In order to grasp the true significance of the second formula it must be translated from the domain of biology into the region of economics. Malthus evidently thought of it as the amount of corn yielded by a given quantity of land. The English economists could think of nothing except in terms of corn! What he wished to point out was that the utmost we can expect in this matter is that the increase in the amount of the harvest should be in arithmetical progression—say, an increase of two hectolitres every twenty-five years. This hypothesis is really rather too liberal. Lavoisier in 1789 calculated that the French crop yielded on an average about 7¾ hectolitres per hectare. During the last few years it has averaged about 16, and if we admit that the increment has been regular throughout the 120 years which have since elapsed we have an increase of 2 hectolitres per 25 years. This rate of increase has proved sufficient to meet the small increase which has taken place in the population of France. But would it have sufficed for a population growing as rapidly as that of England or Germany? Assuredly not, for these countries, despite their superior yields, are forced to import from outside a great proportion of the grain which they consume. The question arises whether France can continue indefinitely on the same basis during the course of the coming centuries. This is, indeed, unlikely, for there must be a physical limit to the earth’s capacity on account of the limited number of elements it contains. The economic limit will be reached still earlier because of the increasing cost of attempting to carry on production at these extreme limits. Thus it seems as if the law of diminishing returns, which we must study later, were the real basis of the Malthusian laws, although Malthus himself makes no express mention of it. It is a truism that the number of people who can live in any place cannot exceed the number of people who can gain subsistence there. Any excessive population must, according to definition, die of hunger.[297] This is just what happens in the animal and vegetable kingdoms. Germs are extraordinarily prolific, but their undue multiplication is pitilessly retarded by a law which demands the death of a certain proportion, so that life, like a well-regulated reservoir, always remains at a mean level, the terrible gaps made by death being replenished by a new flow. Among savages, just as among animals, which they much resemble, a large proportion literally dies of hunger. Malthus devoted much attention to the study of primitive society, and he must be regarded as one of the pioneers of prehistoric sociology—a subject that has made much headway since then.