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THE AMERICAN NATURALIST [Vol. XLV 



We may now consider the behavior of these several 

 measures of physiological vigor in relation to the theory 

 that distinct genotypes of maize are gradually segre- 

 gated from their hybrid combinations, by self-fertiliza- 

 tion, and that the degree of vigor is correlated with the 

 degree of heterozygosis. 



I have kept families selected to given numbers of rows 

 on the ears — one series of families repeatedly self-fertil- 

 ized and another series repeatedly crossed with mixed 

 pollen in such a manner that self-fertilization is pre- 

 cluded by artificial means. It is not practicable to do 

 this crossing with mixed pollen in such a manner as to 

 duplicate the conditions found in an ordinary corn-field 

 for the simple reason that the number of individuals 

 which contribute the pollen must be more greatly re- 

 stricted than is true in the open field. While self-fertil- 

 ization has been entirely prevented, there has been a 

 degree of in-breeding somewhat greater therefore than 

 will occur under non-experimental conditions. This 

 degree of in-breeding is sufficient to slowly eliminate 

 some of the hybrid elements which were originally in my 

 strain of corn and should consequently lead to a gradual 

 deterioration in case my theory of the relation between 

 vigor and hybridity is correct. Asa matter of fact, such 

 deterioration has become apparent in the "cross-bred" 8 

 families, when measured either by height of stalk or 

 yield per acre, though both of these measures show that 

 the deterioration has been slight. Tt is so slight, indeed, 

 that it is very much exceeded by the fluctuations from 

 season to season, and may only be demonstrated by the 

 application of a correction which approximately elimi- 

 nates this seasonal fluctuation. When we compare this 

 continual slight fall in physiological vigor of the cross- 



