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 X XV). 
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. Onewas 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 
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