8 A. FRANKLIN SHULL. 



another character. A similar view is advocated by East and 

 Hayes (1912), but these authors specify that heterozygosis is 

 responsible for only part of the vigor of an individual. The 

 remainder they speak of as "inherent natural vigor" and leave it 

 unexplained. 



The other Mendelian explanation is that of Bruce 1 (1910). 

 According to Bruce's view, there is an indefinite number of genes 

 concerned with each element of vigor, for example, size. Each 

 element of vigor depends on the number of genes present, but 

 dominance is complete, or nearly complete, so that MmNn con- 

 tributes as much, or nearly as much, to vigor as does MMNN. 

 All these genes are held to be equipotent so that MmNn con- 

 tributes twice as much to vigor as MM, and just as much as 

 XxYy. Vigor is therefore proportional, on this view, to the 

 number of different kinds of gene present, whether in homozygous 

 or heterozygous condition. Essentially the same explanation 

 the bringing together of dominant characters in the zygote, some 

 of which existed in one parent, others in the other parent was 

 later offered by Keeble and Pellew (1910) to explain greater 

 stature in FI hybrids of certain peas. This would, of course, pro- 

 duce heterozygosis in these characters, but it was the presence 

 of the genes, not their heterozygous condition, to which the 

 authors appealed as an explanation. 



As between these last two Mendelian views, the evidence does 

 not now decide; but if either one is correct, the other can, with 

 sufficient work, be proven incorrect. 



On Shull's view, according to which vigor depends on the 

 number of genes for which the individual is heterozygous, al- 

 though a single inbreeding of a heterozygous line or a single selfing 

 of a heterozygous individual might, in a few cases, produce 

 offspring heterozygous in just as many genes, and therefore just 

 as vigorous, as its parents; yet successive inbreeding or selfing 

 must, by the laws of chance, eventually result in pure homozygous 

 individuals (homozygous for presence or absence it matters not 

 which). Thus in every pure line (which by definition is homo- 

 zygous) the minimum of vigor has been reached, and that mini- 



1 This statement of Professor Bruce's view is taken in part from his paper, in 

 part from correspondence with the author. 



