1. Introduction. A genetical population in which different genotypes have different viabilities will undergo, by selection, changes in its genetical constitution which can be expressed in a simple mathematical form. We here consider a system of m alleles A lt A z , ...,A m segregating at one locus, and we assume that the genotype A^j has viability a ip where a it = a n ^ 0. We assume discrete, non-overlapping generations and random mating.In an infinite population there will be no random fluctuations, and, as in Mandel's paper (l), the frequencies p t , p' t of the gene A { in two successive generations will be connected by