: NATURE OF THE GENETIC EFFECTS 435 



of such cases would be that the frustration of the present potential 

 mothers would have been exchanged at the expense of an equal or 

 greater total amount of human frustration in later generations. 



From these calculations it will be seen that the procedure, followed by 

 some physicians, of exposing the testes of relatively young men to a dose 

 of some 500 r, in order temporarily to check the reproduction of those who 

 intend to beget children at a later period, would probably, according to 

 the best but admittedly not yet good enough present indications, result 

 in the equivalent of several frustrated future lives, on the average, for 

 each person enjoying this convenience. It should further be observed 

 that the eventual individual victims in all these cases would not have been 

 spared by the exercise of more caution in the exposure of the rest of the 

 population, with the intention of ensuring that the average dose received 

 by the population as a whole did not rise above the agreed upon limit of 

 20 r, or whatever it may have been taken to be. Moreover, if less 

 radiation were really used in other cases, to compensate for the large 

 doses necessary for these partial sterilizations, many people might be 

 penahzed by being deprived of the benefits of radiation which could have 

 been put to better use in their cases. 



When the dose received is less than enough to produce one mutation 

 per germ cell, it can nevertheless be expressed as a risk, or as resulting in 

 an average of one mutation in a certain number of offspring, in a series 

 of cases of the given kind. This damage or risk of damage (Muller, 

 1951a) is then again to be compared with the average benefit, or rather, 

 with the amount by which the average benefit would exceed that to be 

 derived from the best alternative procedure. In many cases the alterna- 

 tive procedure may itself involve irradiation, but irradiation applied with 

 special precautions, such as (where it is intended primarily to expose 

 somatic parts) shielding of the gonads, shuttering down of the field, limi- 

 tation of the time and amperage of exposure, and adjustment of the volt- 

 age so as to give the least exposure to other parts that is commensurate 

 with sufficient exposure of the parts in question. In such cases a con- 

 siderable decrease in genetic risk may sometimes be achieved while the 

 only decrease in benefit to be set against it will be the inconvenience 

 attached to the taking of the special precautions. Even for very small 

 exposures such precautions are usually justified, for these are the very 

 ones which are most likely to be relatively often repeated, and it is the 

 total accumulated exposure, received over an unlimited period prior to 

 reproduction, which counts in determining the chance of producing a gene 

 mutation which will be inherited (as explained in Chap. 8). 



22. SPEEDING UP OF EVOLUTION BY IRRADIATION 



In all the above discussions only the harmful mutations have been 

 taken into consideration. Even though beneficial mutations form but an 



