THE LAWS OF HEREDITY 



tion. And if the modification is neither favorable 

 nor unfavorable to the race, it may chance that 

 two varieties, one showing the modification and 

 the other without it, will continue to flourish in 

 the same environment. 



As a typical instance taken at random you 

 may find gray screech-owls and red ones in the 

 same brood. 



If, however, the modification is favorable, the 

 individuals possessing it will ultimately prevail 

 over those lacking it, and it will be added to the 

 regular equipment of the species. If, contrari- 

 wise, the unmodified character was better adapted 

 to meet the requirements of environment, it will 

 prevail, and the individuals having the modifica- 

 tion will be weeded out. 



A pure "recessive," as we have seen, carries 

 no factors for the antagonistic "dominant" qual- 

 ity. Thus it may come about that certain traits 

 of a given organism are absolutely eliminated 

 from the germ-plasm of a fixed proportion of the 

 descendants of that organism. Thus anomalous 

 though it seem an individual may be of "pure" 

 strain notwithstanding the fact that one of his 

 grandparents was of different strain; and the 

 known laws of heredity pretty clearly explain the 

 anomaly. 



This is the one really new conception in the 

 present-day interpretation of the laws of heredity. 

 The older idea was that any trait once impressed 

 on the organism remains forever as a latent char- 

 acter in the germ-plasm, and is susceptible of 



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