NATURAL SELECTION—DOBZHANSKY AND ALLEN 3/1 
in their own marriage and reproduction. If so, the reproductive suc- 
cess of an individual may be more adequately gaged by the number 
of his grandchildren than by the number of his children. Also, if 
parental influence is so important, the existing negative correlation 
between intelligence and family size may be compensated in some cul- 
tural groups by a positive correlation between intelligence and suc- 
cessful preparation of one’s offspring for adult adjustment. This ex- 
tension of parental functions appears to represent a trend in human 
cultural evolution; in our own society, as class differentials in fertility 
diminish, it may restore some of the biological value which intelli- 
gence seems to have lost. 
Under civilization, reproduction and successful child rearing have 
come to depend more upon individual adjustment patterns and less 
on survival or reproductive capacity. Individual and family adjust- 
ment is the modern theater of the “struggle for existence.” In our 
culture biological adaptedness, that is, optimal reproduction and child 
rearing, seems to bear no direct relation to economic or educational 
status, but probably depends in part upon personal and social ad- 
justment patterns. Though physical and mental handicaps rarely 
eliminate persons completely, they probably affect such adjustment. 
Among the traits capable in this way of influencing reproduction, the 
relative importance of physical health is presumably diminished and 
that of mental health magnified in comparison with selection in prim- 
itive man. In addition, some physiological defects would appear 
to contribute to personal maladjustments more frequently in a mod- 
ern than in a primitive culture. Likely examples of such defects 
are color-blindness, left-handedness, and allergic diathesis. With re- 
spect to genetic factors underlying these traits, present-day natural 
selection may be reinforced both relatively and absolutely. Finally, 
the capacity to compensate for gross physical or sensory handicaps 
probably has more selective value now than it did under conditions of 
existence which eliminated most cripples completely. 
Further speculation is unwarranted here, but it seems safe to as- 
sume that most sensory or mental characteristics that were developed 
in our primate ancestors in response to the demands of an increas- 
ingly complex, variable environment, are even more important to 
civilized man. If so, they surely play some role in determining which 
persons shall marry, which shall have stable families, and which 
shall raise more children. When handicapped individuals defy these 
determinants and become parents, their children pay the price in a 
relatively severe selection by the adverse physical and social en- 
vironment. Asa result of this stringent selection in such families, on 
the average, survivors in the third generation are probably superior 
to the grandparents genetically. 
