Hybrid Vigor 163 



tion of data, he gives the summarizing statement that 

 59 out of 85 angiosperm crosses showed a noticeable 

 increase in vigor. East of course did not continue to 

 investigate all of these 85 types, but concentrated upon 

 two representatives chiefly. Corn was selected as rep- 

 resenting species normally cross-fertilized in nature, 

 while tobacco was used as representing those species 

 generally self-fertilized in nature. East's results with 

 corn need not be discussed, for they confirmed in every 

 point Shull's results. One very valuable addition, 

 however, was made; namely, that some crosses resulted 

 in relatively less hybrid vigor than others. For this re- 

 sult East developed a very significant explanation, which 

 has revolutionized ideas on hybrid vigor, and that is that 

 hybrid vigor is proportional to the number of factors in 

 which parents differ. This situation may be visuahzed 

 from the diagram, in coimection with which certain 

 situations may be developed. 



Parents F* 



AABBCCDDXAABBCCdd =AABBCCDd =little hybrid vigor 

 AABBCCDDXAABBccdd =AABBCcDd = more hybrid vigor 

 AABBCCDDXAAbbccdd =AA BbCcDd = still more hybrid 



vigor 

 AABBCCDDXaabbccdd =AaBbCcDd =most hybrid vigor 



It is the Fi generation of course that shows the vigor, 

 but what index can be obtained from the germinal 

 formula of the Fi generation as to the amount of hybrid 

 vigor that it will show ? It is evident that the index is 

 that hybrid vigor is proportional to the number of 

 factors in the Fi generation in the heterozygous con- 

 dition. Thus in the first case shown in the diagram 

 there is only a single heterozygous set (Dd), and the 



