But by what means will multiplication be restrained?
Malthus points out two general obstacles to indefinite multiplication, which he has denominated the preventive and repressive checks.
As population can be kept down below the level of its physiological tendency only by a diminution of the number of births, or an increase of the number of deaths, the nomenclature of Malthus is undoubtedly correct.
Moreover, when the conditions as regards space and nourishment are such that population cannot go beyond a certain figure, it is evident that the destructive check has more power, in proportion as the preventive check has less. To allege that the number of births may increase without an increase in the number of deaths, while the means of subsistence are stationary, would be to fall into a manifest contradiction.
Nor is it less evident, à priori, and independently of other grave economic considerations, that in such a situation voluntary self-restraint is preferable to forced repression.
As far as we have yet gone, then, the theory of Malthus is in all respects incontestable.
He was wrong, perhaps, in adopting this period of twenty-five years as the limit of human fecundity, although it holds good in the United States. I am convinced that in assuming this period he wished to avoid the imputation of exaggeration, or of dealing in pure abstractions. “How can they pretend,” he may have thought, “that I give too much latitude to the possible if I found my principle on what actually takes place?” He did not consider that by mixing up in this way the virtual and the real, and representing as the measure of the law of multiplication, without reference to the law of limitation, a period which is the result of facts governed by both laws, he should expose himself to be misunderstood. This is what has actually happened. His geometrical and arithmetical progressions have been laughed at; he has been reproached for taking the United States as a type of the rest of the world; in a word, the confusion he has given rise to by mixing up these two distinct laws, has been seized upon to confute the one by the other.
When we seek to discover the abstract power of propagation, we must put aside for the moment all consideration of the physical and moral checks arising from deficiency of space, food, or comfortable circumstances. But the question once proposed in these terms, it is quite superfluous to attempt an exact solution. This power, in the human race, as in all organized existences, surpasses, [p407] in an enormous proportion, all the phenomena of rapid multiplication that we have observed in the past, or can ever observe in the future. Take wheat, for example: allowing five stalks for every seed, and five grains for every stalk, one grain has the virtual power of producing four hundred millions of grains in five years. Or take the canine race, and suppose four puppies to each litter, and six years of fecundity, we shall find that one couple may in twelve years produce eight millions of cubs.
As regards the human race, assuming sixteen as the age of puberty, and fecundity to cease at thirty, each couple might give birth to eight children. It is making a large deduction to reduce this number to one-half on account of premature deaths, since we are reasoning on the supposition of absolute comfort and all wants satisfied, which greatly limits the amount of mortality. However, let us state the premises in this way, and they give us in twenty-five years 2—4—8—16—32—64—128—256—512, etc.; in short, two millions in two centuries.
If we make the calculation on the basis adopted by Euler, the period of doubling will be twelve years and a half. Eight such periods will make exactly a century, and the increase in that space of time will be as 512:2.