CHAPTER XIII 

 PURE LINES 



For half a century succeeding Darwin, it was assumed that by 

 selecting a certain type of individual for propagation, the species or 

 variety would be continually transformed in the direction of the selec- 

 tion. Such a conception was a natural result of the widespread 

 acceptance of Darwin's theory of the method of evolution and later 

 of Galton's "law of inheritance" as applied to selection. Experience 

 seemed to bear out this idea also, inasmuch as continual selection of the 

 best plants for seed and the best animals for mating was found to be 

 profitable. But it was not until Johannsen decided to test the power of 

 selection by keeping the pedigrees of individual plants and their descend- 

 ants that the truth concerning the composition of varieties of cultivated 

 plants became known. Heterogeneity within single botanical species 

 had already been discovered, but that horticultural varieties were also 

 heterogeneous but with respect to less easily distinguishable characters 

 had not been realized. Definite knowledge concerning the composition 

 of horticultural varieties threw light on the problem of selection by ex- 

 plaining why continuous selection within a variety is necessary in some 

 crops while it has little or no effect in the case of certain other crops. 

 This discovery was of tremendous significance to genetics, particularly 

 to breeding. For this reason the following account of Johannsen's 

 classical experiments is based directly upon his own presentation of the 

 matter. 



Discovery of Pure Lines. Johannsen chose a certain brown variety 

 of the common garden bean (Phaseolits vulgaris nana) known as the 

 Princess bean. In 1901 he harvested 287 plants which had grown from 

 selected seeds of very different sizes and of known weights. The har- 

 vested beans from each plant were weighed separately. They were then 

 divided into classes with an interval of 10 eg., the class center values 

 ranging from 30 to 80 eg. Next he determined the mean weights of all 

 the beans from the plants grown from mother beans falling in the first 

 class (25-35 eg.) and similarly for the progeny of each of the groups 

 of the mother beans. The result is shown in the following table. 



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