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effectively realized only if the stocks with large families possessed a
markedly different hereditary constitution from those with few chil-
dren ; but the high differential fertility of unskilled workers as against
the professional classes in England, or of the French Canadians against
the rest of the inhabitants of Canada, demonstrates how rapidly
populations may change by this means.
Still another point in which man is biologically unique is the length
and relative importance of his period of what we may call "post-
maturity.55 If we consider the female sex, in which the transition from
reproductive maturity to non-reproductive post-maturity is more
sharply defined than in the male, we find, in the first place, that in
animals a comparatively small percentage of the population survives
beyond the period of reproduction; in the second place, that such
individuals rarely survive long, and so far as known never for a period
equal to or greater than the period during which reproduction was
possible; and thirdly, that such individuals axe rarely of importance
in the life of the species. The same is true of the male sex, provided
we do not take the incapacity to produce fertile gametes as the
criterion of post-maturity, but rather the appearance of signs of age,
such as the beginnings of loss of vigour and weight, decreased sexual
activity, or greying hair.
It is true that in some social mammals, notably among ruminants
and Primates, an old male or old female is frequently found as leader
of the herd. Such cases, however, provide the only examples of the
special biological utility of post-mature individuals among animals;
they are confined to a very small proportion of the population, and
it is uncertain to what extent such individuals are post-mature in the
sense we have defined. In any event, it is improbable that the period
of post-maturity is anywhere near so long as that of maturity. But in
civilized man the average expectation of life now includes over ten
post-mature years, and about a sixth of the population enjoys a longer
post-maturity than maturity. What is more, in all advanced human
societies, a large proportion of the leaders of the community are
always post-mature. All the members of the British War Cabinet
are in their post-maturity.
This is truly a remarkable phenomenon. Through the new social
mechanisms made possible by speech and tradition, man has been
able to utilize for the benefit of the species a period of life which in
almost all other creatures is a mere superfluity. We know that the
dominance of the old can be over-emphasized; but it is equally
obvious that society cannot do without the post-mature. To act on
the slogan " Too old at forty"—or even at forty-five—would be to rob