239 THE AMERICAN NATURALIST [VOL. XLV 



several other characters, are capable of accentuation by 

 selection, so that different degrees of these qualities 

 are capable of being made characteristics of particular 

 strains of corn, without there being the least evidence as 

 yet that these last-mentioned qualities bear any relation 

 to the unit-characters with which the student of genetics 

 generally deals. A further point in favor of maize as a 

 subject for the study of genotypes among cross-breeding 

 organisms lies in the fact that its flowers are so arranged 

 that, while self-fertilization is possible, it is naturally 

 almost completely excluded, thus ensuring the same re- 

 lations as are presented by bi-sexual or dioecious plants 

 and animals, while retaining the means of conveniently 

 testing the genotypic nature of each individual by con- 

 trolled self-fertilizations. 



I think I have demonstrated during the last five years 

 that there are many genotypes of Indian corn which, 

 although they can not always be distinguished by defin- 

 able external characteristics, can be proved to be just as 

 certainly and permanently discrete as the types whose 

 distinguishing features can be recognized as Mendelian 

 unit -characters. I shall endeavor to show, in what fol- 

 lows, a portion of the evidence which leads me to this 

 conclusion. 



In 1905 I undertook a rather extensive series of com- 

 parisons between cross-bred and self -fertilized strains of 

 Indian corn for the purpose of discovering the effects of 

 these methods of breeding upon variability, and these 

 investigations have been continued each year since that 

 time. Two phenomena immediately attracted my atten- 

 tion : First, the well-known fact that the children of self- 

 fertilized parents are inferior to those of cross-fertilized 

 parents in height, yield and other characters dependent 

 in any way upon physiological vigor. In every instance 

 this phenomenon was plainly evident in the very first 

 generation after self-fertilization. This decrease in 

 physiological vigor due to self-fertilization has become an 



