ii6 



Heredity and Eugenics 



If, for example, a commercial variety of maize is self- 

 fertilized for a number of 

 generations, the plants 

 tend to become homo- 

 zygous, to lose the vigor 

 due to heterozygosity 

 and to become smaller 

 and less productive. 

 This loss of vigor was 

 for years interpreted as 

 the direct efTect of self- 

 fertilization. Now we 

 know that it is simply 

 the withdrawing of pure 

 strains from hybrid com- 

 binations. In a few 

 generations the strains 

 become practically pure 

 and the loss of vigor 

 ceases. Some strains of 

 maize still yield remark- 

 ably well after many 

 generations of self- 

 fertilization. Other 

 strains are so poor that 

 they can scarcely be 

 kept alive. In fact it is 

 evident that they are 

 kept alive merely by 

 the increased vigor of 

 growth due to continual 

 natural hybridization 

 with other strains. 



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