THE PURE LINE AND SELECTION 



from plus parents, granting that plus parents produce 

 plus offspring and making allowance for some regres- 

 sion to type, to shove over the offspring more and more 

 into the plus territory and so to establish a plus race. 



To test this hypothesis, Johannsen selected beans, 

 Phaseolus, with which to experiment, since this group 

 of plants is self-fertilizing, prolific, and easily measur- 

 able. Somewhat to his surprise, the beans refused to 

 shove over as much as expected. That is, big beans 

 did not yield principally big offspring, nor little beans 

 little offspring, according to the expectation, although 

 they each produced offspring that varied in the manner 

 of fluctuating variability around an average unlike the 

 parental type. This gave Johannsen the idea that he 

 was using mixed material, so he next isolated the prog- 

 eny of single beans, which, being self -fertilized for many 

 generations, each constituted unmistakably a single 

 hereditary line. In this way nineteen beans, now fa- 

 mous, became the known ancestors of Johannsen's 

 original nineteen "pure lines," a further study of which 

 has led the way to some of the most brilliant biological 

 discoveries of recent years. 



A pure line has been defined by Johannsen as "the 

 descendants from a single homozygous organism ex- 

 clusively propagating by self-fertilization," and more 

 briefly by Jennings as "all the progeny of a single 

 self-fertilized individual." 



It should be pointed out, however, that this technical 

 idea of a "pure line" is not at all the same as that which 

 the breeder has in mind when he uses the same term. 

 The nearer individuals can be bred to conform to an 



