Hybrid Vigor i^y 



Even ]\Iendel's classic pea hybrids supplic-d furllier 

 instances of increase in size resulting from crossing. 

 ''Stems of I foot and 6 feet in length yielded without 

 exception hybrids which varied in length between 6 feet 

 and 7I feet" (see East and Jones 6). 



Among the modern investigators of hybrid vigor. 

 Shull, East, and Jones have contributed much toward 

 an explanation of the phenomenon. 



Skull's (12) conclusions up to the year igio 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-fertilized corn plant 

 is of inferior size, vigor, and productiveness, as com- 

 pared with the progeny of a normally cross-bred plant 

 derived from the same source." In general this con- 

 clusion 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 

 result may be effected by a cross between two inbred 

 races which have not run out, but remain in normal \igor. 

 Shull answers that hvbrid vigor is exhibited when both 

 parents are above the average condition as well as when 

 they are below it. 



2. Another question which naturally ari.^es is as 

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

 Fx generation that shows the hybrid vigor. If the 

 Fi generation is inbred, what is the status of the Fj 

 and later generations with reference to vigor? SiiULL 



