258 GENETICS IN RELATION TO AGRICULTURE 
generations, the character under observation being degree of mealiness 
of the kernel (Schartigkeit), Johannsen concluded that the selection 
had produced no effect. Moreover the Swedish plant-breeding station 
at Svalof has been guided for years by the knowledge that their pedigree 
cultures, 7.e., pure lines, were not changed by selection. A similar con- 
clusion was reached by Tower after four to ten generations of rigorous 
selection of albinic individuals in three different attempts to establish 
an albinic race from a stable race (pedigree material) of the Colorado 
potato beetle (Leptinotarsa decemlineata). The history of these three 
Fic. 106.—Four pure lines of wheat which have been grown by Vilmorin for 50 years. 
The original specimen in the seed museum is shown on the left in each case. The close 
similarity of the pairs of heads indicates that pure lines remain constant indefinitely. 
(After Hagedoorn.) 
experiments are shown at A, B and C in Fig. 107. The small black 
polygons show for each generation the individuals selected to become 
the parents of the next generation. It will be noted that neither the 
range nor the mode of the population is permanently shifted in the 
direction of the selection. Thus we find that in races or varieties which 
are constant (homozygous) selection has no effect unless mutations occur. 
Various evidence has been brought forward to show that the principle 
does not hold for all organisms. But in all such cases among sexually 
propagated species we may assume that the material used was hetero- 
zygous for certain factors. Such has been shown already to be a satisfac- 
tory explanation of Castle’s results in selecting for plus and minus 
strains in the hooded rats which is one of the cases originally advanced 
as evidence against the pure line theory. 
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