370 GENETICS IN RELATION TO AGRICULTURE 



Both the best and the poorest families transmit average qualities, so that 

 continuous selection is not an efficient means of improvement. The 

 isolation of mutants, on the other hand, is thought by Pritchard to 

 offer promise of improvement, but if the mutation method is to be used, 

 it is deemed essential that more efficient experimental methods be 

 devised to reduce the effects of soil differences and thus make it possible 

 to distinguish real differences more clearly (see Chapter XXV). 



Other normally cross-fertilized crop plants in which mutations are 

 known to have occurred are cotton, hemp, rye and the sunflower. 



The Search for Mutations. It has long been thought that the two 

 most effective methods of inducing heritable variations in plants are 

 hybridization and change of environment. Regarding the importance of 

 the first there is of course no question, and there is evidence that very 

 radical changes of environment such as Tower applied to his beetles and 

 White to his tomatoes may induce germinal variation. But the idea that 

 mere change of location from warmer to cooler climates or from poorer to 

 richer soils, or vice versa, is very effective in " breaking the type " finds very- 

 little to support it. This notion that culture induces germinal variation 

 doubtless finds its explanation in the fact that sooner or later after a plant 

 is subjected to intensive culture and close observation new heritable varia- 

 tions appear. But why conclude that these variations are induced by 

 culture? During the first season of garden cultivation of a species of 

 tarweed two mutations were discovered. One was a change in the color of 

 the stamens, the other was petallody in the ligulate flowers. It seems 

 very probable that these variations would have occurred had the plants 

 been growing in the wild. They were found because the plants were 

 closely inspected. But is their not fair evidence that cultivation of the 

 same species in different regions gives rise to different mutations? There 

 is danger of befogging the issue by this question unless we distinguish 

 clearly between the origin of mutations and the origin of varieties. To 

 consider only one of many possible illustrations: the native sorghums of 

 South Africa, the Sudan, Egypt, Arabia and Persia, India, and Manchuria 

 are a diverse lot of forms; yet sorghum undoubtedly originated in 

 Africa and spread thence to the various regions where it now exists as 

 distinct varieties. It must be admitted that different varieties have 

 developed in different regions, but does this necessarily indicate that geo- 

 graphical differences actually caused the original germinal alterations 

 which resulted in the different varieties of sorghum? Such a conclusion 

 seems unwarranted in view of what is actually known concerning the 

 occurrence of mutations under both natural and artificial conditions. 

 Moreover, it is not improbable that the progenitors of existing varieties 

 of sorghum all originated in Africa, although geographical differences may 

 have been the determining factor in the survival of those mutations 



