THE PURE LINE 103 



nineteen beans, now famous, became the known 

 ancestors of Johanssen's original nineteen "pure 

 lines," a further study of w^hich has led the way to 

 some of the most brilliant biological discoveries of 

 recent years. 



A pure line has been defined by Johanssen as "the 

 descendants from a single homozygous organism 

 exclusively propagating by self-fertilization," and 

 more briefly by Jennings as "all the progeny of a 

 single self -fertilized individual." 



4. Johanssen's Nineteen Beans 



It was found by Johanssen that the progeny of 

 each of these pure lines of beans varied around its 

 own mean, which was different in each of the nine- 

 teen instances. When, however, extremes from any 

 pure line series were selected and bred from, the prog- 

 eny, instead of showing two thirds inheritance and 

 one third regression with respect to the extremeness 

 of a particular character, as Galton found was true in 

 the case of human stature, showed no inheritance and 

 complete regression away from the extreme condition 

 of the parent bean back to the type for the entire 

 pure line in question. That is, selection icithin a 

 pure line is absolutely without effect in modifying a 

 particular character in the offspring of the line in 

 question. 



This is illustrated in Figure 34 in which the results 

 of selecting for size in the year 1902 is shown for four 

 pure lines only. The average for each pure line is 

 given at the top of its column. When, for example, 



