degree of possible rapidity is different in different orders of beings; but in all it is sufficient, for the earth to be very speedily filled up. There are many species of vegetables of which a single plant will produce in one year the germs of a thousand; if only two come to maturity, in fourteen years the two will have multiplied to sixteen thousand and more. It is but a moderate case of fecundity in animals to be capable of quadrupling their numbers in a single year; if they only do as much in half a century, ten thousand will have swelled within two centuries to upwards of two millions and a half. The capacity of increase is necessarily in a geometrical progression: the numerical ratio alone is different.
To this property of organized beings, the human species forms no exception. Its power of increase is indefinite, and the actual multiplication would be extraordinarily rapid, if the power were exercised to the utmost. It never is exercised to the utmost, and yet, in the most favourable circumstances known to exist, which are those of a fertile region colonized from an industrious and civilized community, population has continued, for several generations, independently of fresh immigration, to double itself in not much more than twenty years.[1] That the capacity of multiplication in the human species exceeds even this, is evident if we consider how great is the ordinary number of children to a family, where the climate is good and early marriages usual; and how small a proportion of them die before the age of maturity, in the present state of hygienic knowledge, where the locality is healthy, and the family adequately provided with the means of living. It is a very low estimate of the capacity of increase, if we only assume, that in a good sanitary condition of the people, each generation may be double the number of the generation which preceded it.
