54 GENETIC VARIATIONS 



that the natural radiations are indeed effective in producing 

 mutations. But a very careful study by MuUer and Mott- 

 Smith, based on the fact that the proportional number of 

 mutations varies as the intensity and duration of the radiation, 

 showed that the number of mutations that occur under nat- 

 ural conditions is much greater than can be accounted for by 

 the frequency of natural radiations. It is clear therefore that 

 some other agent is at w^ork. 



And another agent has been identified. Muller found that 

 the number of mutations is slightly increased when organ- 

 isms 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 degrees. Again, as in the 

 case of radiation, most of the organisms are killed. Again the 

 survivors show injuries and weaknesses of various kinds. And 

 again the progeny of survivors show inherited abnormalities 

 and peculiarities which are found to be due to gene mutations. 

 The same types of mutations are produced by heat as occur 

 spontaneously and under radiation; they are inherited in the 

 same manner. 



Certain other matters of great interest have been reported 

 by Goldschmidt and Jollos in connection with the mutations 

 induced by heat. In some of their experimental cultures very 

 great numbers of mutations were produced. It was further 

 reported by these authors that the heat produces changes in 

 the bodies of the individuals subjected to the heat, and that 

 in later generations similar inherited changes appear as a 

 result of gene mutations. However, these later mutations did 

 not as a rule appear in the descendants of the particular indi- 

 viduals that had been directly changed by the heat. 



In the continuation of this work by Jollos, a further rela- 

 tion, of the very greatest interest, is reported. Subjection of 

 one generation to a high temperature causes a slight inherited 



