326 GENE MUTATIONS CAUSED BY RADIATION 



of viability in an equilibrium population [Muller (54)]. According to 

 this estimate, which has a range of error corresponding to a factor of 

 5, there is one new spontaneous mutation, on the average, for every five 

 offspring, as a minimum rate, up to possibly one mutation for each off- 

 spring, on the average, as a maximum rate. It makes a great deal of 

 difference, however, just where within this range the actual rate lies, 

 for with the small size of families obtaining under modern civilized con- 

 ditions there w^ould not be enough surplus to allow the genetic elimina- 

 tion necessary for the maintenance of genetic equilibrium and for the 

 long-term survival of the species unless the mutation rate were at the 

 minimum end of this range of possibilities. Hence in any case it is prob- 

 ably not far from what may be called the "critical level," the level 

 beyond which selection, so long as it is limited by the existing birth rate, 

 cannot keep up with deterioration. 



In view of the above situation, even a comparatively small increase in 

 the mutation rate, artificially induced, might cause it to transgress the 

 critical level, and so, if it were indefinitely continued, it could have 

 disastrous consequences. It could be counteracted only by a corre- 

 sponding rise in the birth rate, accompanied by more stringent selection. 

 On the other hand, if the level of mutation which was "critical," in re- 

 lation to the existing level of reproduction and selection, were already 

 being exceeded by the spontaneous rate, then the induced mutations 

 would occasion a correspondingly faster decline. 



It is true that human policies can hardly be expected to continue suf- 

 ficiently unchanged, over so many centuries, as to lead to the long-term 

 results in question. But, as has been pointed out, the results of an agent 

 operating for a briefer time can be measured in a proportionate manner, 

 in terms of those results which would have been produced had it been 

 of longer duration. Moreover, from an ethical standpoint, policies 

 should be framed according to this yardstick. 



REFERENCES 



1. Altenburg, L., F.. Altenburg, H. U. Meyer, and H. J. Muller, The lack of pro- 

 portionality of mutations recovered to dosage of ultraviolet administered to the 

 polar cap of Drosophila (abstract), Rec. Genetics Soc. Amer., 18: 75, 1949. 



2. Auerbach, C, Chemical induction of mutations, Proc. 8th Intern. Cong. Genetics, 

 Stockholm, 19Jt8 (Supplement to Hereditas), 128-147, 1949. 



3. Baker, W. K., and E. Sgourakis, Alteration of the X-ray sensitivity of Drosophila 

 by means of respiratory inhibitors (abstract), Rec. Genetics Soc. Amer., 35:96, 

 1949. 



4. Baker, W. K., and E. Sgourakis, The effect of oxygen concentration on the rate 

 of X-raj'-induced mutations in Drosophila, Proc. Natl. Acad. Sci., 36: 176-184, 

 1950. 



