The choice of egg weight in the domestic fowl as the focal point of these studies is based upon several features. It is, firstly, a characteristic of major economic importance in the field of poultry breeding. Resultantly the already extensive literature on the subject of its inheritance provides a convenient picture of present century developments in the genetical study of continuous variation. As a trait displaying marked variation due to both genetic and environmental agencies it affords a convenient model for illustrating the utility, validity, and limitations, of current biometrical methods in applied population genetics. Finally its dependence on, or association with, other important production traits in the fowl - age at sexual maturity and body weight - leads naturally to a study of the associated variations of these traits, and to a study of their individual properties. The ultimate object of these investigations, however far from complete, is increased efficiency in the genetic control of such a system of interrelated and individually complex traits. The problems arising, though here confined to poultry breeding, have analogues in diverse fields of animal, plant and human genetics.