326 GENETICS IN RELATION TO AGRICULTURE 



the rapidity and amount of cell division. This phenomenon continues 

 only to a certain point and is in no sense an actual degeneration. 



2. There is an isolation of biotypes differing in morphological char- 

 acters accompanying the loss of vigor. 



3. The hereditary differences between these biotypes is often indi- 

 cated by regression away from instead of toward the mean of the general 

 population. 



4. As these biotypes become more constant in their characters the 

 loss of vigor ceases to be noticeable. 



5. Normal biotypes with such hereditary characters that they may be 

 called degenerate strains are sometimes, though rarely, isolated. 



6. It is possible that pure strains may be isolated that are so lacking 

 in vigor that the mechanism of cell division does not properly perform 

 its function, and abnormalities are thereby produced. 



Thus we know that any commercial variety of corn is a mixture of 

 different genotypes and that inbreeding tends to isolate pure genotypes, 

 i.e., inbred strains tend to become homozygous. Thus it is evident that 

 the cross-bred progeny of two different inbred strains will be heterozy- 

 gous for many factors. That cross-bred maize frequently displays greater 

 vigor than either parent was first demonstrated by Beal of Michigan in 

 1878. But it was not until Shull and East demonstrated the existence 

 of genotypes in maize that the genetic significance of this phenomenon 

 became evident. The actual cause of the increased vigor has been ex- 

 plained in various ways. Both Shull and East held that decrease in vigor 

 in inbred strains is due to reduction in the number of heterozygous 

 factor combinations and that increase in vigor in Fi hybrids is the result 

 of increase in the number of such combinations. The general occurrence 

 of decrease in vigor upon inbreeding naturally cross-bred species and of 

 increase in vigor upon crossing closely related forms led them to conclude 

 that heterozygosis is the cause of increased physiological vigor in Fi 

 hybrids. Other explanations of this phenomenon have been offered, 

 one of which was that of Keeble and Pellew, to the effect that it 

 "may be due to the meeting in the zygote of dominant growth factors 

 of more than one allelomorphic pair, one (or more) provided by the 

 gametes of one parent, the other (or others) by the gametes of the other 

 parent." East and Hayes reject this hypothesis on the grounds that this 

 increase in vigor "is too universal a phenomenon among crosses to have 

 any such explanation. Furthermore, such interpretation would not 

 fitly explain the fact that all maize varieties lose vigor when inbred." 

 But there is good evidence that all maize varieties do not lose vigor to 

 the same extent when inbred and that certain genotypes produce much 

 more vigorous FI hybrids when crossed than other genotypes. As was 

 stated in Chapter XII, D. F. Jones has explained this increased vigor 



