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represent the actual age which all persons reach, but that
in contributing to maintain it many persons live beyond it,
many fall short of it. While, however, the former consideration
would tend to reduce the possible increase of
populations by births below my estimates, the last demonstrates
that even in those cases in which the mean duration
of life is under 20, and in which, therefore, on my original
assumption there is little or no reproductive material, there
would be actually living a considerable number of women
above the age of 20, and that generally in the series of cases
which I have just quoted there would certainly be a considerably
larger percentage of adult population than my
figures show. At any rate, the practical conditions which
tend to reduce the possible birth-rates under the assumed
relations of the birth and death-rates, and those which tend
to increase them, balance one another to some extent, and
thus tend to confirm the possibility of the conclusions to
which my calculations lead.
I have drawn in each column of Table I. a horizontal
line immediately above the highest birth-rate which, on the
grounds above given, seems to be reasonably attainable in
connection with the death-rate belonging to that column.
The mean duration of life in England has been
calculated to be nearly 41 years. It is impossible to predict
to what extent sanitary science will ultimately succeed in
raising it. It is, certain, however, that its rise must
depend, less on increasing the duration of life beyond what
is commonly regarded as the normal span, namely 70 or 80
years, than on the prevention of deaths in the earlier
periods of life, and especially in early infancy. If we
assume that, in consequence of the prevention of premature