247 THE AMERICAN NATURALIST [VOL. XLV 



bred families with the changes produced in the self- 

 fertilized families during the same period, there is a strik- 

 ing contrast, for in the latter case there was great de- 

 crease in height and yield in the first year, a consider- 

 ably less decrease in the second year of self-fertilization, 

 still less in the third year, and so on, and while I have 

 evidence that none of my self -fertilized families has yet 

 reached a state of perfect stability, they are at the present 

 time decreasing in regard to both of these measures of 

 vigor somewhat less rapidly under continued self-fertil- 

 ization than are the families in which self-fertilization 

 has been absolutely precluded. 



Necessary corollaries of the view that the degree of 

 vigor is dependent on the degree of hybridity, or, in other 

 words, that it is dependent roughly upon the number of 

 heterozygous elements present and not upon any injuri- 

 ous effect of in-breeding per se, are (a) that when two 

 plants in the same self-fertilized family, or within the 

 same genotype, however distantly the chosen individuals 

 may be related, are bred together, there shall be no in- 

 crease of vigor over that shown by self-fertilized plants 

 in the same genotype, since no new hereditary element is 

 introduced by such a cross; (b) that first generation 

 .hybrids produced by crossing individuals belonging to 

 two self -fertilized lines, or pure genotypes, will show the 

 highest degree of vigor possible in progenies represent- 

 ing combinations of those two genotypes, because in the 

 first generation every individual will be heterozygous 

 with respect to all of the characters which differentiate 

 the two genotypes to which the chosen parents belong, 

 while in subsequent generations, recombination of these 

 characters will decrease the average number of hetero- 

 zygous genes present in each individual; (c) that crosses 

 between sibs among the first-generation hybrids between 

 two genotypes will yield progenies having the same char- 

 acteristics, the same vigor, and the same degree of hetero- 

 geneity, as will be shown by the progenies of self-fertil- 

 ized plants belonging to the same first-generation family. 



