

 in the same ear endosperms obviously hybrid and en- 

 dosperms obviously homozygous. When he weighed 

 these two types he found that the hybrids exceeded the 

 homozygotes in weight by from 5 to 35 per cent, or an 

 average of 20 per cent. 



He then made the reciprocal cross, using a similar 

 mixture of yellow and white pollen on silks of the yellow 

 race. Of course all the resulting endosperms were 

 yellow, but the hybrids which had the yellow factor only 

 from the female side were distinctly lighter yellow than 

 the homozygotes which had the yellow factor from both 

 male and female sides. Weighing these two types JONES 

 obtained the same results as before, the hybrids exceeding 

 the others in weight by an average of 20 per cent. This 

 is the clearest demonstration of heterozygosis that has 

 yet been given, for the conditions of the experiment 

 were ideally constant. 



The theory of heterozygosis accounting for the 

 phenomenon of hybrid vigor is the one most generally 

 accepted. Its claim is that hybrid vigor appears in 

 proportion to the number of factors in which the parents 

 of the cross differed. 



This claim may be considered briefly. Is hetero- 

 zygosis really an explanation of the phenomenon of 

 hybrid vigor? It seems obvious that it is not. It 

 was known that hybrids are vigorous because they are 

 hybrids. Heterozygosis states that hybrids are vigorous 

 to the degree that their parents differed in hereditary 



