Hybrid Vigor 173 



be more reasonable and accurate to suppose that certain 

 factors induce more vigor than others. East recognizes 

 this in the following statement: "This stimulus to 

 development is cumulative up to a limiting point, and 

 varies directly with the number of heterozygous factors 

 in the organism, although some factors may have 

 more powerful action than others." 



It is obvious that the suggestion made above is that 

 of a real explanation of hybrid vigor and not merely a 

 restatement. It is rather an obvious explanation that 

 has probably occurred to a number of geneticists. 

 Keeble and Pellew (5) suggested it in 19 10, and 

 since that time it has been somewhat discussed in the 

 literature, being referred to as "the hj^othesis of 

 dominance (accounting for hybrid vigor)." At first 

 statement the theory seems sound, but actually it does 

 not fit the facts. The two chief objections to this 

 theory of dominance may be found in the publications 

 of Shull, Emerson, and East (2). 



1. If hybrid vigor were due to dominance it would be 

 possible in generations subsequent to the F^ to recom- 

 bine in one race all of the dominant factors. Thus 

 there could be isolated a race that was " 100 per cent 

 vigorous," and since it would be homozygous its vigor 

 would not be lost by inbreeding. Actually, though, 

 hybrid vigor cannot be fixed in this way; "all maize 

 varieties lose vigor when inbred." 



2. Experience assures us that the distribution of 

 individuals in the F2 generation with reference to hybrid 

 vigor is represented graphically by a symmetrical curve, 

 similar to the normal probabilities curve; the class 

 containing the greatest number of individuals is that 



