With world population today edging over seven billion, and with projections for it to reach nine billion by mid-century, the ideas of eighteenth-century English cleric Thomas Malthus-and his grim prediction that war, plague, and famine are the inevitable response to overpopulation--loom ever larger on the horizon. But if Malthus is a familiar name to most educated people, few of us have read his famous and controversial work, Essay on the Principle...