CAUSE OF HYBRID VIGOR 181 



hereditary resources, so that the combined effect is better 

 than either could produce alone. 



This complementary action can be illustrated by as- 

 suming three linked factors, all of which are essential for 

 best development. In one organism there is AbC ; in an- 

 other there is ac. Dominance, either partial or complete, 

 is characteristic of each. Now in the former interpretation 

 of heterosis, where a physiological stimulation was as- 

 sumed, the heterozygous combination, Aa, for example, 

 evolved developmental energy and differed in that respect 

 from either AA or aa. Moreover, this stimulation was 

 considered to be of a general nature, affecting the organ- 

 ism as a whole, and was thus differentiated from the spe- 

 cific effect which each had as hereditary factors. With 

 linkage, one may consider heterosis to be due to the action 

 of heredity alone : the hybrid union A a is. not superior to 

 either of the homozygous combinations, A A or aa, but is 

 more or less intermediate. This view has a very great 

 theoretical and practical importance, because one may ex- 

 pect to obtain homozygous instead of heterozygous com- 

 binations of the factors which bring about increased vigor 

 in crosses and thus obtain individuals which will have a 

 vigor equal or even superior to the first cross and which 

 will not be affected by future inbreeding. 



Such a happy result was not possible on the stimula- 

 tion hypothesis. This hypothesis was invented to account 

 for the frequency of heterosis, the loss of vigor due to 

 inbreeding, and the extreme rarity of homozygous com- 

 binations approaching those of the first hybrid generation 

 in vigor. With a knowledge of independent Mendelian 

 heredity only it was necessary. But if in our illustration 

 the individual AbC-aBc is vigorous because of heredity 



