Hybrid Vigor 171 



the plant that contains it, and if vigor is increased such 

 things as size and productiveness will also be increased. 

 Those plants, therefore, will be most vigorous which have 

 in combination the greatest number of desirable char- 

 acters, and it is obvious that the plants which have the 

 greatest combination of such characters are hybrids. 



A diagram similar to that which was used to explain 

 heterozygosis may be considered: 



P, P, Ft 



AABBCCDDXAABBCCdd =AABBCCDd = little hybrid vigor 

 AABBCCDDXAAbbccdd =AABbCcDd = still more hybrid 



vigor 



In that explanation it was stated that the first case 

 showed little hybrid vigor because it had only one 

 heterozygous set (Dd), while the other case showed 

 more hybrid vigor because it had three such hetero- 

 zygous sets. Hybrid vigor, therefore, appeared in 

 proportion to the number of heterozygous sets in the 

 hybrid. This diagram served the purpose in explaining 

 heterozygosis, but it will now be discarded because 

 it does not represent the most important result when 

 two races are crossed. The important result is repre- 

 sented in the following diagram: 



F, 

 P^-aabbccDDEEFF JAaBbCcDdEeFf =more hybrid vigor 



Px-AABBccddeeff \ 



P 2 -aabbCCDDeeff J AaBbCcDdeeff = less hybrid vigor 



The thought is that in each of these two cases the 

 hybrid is more vigorous than either parent, not because 

 it contains more heterozygous sets but because it 

 contains more positive, dominant factors. For example, 



