JSTATURE OF THE GENETIC EFFECTS 439 



brought about there was to be, as in the flies, a natural replacement of 

 the original type by multiplication of the very rare ''superior" mutants, 

 we should have to add the lives of numerous "normal" individuals, frus- 

 trated by that competition, to those of the individuals carrying an exces- 

 sive load of detrimental mutations. 



Of course nothing like so great an increase in elimination rate could 

 be tolerated by any human population, still less by one living after the 

 fashion of modern civihzed communities. For the rate of human repro- 

 duction, especially under modern conditions, is so low that it would be 

 quite impossible for a minute fraction of the population to multiply 

 enough in each generation to make up for the loss of the remainder. In 

 fact, even at present the populations of the technically most advanced 

 countries are not much more than maintaining themselves. This is 

 another way of saying that human beings are already near if not at or 

 beyond the mutation rate which, in relation to their conditions of living 

 and breeding, is the "critical" one. Nevertheless it might well be that 

 a small increase in the mutation rate, such as that brought about by an 

 average dose of only 20 r, could be tolerated. It would however take its 

 proportionate toll in genetic deaths and load, as has been explained. At 

 the same time, the relatively small increase in the potential speed of bio- 

 logical evolution which it would theoretically give rise to could not, under 

 such circumstances as those in which modern man lives, redound to his 

 actual improvement but rather to his deterioration. 



The reason for this reverse influence of mutation rate on biological 

 progress in the case of modern man is that he uses artificial means to 

 counteract and in some important ways even to reverse the usual mode 

 of operation of natural selection. Not only do modern mechanical and 

 social devices for rendering living easier combine with medical methods 

 to save for reproduction an increasingly large proportion of those who 

 formerly would have met genetic death — a procedure which when not 

 combined with eugenic ones tends greatly to raise the frequency of 

 essentially detrimental genes— but the more distinctively human, or 

 (to us) "higher" characteristics, of intelligence, foresight, and social 

 behavior, in so far as they have genetic bases, are, according to the con- 

 clusions of the great majority of the serious students of this subject, 

 actually at a disadvantage in reproduction, under modern conditions, in 

 competition with their opposites. In "advanced" countries and classes 

 it is on the whole the wiser, the more humane, and the more progressive 

 who artificially restrict their families more, while those who have less fore- 

 sight, less interest in the education and welfare of their children, greater 

 clumsiness in techniques, more thoughtless profligacy and more super- 

 stitution, leave a larger retinue of offspring, whose lives are largely saved, 

 by modern methods, to repeat the process. Not that such differences in 

 behavior are exclusively or even mainly genetic in their basis, but they 



