Rate of aging, rate of dying and the mechanism of mortality.

The history of the search for the law and the mechanism of mortality is reviewed. Recent evidence is summarized showing that the Gompertz law of exponentially increasing force of mortality is only an approximate model of mortality kinetics; various other models also provide a more or less satisfactory fit of mortality kinetics data. In particular, a simple model proposed by the author contains the Gompertz model as a special case and is of general validity: it consists of exponentially increasing cumulative mortality in an initial age range followed by exponentially decreasing survivorship. The various proposed mechanisms underlying mortality kinetics are reviewed, with emphasis on their origin and similarities, and a mechanism is proposed mending two basic classical ideas which are only partially valid: (1) Gompertz's accelerated decline of vitality coupled with identical aging rates of the individuals of a population; and (2) Simms' idea of statistically distributed individual aging rates with a uniform average aging rate (linear decline of physiological vitality). This theory provides a basis for analyzing the relationship between rates of aging and rates of dying.