Malthus afterwards pointed out, in the course of controversy,[120] that there is strictly speaking no question here of the comparison of two tendencies, for we cannot speak of a tendency to increase food in the same sense as a tendency to increase population. Population is increased by itself; food is increased not by food itself, but by an agency external to it, the human beings that want it; and, while the former increase is due to an instinct, the latter is (in a sense) acquired. Eating is instinctive, but not the getting of the food. We have, therefore, to compare an increase due to an instinctive desire with an increase due to labour, and “a slight comparison will show the immensity of the first power over the second.” Malthus allows that it is difficult to determine this relation with exactness;[121] but, with the natural liking of a Cambridge man for a mathematical simile,[122] he says that the one is to the other as an arithmetical to a geometrical ratio,—that is to say, in any given time (say a century) the one will have increased by multiplication, the other only by addition. If we represent both the population and the food at the beginning of that century by ten, then the population will double itself in twenty-five years; the ten will become twenty in the first twenty-five years, forty in the second, eighty in the third, one hundred and sixty in the last, while the food will only become twenty in the first, thirty in the second, forty in the third, fifty in the fourth. If this is true, it shows the tendency of population to outrun subsistence. But of course it needs to be shown from experience that, while the strength of the desire remains the same in the later stages of the growth of population as in the earlier, the laboriousness of the labour is greater in the later stages of the increase of food than in the earlier. It is the plain truth, says Malthus, that nature is niggardly in her gifts to man, and by no means keeps pace with his desires. If men would satisfy their desire of food at the old rate of speed, they must exert their mind or their body much more than at first.[123] An obvious objection presents itself. Since man’s food consists after all of the lower forms of life, animal and vegetable, and since these admittedly tend to increase, unchecked by themselves, in a geometrical ratio, it might be thought that the increase of human population and the increase of its food could proceed together, with equal ease. But the answer is that this unchecked geometrical increase of the first could go on only so long as there was room for it. It could only be true, for example, of wheat in the corn-field at the time when the seed of it was sown, and the field was all before it. The equality of the two ratios would only be true of the first crop.[124] At first there might be five or sixfold the seed; but in after years, though the geometrical increase from the seed would tend