HERMANN J. MULLER 
natural selection for truly social propensities; there is also a 
disappearance of the circumstances that have favoured the 
survival and multiplication of individuals genetically better 
fitted to cope with difficulties and that, conversely, have led 
to the dying out of lines deficient in these faculties. For society 
now comes effectively to the aid of those who for whatever 
reason, environmental or genetic, are physically, mentally, or 
morally weaker than the average. True, this aid does not at 
present afford these people a really good life, but it does usually 
succeed in saving them and their children up to and beyond the 
age of reproduction. 
It is probable that some 20 per cent, if not more, of a human 
population has received a genetic impairment that arose by 
mutation in the immediately preceding generation, in addition 
to the far larger number of impairments inherited from earlier 
generations. If this is true, then, to avoid genetic deterioration, 
about 20 per cent of the population who are more heavily laden 
with genetic defects than the average must in each generation 
fail to live until maturity or, if they do live, must fail to repro- 
duce. Otherwise, the load of genetic defects carried by that 
population would inevitably rise. Moreover, besides deaths 
occasioned by circumstances in which mutant genes play a 
critical rdle, there is always a large contingent of deaths resulting 
from environmental circumstances. Consequently, the number 
of individuals who fail to “carry along’? must considerably 
exceed 20 per cent, if genetic equilibrium is to be maintained, 
and merely maintained. Yet among us today, in industrialized 
countries, the proportion of those born who fail to reach maturity 
has fallen to a small percentage, thanks to our present high 
standards of medicine and of living in general. This situation 
would, other things being equal, spell genetic deterioration, at 
a roughly calculable rate. 
However, it has sometimes been surmised that the present 
excess of genetically defective adults—those whose lives have 
been saved by modern techniques—may somehow be screened 
out, after maturity, through the automatic operation of an 
increased amount of reproductive selection, in that these 
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