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, i.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 decemlineatd) . The history of these three 



FIG. 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. 



