202 



HEREDITY AND SEX 



Darwin's result in more general terms. The hetero- 

 zygous plant is more vigorous than the homozygous 

 plant. Moreover, in this case it is not the presence 

 of the dominant factors that makes greater vigor (for 

 the short-styled plant containing both dominants is 

 less vigorous than the heterozygous), but the presence 

 of two different factors that gives the result. 



FIG. 97. At left of figures there are two strains of pure bred corn and 

 at right the hybrids produced by crossing those two pure strains. (After 

 East.) 



The most thoroughly worked out case of the effects 

 of inbreeding and cross-breeding is that of Indian corn. 

 In recent years East and G. H. Shull have studied on 

 a very large scale and with extreme care the problem 

 in this plant. Their results are entirely in accord on 

 all essential points, and agree with those of Collins, 

 who has also worked with corn. 



East and Shull find that when two strains of corn 



