THE PURE LINE 117 



obtained, in the direction of the selection simply 

 by isolating out a pure line of one type. Thus 

 beans chosen from the extreme left-hand test tube 

 in the population cited would belong only to pure 

 line 2, while those taken from the extreme right- 

 hand test tube could belong only to pure line 3. 



Galton's "law of regression," namely, that minus 

 parents give minus offspring and plus parents plus 

 offspring, with a tendency to reversion from genera- 

 tion to generation, depends simply upon a partial 

 but not complete isolation of pure lines out of a 

 population. 



From this distinction between pure lines and popu- 

 lations it is clear why breeders in selecting for a 

 particular character out of their stock need to keep 

 on selecting continually in order to maintain a cer- 

 tain 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 population which inevi- 

 tably results as soon as selective isolation 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 mem- 

 bers of it, large as well as small, are equally efficient 

 in maintaining the pure line in question, regardless 

 of their phenotypical constitutions. 



