answers to the essay had been written; and they have continued to appear. The chief contributions have been by A. H. Everett, “New Ideas on Population” (1823), who believed that an increase of numbers increased productive power; by M. T. Sadler, “Law of Population” (1830), who taught that human fertility varied inversely with numbers, falling off with density of population; by Sir Archibald Alison, “Principles of Population” (1840), who reasoned inductively that the material improvement of the human race is a proof that man can produce more than he consumes, or that in the progress of society preventive checks necessarily arise; by W. R. Greg, “Enigmas of Life” (1873); and by Herbert Spencer, “Westminster Review” (April, 1852), and “Principles of Biology,” (part vi, ch. xii and xiii), who worked out a physiological check, in that with a mental development out of lower stages there comes an increased demand upon the nervous energy which causes a diminution of fertility. Since Darwin's studies it has been very generally admitted that it is the innate tendency of all organic life to increase until numbers press upon the limit of food-production; not that population has always done so in every country.31 Malthus's teachings resulted in the modern poor-house system, beginning with 1834 in England, and they corrected some of the abuses of indiscriminate charity.
While Adam Smith had formulated very correctly the