348 GENETICS 



any before observed. The same gene may be caused by radi- 

 ation to produce repeatedly in different individuals the same 

 mutation. Since all kinds of mutations are induced by radia- 

 tion, it is clear that in these cases there is no relation be- 

 tween the kind of mutation produced and the kind of 

 environmental agent that induces them. The mutations in- 

 duced by radiation are inherited in later generations in the 

 same manner as are those that occur spontaneously. 



The number of mutations produced in a given number of 

 individuals is found to increase in proportion to the energy 

 of the radiation; that is, to its intensity and duration, up to 

 an energy that causes injury so great that the radiated germ 

 cells do not develop. 



The question arises as to whether the "spontaneous" 

 mutations are not partly or wholly the result of radiations 

 occurring under natural conditions. It was found that fruit- 

 flies kept in a mining shaft in which natural radiation from 

 the minerals present was abundant gave a larger proportion 

 of mutations than those kept under usual conditions, indicat- 

 ing that the natural radiations are indeed effective in pro- 

 ducing mutations. But a very careful study by MuUer and 

 Mott-Smith,^ based on the fact that the proportional num- 

 ber of mutations varies as the intensity and duration of the 

 radiation, showed that the number of mutations that occur 

 under natural conditions is much greater than can be ac- 

 counted for by the frequency of natural radiations. It is 

 clear therefore that some other agent is at work. 



Another agent has been identified. Muller found that the 

 number of mutations is slightly increased when organisms 

 are kept at a high temperature — 29 degrees in place of 20 

 degrees. Then Goldschmidt and after him Jollos '^ employed 

 heat that was destructive, up to 37°. Again, as in the case of 

 radiation, most of the organisms are killed. Again the sur- 

 vivors show injuries and weaknesses of various kinds. And 

 again the progeny of survivors show inherited abnormalities 



