INBREEDING EXPERIMENTS 135 



constant hereditary factors. Some of the characters 

 which appear after long-continued inbreeding are seldom 

 seen in continually cross-pollinated plants, and never are 

 so many seen in combination. This is because they are 

 recessive in nature and complex in mode of inheritance. 

 The most significant feature about the characters which 

 make their appearance in inbred plants is that none of 

 them can be attributed directly to a loss of a physiological 

 stimulation, although undoubtedly many of them may be 

 modified by the vigor of the plants upon which they are 

 borne. There is no one specific feature common to all 

 inbred strains, but simply a general loss of vigor, a gen- 

 eral reduction in size and productiveness accompanied by 

 specific characters more or less unfavorable to the 

 plant's best development. But these unfavorable char- 

 acters are never all found in one inbred strain, nor is any 

 one of them found in all inbred strains. 



Although no systematic selection has been practiced 

 throughout these inbreeding experiments, a great deal of 

 selection upon many characters has been unavoidable as 

 is the case in any inbreeding experiment. In maize, the 

 difficulties of hand pollination result in the selection of 

 plants whose staminate and pistillate parts are matured 

 synchronously. Any great difference in this respect, par- 

 ticularly towards protandry, renders self-fertilization 

 difficult or impossible as the pollen is viable but a short 

 time. Of course, all plants which are weak, sterile, dis- 

 eased or in any way abnormal, tend to become eliminated 

 wherever these causes reduce the chance of obtaining seed. 

 This unconscious selection becomes more rigid in the later 

 generations of inbreeding as reduction in vigor and pro- 

 ductiveness becomes more pronounced. Again, the small 



