THE METHOD OF BIOMETRY 



461 



" fix " a new average. From any group oi individuals many pure 

 lines can be sorted out, and such a mixture of pure lines is 

 known as a " population." 

 Johannsen demonstrated Pure Line 



the relationship of pure 

 lines to population (Fig. 

 241). If one starts with a 

 population one can, by selec- 

 tion, sort out pure Hues 

 having any characteristics 

 that may be desirable. 

 Such pure lines are stable 

 only if mixing with other 

 lines is prevented. Galton 

 was, of course, dealing 

 with populations in his 

 work. Just as each pure 

 line varies around its aver- 

 age, so the population 

 varies around the average 

 of its pure lines. Other 

 workers have tested selec- 

 tion in pure lines. These 

 experiments have been 

 made on size variation in 

 different organisms, chemi- 

 cal content of potatoes and 

 sugar beets, egg-laying ca- 

 pacity in poultry, time of 

 maturity of seeds, distribu- 

 tion of color in coats of 

 mammals, and many other 

 characteristics. In no case 

 has selection been shown 

 to have shifted the average 

 about which a pure line 

 varies. These variations, 

 which are known as " fluc- 

 tuations," are the result of 

 ment and do not influence 



Population 



Fig. 241. — Diagram illustrating the varia- 

 tions in weight of five "pure lines" of 

 beans, and in the "population" formed 

 when all are put together. Beans of the 

 same weight are shown in the test tubes of 

 each vertical hne (c/. Figs. 307 and 308). 



(After Johannsen, from Walter, "Genetics," 

 copyright, 1922, by The Macmillan Co., reprinted 

 by permission.) 



environmental effects during develop- 

 heredity in succeeding generations. 



