causes may be the greater danger which the officer-class incurs in war,
and, in former times, the executioner's axe. In our own day the
reluctance of rich and self-indulgent women to bear children is
undoubtedly a factor in the infertility of the leisured class.

This brings us naturally to the second part of our discussion--the
consideration of the causes which lead to the increase or decrease of
population. It is the most important part of our inquiry; for it is
usually assumed that the British Isles will continue to send out
colonists in large numbers, as it did in the last century, and the hopes
of the imperialist that a large part of the world will speak English for
all time depend on the untested assurance that the swarming-time of our
race is not yet over. Our starting-point must be that the pressure of
population upon the means of subsistence is a constant fact in the human
race, as in every other species of animals and plants. There is no
species in which the numbers are not kept down, far below the natural
capacity for increase, by the limitation of available food. It may not
always be easy to trace the connection between the appearance of new
lives and the passing away of old, nor to say whether it is the
birth-rate which determines the death-rate, or the death-rate the
birth-rate. But it is well known that, wherever statistics are kept, the
numbers of births and of deaths rise and fall in nearly parallel lines,
so that the net rate of increase hardly alters at all, unless some
change, which can easily be traced, occurs in the habits of the people
or in the amount of the food supply. In civilised countries the greater
care taken of human life, and its consequent prolongation, has reduced
the birth-rate, just as in the higher mammals we find a greatly
diminished fertility as compared with the lower, and a much higher
survival-rate among the offspring born. The average duration of life in
this country has increased by about one-third in the last sixty years,