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, 
z.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 F 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 /’; hybrids when crossed than otHer genotypes. As was 
stated in Chapter XII, D. F, Jones has explained this increased vigor 
Digitized by Microsoft® 
