During the past century the population of Europe increased by about
160,000,000, but it is utterly unreasonable to assume that this rate of
increase will be maintained during the present century. It would be as
sensible to argue that because a child is four feet high at the age of
ten he will be eight feet high at the age of twenty. Moreover, there is
evidence that, apart altogether from vice, the fertility of a nation is
reduced at every step in civilisation. The cause of this reduction in
fertility is unknown. It is probably a reaction to many complex influences,
and possibly associated with the vast growth of great cities. This decline
in the fertility of a community is a natural protection against the
possibility of overpopulation; but, on the other hand, there is a point
beyond which any further decline in fertility will bring a community within
sight of depopulation and of extinction.