With the introduction and
improvement of instrumental and induced deliveries, many of these women
are enabled to survive, with the necessary consequence that their
daughters will in many cases have a similarly narrow pelvis, and
experience similar difficulty in childbirth. The percentage of
deliveries in which instrumental aid is necessary is thus increasing
from generation to generation, and is likely to continue to increase
for some time. In other words, natural selection, because of man's
interference, can no longer maintain the width of woman's pelvis, as it
formerly did, and a certain amount of reversion in this respect is
probably taking place--a reversion which, if unchecked, would
necessarily lead after a long time to a reduction in the average size of
skull of that part of the human race which frequently uses forceps at
childbirth. The time would be long because the forceps permit the
survival of some large-headed infants who otherwise would die.
But it must not be supposed that lethal, non-sustentative selection
works only through forms of infant mortality. That aspect was first
discussed because it is most obvious, but the relation of natural
selection to microbic disease is equally widespread and far more
striking.
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