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 (Phaseolus 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 cg., the class center values 
ranging from 30 to 80 cg. Next he determined the mean weights of all 
the beans from the plants grown from mother beans falling in the first 
class (25-35 cg.) and similarly for the progeny of each of the groups 
of the mother beans. The result is shown in the following table. 
Weight of mother beans...............] 30 | 40 50 60 70 | 80 
Mean weight of progeny...............| 37.1 | 38.8 | 40.0 | 43.4 | 44.6 | 45.7 
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