NATURE OF THE GENETIC EFFECTS 433 



ants, with perhaps not more than 50 of the 5000 induced genetic deaths 

 occurring in any single generation. Since the natural genetically caused 

 elimination rate of the population in the equilibrium state is 20,000 per 

 generation (a collective figure involving the elimination of some genes 

 derived from mutations in each of scores or hundreds of past generations), 

 the addition to it of this 50 or fewer genetic deaths would probably be 

 imperceptible. So too would the added disabilities, which would, on the 

 average, increase very slightly (by but a few per cent) the "load" carried 

 by 5000 X p or (substituting 100 for p) 500,000 persons, scattered over 

 many centuries. That is, the damage would be entirely real, and of 

 great over-all magnitude, yet not to be detected because so exceedingly 

 dispersed. 



If, however, the 20 r exposure was continued generation after genera- 

 tion, the effect of the added mutations would eventually become important 

 for each generation. A new equilibrium rate of elimination would finally 

 be reached, some 25 per cent higher than the original one, to match the con- 

 stantly higher mutation rate. At the new rate 25,000 persons would be 

 genetically eliminated in each generation instead of 20,000, a quite notice- 

 able increase. At the same time the average amount of disaljility per 

 person would also have grown so that, if before it could be represented 

 by the figure 20 per cent, it would now have risen to 25 per cent. 



It is not known how much permanent increase in mutation rate any 

 given species is capable of enduring without decreasing in numbers and 

 finally dying out, but each species has its own limit, determined by the 

 magnitude of its original mutation rate and the rapidity with which it 

 can replace lost numbers by selective multiplication. There are grounds 

 for inferring that man, by reason of the high spontaneous mutation rate 

 that he already has, coupled with his slow natural rate of multiplication, 

 particularly under conditions of modern civilization where the birth rate 

 is artificially reduced and genetic deaths interfered with, may already be 

 near if not beyond that limit, which may be called the critical mutation 

 rate. Much research is required before the facts relevant to a decision 

 of this question can be determined with sufficient exactness. If it is true 

 that the Umit has been closely approached, then even such an increase of 

 mutation rate as the 25 per cent above postulated might transgress it, 

 and could not be tolerated indefinitely. 



It might be thought that since the genetic damage of radiation becomes 

 so \videly dispersed, without greatly affecting the immediate descendants 

 of an exposed individual, consideration should be given only to the total 

 amount of exposure of the population as a whole, and perhaps only to the 

 total incurred over the course of many generations, so as to keep this 

 total within limits that are not dangerous to mankind as a w^hole. This 

 limiting "permissible total dose," whatever it is taken to be, when divided 

 by the total estimated population of all the parental generations in ques- 



