388 HEREDITY AND ENVIRONMENT 



which is nearly all-black and another nearly 

 all-white. He maintains that not only inheri- 

 ted characters, but also their factors are vari- 

 able and that by means of selection of plus or 

 minus variations the mode of a character may 

 be shifted in one direction or the other. This is 

 the old view which flourished before a distinc- 

 tion was made between fluctuations and muta- 

 tions. Breeders have long been acquainted 

 with similar results of selection from a mixed 

 population containing different hereditary 

 lines; however Castle has been careful to em- 

 ploy as pure a race of rats and of rabbits as he 

 could obtain, but it is not possible to get as pure 

 a race of these animals or of any organisms in 

 which cross fertilization occurs as in the case of 

 self fertilizing plants such as beans. Johann- 

 sen defines a "pure line" as "all individuals 

 which are derived from a single, abso- 

 lutely self-fertilizing, homozygous individual." 

 Within such a pure line he maintains that se- 

 lection is unable to change any character. 



Adherents of the "pure line" hypothesis ex- 

 plain Castle's results in one of two ways: either 

 his material may not have been genetically 



