There is nothing to prevent the increase of human numbers if we suppose reason to have no need (as in the lower creatures it has no opportunity) to interfere. To understand the situation, however, it is best not to assume the truth of this parallelism, but to take the actually recorded instances of human increase under the nearest known approaches to absolute plenty combined with moral goodness, that is to say, with a state of society in which vice is at a minimum. “In the northern states of America, where the means of subsistence have been more ample, the manners of the people more pure, and the checks to early marriages fewer than in any of the modern states of Europe, the population was found to double itself for some successive periods every twenty-five years.”[118] From this near approach to an unchecked increase, we infer that the unchecked would mean a doubling in less than twenty-five years (say twenty, or perhaps fifteen), and that all population, in proportion as it is unchecked, tends towards that rate of increase. If it is difficult to find an unchecked increase of population, it is still harder to find an unchecked increase of food; for what is meant is not that a people should find their food in one fertile country with as much ease as in another, but that, for a new people, new supplies should always be found with the same ease as the old ones. Now it is not necessary to suppose that the most fertile land is always used first;[119] very often it might only be used late after the rest, through political insecurity, imperfect agriculture, incomplete explorations, or the want of capital; but, when it is once occupied, the question is, will it supply new food to new-comers without any limit at all? This would be an ideally fertile land corresponding to the ideally expanding population. And on some such inexhaustible increase of food an unchecked increase of population would depend, unless men became able to live without food altogether.