The passion between the sexes has appeared in every age to be
so nearly the same that it may always be considered, in algebraic
language, as a given quantity. The great law of necessity which
prevents population from increasing in any country beyond the
food which it can either produce or acquire, is a law so open to
our view, so obvious and evident to our understandings, and so
completely confirmed by the experience of every age, that we
cannot for a moment doubt it. The different modes which nature
takes to prevent or repress a redundant population do not appear,
indeed, to us so certain and regular, but though we cannot always
predict the mode we may with certainty predict the fact. If the
proportion of births to deaths for a few years indicate an
increase of numbers much beyond the proportional increased or
acquired produce of the country, we may be perfectly certain that
unless an emigration takes place, the deaths will shortly exceed
the births; and that the increase that had taken place for a few
years cannot be the real average increase of the population of
the country. Were there no other depopulating causes, every
country would, without doubt, be subject to periodical
pestilences or famine.

The only true criterion of a real and permanent increase in
the population of any country is the increase of the means of
subsistence. But even, this criterion is subject to some slight
variations which are, however, completely open to our view and
observations. In some countries population appears to have been
forced, that is, the people have been habituated by degrees to
live almost upon the smallest possible quantity of food. There
must have been periods in such counties when population increased
permanently, without an increase in the means of subsistence.