THE METHOD OF EXPERIMENTAL BREEDING 



471 



are white-eyed and the females are red-eyed. In the F2 generation 

 red-eyed and white-eyed males and females occur in equal num- 

 bers. This does not ssem to substantiate the Mendelian principles, 

 but when the theoretical conceptions are considered in the follow- 

 ing section it will be found to conform. 



The method of experimental breeding, first carefully used by 

 Gregor Mendel, has yielded a vast amount of detailed informa- 



Fio. 249.— Edmund Beecher Wilson, 1922 (on the left). Thomas Hunt Morgan 

 1923 (on the right). (Photos, by courtesy of A. F. Huettner.) 



tion concerning the course of inheritance of specific characteristics 

 by particular individuals through many successive generations. 

 All of this work has verified the Mendelian Law of Heredity. 

 The fundamental point that Mendel brought out was that heredi- 

 tary determiners of characters are not modified by association with 

 other hereditary determiners of characters. This is often spoken 

 of as the '' purity of the germplasm." Because the character 

 determiners are not altered but retain their unmodified indepen- 

 dence, they segregate and recombine in successive generations, 

 producing the characteristic ratios of mono-, di-, and tri-hybridiza- 

 tion experiments. The hereditary units, or determiners, of the 

 characters that are recognizable in the adult individuals are now 



