Extract: The conventional wisdom about why species age and live as long as they do is based in part on the modern version of the very old and now discredited "rate of living" (ROL) theory of aging. According to the old ROL theory, aging is caused by the loss of some vital substance such as water or hormones - the more rapid the vital substance is used up the shorter the lifespan. The modern and more plausible version of ROL is based on a hypothesis formulated by Raymond Pearl (1921) in the early 20th century where it was suggested that the primary determinant of how long species live is influenced by the relative speed of their resting metabolism. That is, metabolic rate is thought to be inversely proportional to maximum lifespan, which means that species that live fast will die young while those that have a slower metabolic rate live slower and longer. The evolutionary theory of why aging occurs that arose in the 20th century conceptually supports predictions from the metabolic rate theory, although the evolutionary line of reasoning is silent about the mechanisms involved. According to evolution theory, animals that face high extrinsic mortality such as predation and infectious diseases must develop quickly (i.e., live fast) in order to pass their genes onto the next generation before death occurs, while animals that face low extrinsic mortality delay development and reproduction, and thus live slower and longer.