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.