Genotypic differences are the raw material on which selection acts but not all kinds of genotypic differences are of equal importance in this connection. FISHER (1930) showed that whether or not mating occurs at random, the population genotypic variance for any quantitative character may be analyzed into a number of components and that one of these, which he called the genetic variance, is of basic importance in selection. It is the purpose of the present article to review this analysis of the genotypic variance and to discuss some of its consequences for natural selection where the character concerned is fitness to survive and reproduce. 2. ANALYSIS OF THE PHENTOTYPIC VARIANCE Consider a quantitative character whose phenotypic value Y in any member of the given population is determined by the individual's genotype and environment which, we shallsuppose, act independently of one another, i.e., we shall assume that the environment provides a system of chances equally applicable to all genotypes. The phenotypic value Y can then be expressed as the sum of a genotypic value G, which is the mean phenotypic value in indi:viduals" of the given genotype in all possible environments, and ar~ independent environmental contribution E due to the particular environment which is experienced. The phenotype variance ~, which is the population variance of the phenotypic values, can then be expressed as the sum of