Thomas Robert Malthus (1766–1834) is remembered primarily for his Essay on the Principle of Population (1798). Malthus was a distinguished scholar at Cambridge, where he concentrated on classics and mathematics. Like many others of his time, he entered the church as a clergyman in order to secure a living. His real interest, however, was in the applied mathematics of economics. In the Essay on the Principle of Population, Malthus made the mathematical point that while food supplies can increase only in a simple arithmetical progression, populations can increase by geometric progression; and the population must, therefore, inevitably outgrow its food supply, with famine the result. This line of reasoning continues to generate controversy to this day, supported by advocates of environmental sustainability and opposed by advocates of a free market. Malthus's forecasts were not fulfilled in the time frame he envisaged because of a combined effect of colonial expansion, the opening up of vast new areas for agriculture in the Americas, Australasia, and parts of Africa; and the development of increasingly efficient farming and animal husbandry practices. These factors successfully maintained food supplies, even through the hyperexponential population surges of the twentieth century, which saw a quadrupling of the world's population from 1.5 billion in 1900 to 6 billion in 1999. Malthusian population forecasts are not flawed, however, if the time scale is correctly estimated and other factors remain unaltered. Malthus's other works include a treatise much admired by economists, Principles of Political Economy (1820).