s itself in the multiplication of the race. To some extent it
depends on climate; but, since the climate of Europe did not sensibly
change between the fourth and the sixteenth centuries, we may regard
this force as having been, on that continent, during the period under
consideration, invariable.

By the resistances to life is meant whatever tends to make individual
existence more difficult of support. Among such may be enumerated
insufficient food, inadequate clothing, imperfect shelter.

It is also known that, if the resistances become inappreciable, the
generative force will double a population in twenty-five years.

The resistances operate in two modes: 1. Physically; since they diminish
the number of births, and shorten the term of the life of all. 2.
Intellectually; since, in a moral, and particularly in a religious
community, they postpone marriage, by causing individuals to decline
its responsibilities until they feel that they are competent to meet
the charges and cares of a family. Hence the explanation of a
long-recognized fact, that the number of marriages during a given period
has a connection with the price of food.

The increase of population keeps pace with the increase of food; and,
indeed, such being the power of the generative force, it overpasses the
means of subsistence, establishing a constant pressure upon them. Under
these circumstances, it necessarily happens that a certain amount of
destitution must occur. Individuals have come into existence who must be
starved.

As illustrations of the variations that have occurred in the population
of different countries, may be mentioned the immense diminution of that
of Italy in consequence of the wars of Justinian; the depopulation of
North Africa in consequence of theological quarrels; its restoration
through the establishment of Mohammedanism; the increase of that of all
Europe through the feudal system, when estates became more valuable in
proportion to the number of retainers they could supply. The crusades
caused a sensible diminution, not only through the enormous army losses,
but also by reason of the withdrawal of so many able-bodied men
from marriage-life. Similar variations have occurred on the American
Continent. The population of Mexico was very quickly diminished by two
million through the rapacity and atrocious cruelty of the Spaniards, who
drove the civilized Indians to despair. The same happened in Peru.

The population of England at