582 RADIATION BIOLOGY 



ash were at the same time made to make sure that the internal concen- 

 tration of radioactive material did not exceed the value assumed. Sub- 

 stantially the same conclusion regarding the ineffectiveness of natural 

 radiation was reached independently by Timofeeff-Ressovsky (1931a) 

 and by Efroimson (1931). 



It is true that Babcock and Collins (1929a, b) and Hanson and Heys 

 (1930) had presented some data which suggested that, in regions of higher 

 natural radiation, more mutations occurred, but their data were incon- 

 clusive, and if the results were to be considered positive, they would be 

 quite out of line with those of exact tests of the amount of mutagenic 

 effect produced by controlled doses of radiation. These strictures like- 

 wise apply to Jollos's (1939) tests of lethal production in Drosophila at 

 high altitudes; the data appeared to show an increased mutation rate, 

 more especially in the neighborhood of masses of heavy metal which would 

 give rise to showers of secondary radiation. The calculations of Muller 

 (1934) and of Delbrlick and Timofeeff-Ressovsky (1936) had shown that 

 even at high altitudes radiation-induced mutations would (barring 

 unknown factors) be an insignificant fraction of the total of spontaneous 

 mutations. Moreover, tests for sex-linked lethals conducted by Friesen 

 (1936) on Drosophila which had been taken into the stratosphere and by 

 Rajewsky and Timofeeff-Ressovsky (1939) on Drosophila kept high up 

 on Mount Jungfrau had failed to show any perceptible increase. As for 

 the possible influence of adjacent masses of dense material, it is now 

 known from physical measurements that this could not increase the 

 amount of radiation received at high altitudes by a factor of more than 

 about 4 at most. 



In recent years more exact information concerning the incidence both 

 of natural radiation and of spontaneous and radiation-induced mutations 

 have made possible more accurate calculations along the above lines. It 

 is known that 0.15 per cent of new sex-linked lethals per gamete per 

 generation is a conservative (low) average for Drosophila of either sex. 

 If all these had been produced by ionizing radiation, received over the 

 comparatively long generation time (for Drosophila) of four weeks, by 

 germ cells which for three weeks had been in gonial and for a week in 

 the (on the average) twice-as-susceptible postgonial stages, a dose of 

 some 80 r would have been required. (The first three weeks would have 

 yielded lethals at the rate of 1 per cent per approximately 730 r or 0.09 

 per cent for the 60 r received during those weeks, and the last week, at 

 twice the rate of production of lethals, would have yielded 0.06 per cent 

 for the 20 r then received, a total of 0.15 per cent.) However, the back- 

 ground radiation received from both cosmic and earth sources together 

 rarely averages more than 0.004 r/day at sea level, or 0.028 r for the 

 assumed four weeks. The potassium in an animal (0.4 per cent at most) 

 would give an equivalent of not more than 0.017 r in four weeks. The 



