"In plants and irrational animals the view of the subject is simple. They are all impelled by a powerful instinct to the increase of their species, and this instinct is interrupted by no doubts about providing for their offspring. Wherever, therefore, there is liberty, the power of increase is exerted; and the superabundant effects are repressed afterwards by want of room and nourishment." Malthus then adduces evidence of the extremely rapid increase of population amongst mankind under conditions in which food is abundant and easily obtainable. He calculates that population, if unchecked, goes on doubling itself every twenty-five years, or increases in a geometrical ratio. But he points out that the food supply can by no means be increased with equal facility. Even if it were possible in one period of twenty-five years to double the amount produced, there is no reason to suppose that the operation could be repeated during the following twenty-five years. As the demand for food increased, less fruitful soils would be taken into cultivation, and the additions that could be made to the former average produce would be gradually and regularly diminishing. Malthus then makes the following calculation: "Let us suppose that the yearly additions which might be made to the former average produce, instead of decreasing, which they certainly would do, were to remain the same; and that the produce of this island might be increased every twenty-five years, by a quantity equal to what it at present produces. The most enthusiastic speculator cannot suppose a greater increase than this. In a few centuries it would make every acre in the island like a garden. "If this supposition be applied to the whole earth, and if it be allowed that the subsistence for man, which the earth affords, might be increased every twenty-five years by a quantity equal to what it at present produces, this will be supposing a rate of increase much greater than we can imagine that any possible exertions of mankind could make it. "It may fairly be pronounced, therefore, that considering the present average state of the earth, the means of subsistence, under circumstances the most favorable to human industry, could not possibly be made to increase faster than in an arithmetical ratio.