INBREEDING EXPERIMENTS 119 



Shamel m ; but the conclusion which he drew was new. 

 The universality of this decrease in vigor was to Shull a 

 proof that the injurious effect of inbreeding could not be 

 due to an accumulation of deficiencies possessed by the 

 parents since superior and inferior parents yielded sim- 

 ilar results. Further, Shull noted that this decrease in size 

 and vigor accompanying self-fertilization, instead of 

 proceeding at a steady or even at an increasing rate as 

 might be expected from this older view, actually became 

 less and less in succeeding generations presumably in- 

 dicating an approach to stability. The neatness with 

 which these observations fit a Mendelian interpretation of 

 inbreeding did not escape notice. It was pointed out how 

 one might consider a corn field to be a collection of com- 

 plex hybrids whose elementary components may be 

 separated by self-fertilization through the operation 

 of the fundamental Mendelian laws of segregation 

 and recombination. 



With this working hypothesis the investigations were 

 continued for several years, papers on the subject ap- 

 pearing in 1909, 1910 and 1911. Evidence of the hybrid 

 nature of ordinary commercial maize plants and their de- 

 pendence upon hybridity for their vigor was found in the 

 decided differences in definite, hereditary, morphologi- 

 cal characters exhibited by self-fertilized families having 

 a common origin, but a further proof of the validity of the 

 hypothesis came in testing the conclusions to which the 

 view leads. Obviously crosses between plants of a single 

 family, which by long-continued self-fertilization has be- 

 come homozygous in nearly all its characters, should show 

 little increase in vigor over self-fertilization ; but crosses 



