Humans need food in order to survive, and most of the food in the modern world is the product of agriculture. In 1798, Thomas Robert Malthus published the famous essay in which he argued that the human population increases geometrically yet food production can increase only arithmetically. What he could not predict at that time was the contribution of science to the increased production of food. As Malthus foretold, the world population has increased at an almost alarming rate. It took slightly more than 100 years to double from the 1.25 billion in Malthus's day to 2.5 billion in 1950, but the next doubling, to 5 billion, was achieved in less than 40 years, as seen in Figure 6.1. However, the yield of major food crops per unit area (represented by wheat in Figure 6.1) has increased at an even steeper rate, tripling in slightly more than 40 years. One of the major contributing factors to this increase has been the development of high-yielding varieties of crops, for example, semi-dwarf varieties of wheat and rice, which direct a larger portion of their energy to the production of seeds (grains) than to plant growth; this development, which occurred in the 1960s and 1970s, is often called the “Green Revolution.” Thanks to this increase in yield, the world production of food (represented by cereals in Figure 6.1) could more than keep pace with the increase in population, in spite of the steadily decreasing total land area devoted to agricultural production.