68 HEREDITY AND EVOLUTION IN PLANTS 



59. Pure Line Breeding. Johannsen also originated 

 the "pure line" theory a theory which has done 

 much toward elucidating the problems of selection. He 

 and his followers regard genetic factors as fixed and un- 

 varying. Hence the results obtained in selective breeding 

 of a given variety of maize for high or low oil content, or 

 of a given variety of beans for larger or smaller size of seed, 

 would be interpreted on this theory, as the isolation or 

 separation of pure strains from a "mixed population" 

 or "impure" variety. In practical language, several true 

 breeding varieties of beans, differing in seed size, might be 

 obtained by selection from what appeared to be a "pure" 

 variety with considerable variation in size of seeds. 1 



60. Value of Mendel's Discoveries. The discoveries 

 that, in inheritance, certain characters are dominant over 

 certain others; that a given inheritance (e.g., conditions 

 associated with seed-color, odor, eye-color, stature, musi- 

 cal ability, insanity, tendency to some disease) may be 

 carried and transmitted to offspring by an adult who gives 

 no outward signs of carrying the inheritance; that, under 

 certain conditions of breeding, some characters (the re- 

 cessive ones), whether good or bad, may become perma- 

 nently lost; that dominant characteristics are certain to 

 reappear in some of the offspring all of these truths, 

 learned by the study of a common garden vegetable, will 

 be recognized at once as of enormous importance to the 

 breeders of plants and animals, and above all to man- 

 kind, in connection with our own heredity. They point 

 the way to the explanation of such enigmas as the pro- 

 verbial bad sons of pious preachers, spendthrift children 



1 A detailed discussion of Johannsen's method of "pure line" breeding 

 belongs to more advanced studies. 

 8 



