Hybrid Vigor 159 



Shull's (6) conclusions up to the year 19 10 may be 

 summarized as follows. His work was entirely with 

 corn, and the conclusions contained some very significant 

 points. 



1. "The progeny of every self-fertihzed corn plant is 

 of inferior size, vigor, and productiveness, as compared 

 with the progeny of a normally cross-bred plant derived 

 from the same source." In general this conclusion 

 would be admitted by everyone, but it raised one 

 question. It was known that when two races have been 

 inbred for many generations they frequently "run out," 

 gradually losing their vigor. In such a case a cross 

 between the two races tends to restore the original vigor. 

 The remaining question, however, is whether the same 

 thing may be effected by a cross between two inbred 

 races which have not run out but remain in normal vigor. 

 Shull answers that hybrid vigor is exhibited when both 

 parents are above the average condition as well as when 

 they are below it. 



2. Another question which naturally arises is as fol- 

 lows. When these crosses are made it is of course the 

 F, generation that shows the hybrid vigor. If the F, 

 generation is inbred, what is the status of the F^ gen- 

 eration with reference to vigor? Shull answers this 

 question in the following general way. "The decrease 

 in size and vigor which accompanies self-fertilization is 

 greatest in the first generation, and becomes less and less 

 in each succeeding generation, until a condition is reached 

 in which there is (presumably) no more loss of vigor." 

 The facts involved in this statement may be represented 

 in fig. 36. In this figure it can be seen clearly how 

 the great loss of vigor comes immediately after self- 



