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persons in this country are concerned, the mental differences which
we observe, after stripping off the obvious acquirements in the form
of knowledge of facts, habits, customs, manners, are due only in very
small part to differences in the physical environment, and in a varying
though never to a large degree to differences in the social environ-
ment, and for the greater part to inherited differences." And he
draws the same general conclusion with regard to the physical differ-
ences. Yet in the few years since Professor Carr-Saunders' book was
written, this conclusion has beome extremely unlikely. For recent
work has shown that vitamins and other accessory food-factors have
physical and mental effects far transcending what we originally
thought possible.
In the early years of vitamin research, attention was concentrated
upon the definitely pathological states resulting from total or almost
total deprivation. During the last ten years, it has been shown that
moderate insufficiency of these accessory food-factors will result in
retardation of growth, stunting, lack of physical and mental energy,
and reduced resistance to infectious disease. Even boys who by all
ordinary canons were regarded as in fine health and well above the
average in physique were shown to benefit both in growth and in
energy from the addition of extra milk to their diet. Sir John Orr
has shown that the diet actually consumed by the poorer classes in
Aberdeen, when given in unlimited quantities to rats, results in poor
physique, small litters, low expectation of life, and proneness to
numerous diseases, while the same diet with the addition of various
vitamins and mineral salts kept the animals in tip-top condition.1
In the face of such facts, it is no longer legitimate to attribute the
observed differences in physique and intelligence between social classes
mainly to genetic factors. Genetic differences may of course exist ;
but the strong probability is that most of the differences are dependent
on differences in nutrition. Further, the defective nutrition of the
poorer classes is in part due to ignorance, but in a large measure to
mere poverty. Until we equalize nutrition, or at least nutritional
opportunity, we have no scientific or other right to assert the constitu-
tional inferiority of any groups or classes because they are inferior
in visible characters.
The extreme importance of applying accurate methods to the
problem is shown by the results of recent investigations on twins.   As
is well known, twins may be identical or monozygotic, always of the
same sex and both derived from the same fertilized egg; or they may
be fraternal or dizygotic, either of like or unlike sex, and derived from
1 Cited in Orr, 1936.