MANNER OF PRODUCTION OF MUTATIONS 479 



without evidence of a change in effectiveness.^ Taking all this work 

 together, an intensity difference of 300,000 has been attained. These 

 results fit in both with those obtained by varying the total dose and with 

 those obtained by varying the voltage or type of radiation (X, 7, or jS) in 

 indicating that either the single ionizations or activations or the indi- 

 vidual clusters of them are the mutagenic agents and that there is no 

 appreciable interaction between them which affects the production of the 

 point mutations. If there were such interaction, it would be very strange 

 if it took place just as effectively when the ions (or otherwise activated 

 particles) had arisen at widely different times and places as when they 

 arose close together. 



As is seen in more detail on pp. 521-524 the final result of the latest 

 work with neutrons applied to Drosophila (Muller, Valencia, and Valencia, 

 1949) was to show that even radiation of this type has sensibly the same 

 effectiveness in producing gene mutations in spermatozoa as X and 7 

 radiation of the types just considered. In the case of neutron irradiation, 

 virtually all the ionizations in the proton tracks are as crowded as those 

 in the "ends" (approximately the last }4 m) of the electron tracks pro- 

 duced by the 7 and X rays. Since the gene-mutation frequency is not 

 differently affected under these two contrasting conditions, it may be 

 concluded that the ionizations in the ends of the major electron tracks 

 arising from 7 and X irradiation are no more effective than the scattered 

 ionizations or those small 5 rays and minute clusters associated with the 

 primary ionizations, which are present in like relative frequencies in 

 both proton and electron tracks. That is, the larger-scale clustering 

 which exists at the ends of the major electron tracks and in the entire 

 course of the proton tracks does not appear to play a significant role in the 

 production of individual gene mutations in Drosophila. 



1 The papers of Hanson and Heys, reporting work performed in St. Louis and later 

 in Paris on problems concerned with the influence of wave length of radiation, timing 

 of dose, aging, starvation, and anesthesia on the induced-mutation frequency, are 

 not cited here, even though they antedate the papers of Timofeefl-Ressovsky and his 

 co-workers on wave length and timing. They are omitted because statistical aspects 

 of all the data presented, especially their extreme "goodness of fit," as noted by Lea 

 (1946), as well as inconsistencies in some of the genetic methods and physical arrange- 

 ments described, make the results reported unacceptable. However, the papers which 

 were published by Hanson under his name alone are rigorous in method and well 

 grounded. These include both his earlier papers, on work done in close consultation 

 with Muller, and the latest one (1935), reporting experiments which led to quite 

 different conclusions from those of Hanson and Heys concerning the influence of 

 aging, nutritional state, and anesthesia on the radiation-induced-mutation frequency. 

 These latter experiments, undertaken to clear up the situation raised by the preceding 

 ones on these subjects, were in fact carried out entirely by C. A. Offermann and his 

 assistant Mrs. L K. Schmidt, while they were working under the direct guidance of 

 Muller in Leningrad and Moscow. 



