THE PURE LINE AND SELECTION 181 



From this distinction between pure lines and popula- 

 tions it is clear why breeders in selecting for a particu- 

 lar character out of their stock need to keep on select- 

 ing continually in order to maintain a certain standard. 

 As soon as they cease this vigilance, there is a "reversion 

 to type" or, as they say, "the strain runs out," which 

 means that the pure lines become lost in the mixed popu- 

 lation which inevitably results as soon as selective iso- 

 lation of the pure line ceases. 



Such reversion must always be the case in dealing 

 with a population made up of a mixture of pure lines, 

 for only by the isolation of pure lines can the constancy 

 of a character be maintained. When, however, a pure 

 line is once isolated, then all the members of it, large 

 as well as small, are equally efficient in maintaining the 

 pure line in question, regardless of their phenotypical 

 constitutions. 



Conceding that natural history and common usage as 

 well as the older theories of heredity are concerned with 

 phenotypic constitution of organisms, we are now 

 coming to see more clearly than before that heredity 

 must always be a case of similarity in origin, that is, 

 in germinal composition, and that similarity in appear- 

 ance by no means always indicates similarity in origin 

 or true relationship. 



The assumption that similarity in appearance does 

 indicate relationship has been made the foundation of 

 many conclusions in comparative anatomy and phy- 

 logeny, but to the modern student of genetics who places 

 his faith in things as they are, rather than in things as 

 they seem to be, conclusions based upon phenotypical 



