All men feel that the duration of life of living creatures is governed by fixed laws.
But theologians, philosophers and scientists have failed to find an answer. In this publication the beaten paths of demographers, actuarians and statisticians are abandoned. It is tried to apply the laws of Nature and in particular the laws of physico-chemical equilibrium. For this purpose censuses of many nations and many biological experiments were investigated. It is maintained that the equilibrium between living creatures and their ecosystem is analogous to the equilibrium between living macromolecules and their biochemical environment.