ii6 



Heredity and Eugenies 



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 effect 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 b}^ 

 the increased vigor of 

 growth due to continual 

 natural hybridization 

 with other strains. 





O 



