In the present publication a relationship is applied to the distributions of life spans of animals, which originally had been found by the authors when studying a quite different problem. The new formula is far simpler and its derivation is more evident than the formulas earlier proposed by GOMPERTZ, MAKEHAM or PERKS 1. Moreover all statistical data taken from literature fit our equation better than any other one. Using the new relationship it is possible to characterize the survival curves of normally ageing biological collections as well as those of collections, the individuals of which are treated with gamma radiation resp. drugs, by one single constant similar to the rate constant in the case of chemical reactions.