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Now, it will be readily perceived from the above
figures that under the same death-rate, namely .5 per cent.,
the average duration of life progressively diminishes as the
birth-rate increases, and conversely progressively increases as
the birth-rate diminishes. And it is obvious as well from an
inspection of the table as from a consideration of the principles
on which it has been constructed, that if the annual
increase of the population be conceived to increase to infinity,
the mean duration of life would sink to zero; and that, on the
other hand, if the birth-rate sink to nil, the mean duration
of life becomes infinite.
The second series of results are those which are
obtained with a death rate of 1 per cent. In this case the
mean duration of life varies from 6 2/3 years with a birth-rate
of 101 and an annual increase of 100, to 100 years when the
birth-rate is 1 per cent. and the population consequently
neither increases nor diminishes; but when the birth-rate
sinks to .5 per cent., and the population decreases at the
same rate, the duration of lite becomes 138.28 years; and
when the birth-rate sinks to .05 per cent., and the population
decreases by .95 the duration of life becomes 313.84.
In the third series of results, which are derived under
a death-rate of 1.5 per cent., the duration of life appears to
be 6.08 years when the birth-rate is 101.5 and the increaserate
100, and 66 2/3 years when the death-rate and birth-rate
are equal and the population is stationary. With a birth-rate
of - 5 per cent. and an annual diminution of 1 per cent., the
duration of life mounts to 109.31, and with a birth-rate of
.05 per cent, and an annual diminution of 1.45 per cent, the
duration of life mounts to 232.86.