2 East and Hayes, Heterozygosis in Evolution and in Plant Breeding 



have a more powerful action than others". Granting the truth of 

 this conclusion, every phenomenon observed in inbreeding and 

 crossbreeding is that which is to be expected from the action of 

 Mendel's law of heredity. 



The extraordinary frequency of observations of hybrid vigor 

 by early authors, including as they do the pteridophytes, the 

 gymnosperms and 59 out of 85 families of the angiosperms in 

 which artificial hybrids have been made, is fully discussed in the 

 historical abstract. Darwin's work is shown to agree with the 

 authors' hypothesis, by a fairly detailed analysis of the results 

 published in "Cross and Self Fertilisation in the Animal Kingdom". 



The experiments on the effects of inbreeding included the self- 

 ing of over 30 varieties of maize for from 1 to 7 generations, and 

 the crossing of selfed strains with each other. Selling was always 

 followed by a loss of vigor, using that term to mean a somewhat 

 less rapid cell division or slower growth and a smaller total amount 

 of cell division resulting in smaller plants and plant organs, but 

 not to mean a pathological degeneration. This should be the result 

 if vigor is an accompaniment of heterozygosis through the ten- 

 dency of inbreeding to produce homozygosis, the probable 

 number of homozygotes and any particular class of heterozygotes 

 in any generation r being found by expanding the binomial 

 [1 -]- (2 r - - l)] n where n represents the number of character pairs 

 involved. 



It was concluded that inbreeding a naturally crossbred plant, 

 has these results: 



"1. There is a partial loss of power of development, causing 

 a reduction in the rapidity and amount of cell division. This 

 phenomenon is universal and therefore cannot be related to inher- 

 itance. Further, it continues only to a certain point and is in no 

 sense an actual degeneration. 



2. There is an isolation of sub varieties differing in morphological 

 characters accompanying the loss of vigor. 



3. There is often regression away from instead of toward the 

 mean of the general population. 



4. As these subvarieties become more constant in their char- 

 acters the loss of vigor ceases to be noticeable. 



5. Normal strains 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." 



