RADIATION AND THE STUDY OF MUTATION IN ANIMALS 1243 



THE TIME FACTOR — SECONDARY REACTIONS 



The relation of time and intensity has been given no exhaustive study; 

 but the fact that similar relations have been obtained in experiments like 

 those of Oliver (124, 125) where only time was varied, or of Hanson and 

 Heys (60) where intensity was varied by means of lead filters, shows that 

 the important element is the total amount of energy delivered. Timof^eff- 

 Ressovsky (185) and Hanson and Heys (64) (Tables 22 and 23, respec- 

 tively), have in particular put this to test over longer periods of time, and 

 found again equal effects for equal amounts of energy. 



Table 22. — The Effects of Equal Energies of X-rays, Applied over Different 

 Time Intervals, on the Percentage of Sex-linked Lethal Mutation in 



Drosophila melanogaster 

 Timofceff-Ressovsky (185) 



Table 23. — Lethal-mutation Rates in the X-chromosome of Drosophila 

 melanogaster When the Same Energy Is Applied but the Time and 



Intensity Are Varied 

 Wild-type males treated (Hanson and Heys, 64) 



What such experiments mean is obvious: Subsequent to the initial 

 radiochemical reactions, there are none which effectively modify them — 

 either removal reactions such as are responsible for thresholds or "back- 

 reactions." In effect, the mutation process is pseudo-irreversible. The 

 most striking evidence for this comes from the experiments of Muller 

 (105), confirmed and extended by Harris (72; Table 24). Males which 



