438 RADIATION BIOLOGY 



simultaneously entailed such retrogression. These factors, then, favored 

 a relatively rapid advance in relation to the special conditions, despite a 

 mutation rate that gave a load which under natural conditions might 

 have placed the species at more disadvantage in relation to its competi- 

 tors than before. 



Nevertheless it is probable that the evolution of most species, even in 

 a state of nature, would in the long run be accelerated by some increase 

 in their mutation frequency, such as could be brought about by moderate 

 doses of radiation. For, to follow up a suggestion made by Sturtevant 

 (1937) and later developed further by the present writer (1950b), natural 

 selection tends, where possible, to keep the mutation rate at a lower level 

 than that conducive to their most rapid evolution. Granting this, how- 

 ever, natural evolution is usually extremely slow, especially in species 

 which, like Drosophila, have for an extended period existed in a form 

 much hke their present one, as long as they continue to live under the 

 same conditions as those which formed them. Thus even if a twenty- 

 fold increase in mutation rate, produced, say, by 3000 r per generation, 

 did result in a twentyfold acceleration of their natural evolution when 

 applied to them while living otherwise in a state of nature, it would be 

 unreasonable to expect a perceptible change in them — much less a meas- 

 urable increase in such over-all, already nicely adjusted characters as 

 viability or fertility — in the course of, say, 50 years (1250 generations) 

 of such treatments. For this would be equivalent to only a thousand 

 years of their ordinary evolution, an insignificant period in most evolu- 

 tionary history. It therefore seems certain that the seeming improve- 

 ments noted in the laboratory populations represented entirely a reorien- 

 tation to their new conditions of life. 



The questions remain to be asked : at what cost to individuals did this 

 accelerated reorientation take place, what would it have meant in human 

 terms, and what would have happened in the case of a modern human 

 population in which it had been attempted to practice irradiation simi- 

 larly, although necessarily much more mildly, for many successive gener- 

 ations? In the first place, it should be recognized that even where, as 

 above, the evolution consists of an adaptation to new conditions, the 

 vast majority of the induced mutations, like the spontaneous ones, are 

 detrimental to life and/or reproduction. Hence the increase of mutation 

 rate entails a corresponding increase in the rate of elimination and in the 

 genetic load; i.e., the price is unavoidably paid in what we have termed 

 "frustrated lives." According to the estimates given above, it would 

 take only some 100 to 300 r applied to all the human population every 

 generation, to result, if it could continue until equilibrium between muta- 

 tion and elimination rate were reached, in genetic death and completely 

 frustrated lives for all but a minute fraction of the population in every 

 generation. If, however, in addition to the increased ehmination thereby 



