102 GENETICS 



Average parents tend to produce average children ; 

 minus parents tend to produce minus children ; plus 

 parents tend to produce plus children ; but the progeny 

 of extreme parents, whether plus or minus, inherit 

 the parental peculiarities in a less marked degree than 

 the latter were manifested in the parents themselves. 



3. The Idea of the Pure Line 



It was Galton's law of regression that suggested 

 to the Danish botanist Johanssen a possible means of 

 controlling heredity. In his mind arose the ques- 

 tion whether it would not be possible by continually 

 breeding from plus parents, granting that plus par- 

 ents produce plus offspring and making allowance 

 for some regression to type, to shove over the off- 

 spring more and more into the plus territory and so 

 to establish a plus race. 



To test this hypothesis, Johanssen selected beans, 

 Phaseolus, with which to experiment, since this 

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

 measurable. Somewhat to his surprise, his 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 ex- 

 pectation, although they each produced offspring 

 that varied in the manner of fluctuating variability 

 around an average unlike the parental type. This 

 gave Johanssen the idea that he was using mixed 

 material, so he next isolated the progeny of single 

 beans, which, being self-fertilized, each constituted 

 unmistakably a single hereditary line. In this way 



