178 Outline of Genetics 



carried on through twelve generations the inbred corn 

 cultures that were started by East. In the course of 

 this experiment, a great number of undesirable recessive 

 types have been thrown off. On the other hand, certain 

 of the lines that have been isolated by this inbreeding 

 are quite normal and healthy, though small in stature 

 and yield. A point of homozygosity has been reached 

 where further inbreeding brings no further loss in vigor. 



On the other hand, hybrid \'igor does not arise from 

 the act of crossing per se, but merely through a combina- 

 tion in the hybrid of the maximum number of desirable 

 factors. 



On this point the proof lies in the fact that crossing 

 brings hybrid vigor only when the parents to the cross 

 differ in their germinal constitution. There is plenty of 

 evidence on this point. In Jones's inbreeding experi- 

 ments, a point of homozygosity has been reached where 

 crosses between different individuals of the same line 

 brings absolutely no hybrid vigor. 



In conclusion, attention should be called to the danger 

 of confusing phenomena of hybrid vigor with those of 

 cumulative factors. Both mechanisms may operate on 

 some generalized quantitative character such as size, but 

 the hereditary behavior is distinctly different. Cumula- 

 tive factors bring an Fi which is no more variable than 

 either parent-type and intermediate in size, and later 

 generations which are highly variable. The average size 

 of the whole population, however, is the same for every 

 generation, including the parental and the Fi generations. 

 The hybrid vigor mechanism also brings an Fi no more 

 variable than either parent-type (as would any Mendelian 

 mechanism for that matter), and later generations which 



