In a population in which the sexual nature of the individuals is not subject to change, the sex-ratio of a given brood as determinable in practice may be different at different times if there is a differential rate of maturation for the two sexes, or a differential death rate. Excluding from present consideration the periods in the life of the population at whi ch these differentials may operate, however, the sex-ratio of the brood at any given time shou ld be identical with that at any other time....