CAUSE OF HYBRID VIGOR 



175 



ured. It attains this same development, however, by the 

 operation of a different set of factors distributed in the 

 three chromosomes and numbered 2, 4, 6 ; 8, 10, 12 ; and 

 14, 16, 18 in the diagram. Both individuals are homo- 



X: 6 



Y:8 



Xx Y: 12 



Fia. 36. To show how factors contributed by each parent may enable the first generation 

 of a cross to obtain a greater development than either parent. 



zygous ; i.e., the allelomorphic pairs are composed of like 

 elements. It is also assumed that all these nine factors 

 are as fully effective in the haploid as in the diploid con- 

 dition ; in other words, they show perfect dominance over 

 their absence. It will be seen from the diagram that when 

 these individuals are crossed together the progeny de- 



