Hereditary differences in the duration of life suggest the existence of a distinct genetic factor or group of factors for the duration of life (Růžička, 1923). Genetic and experimental treatment of such factors in man and in the higher animals is greatly limited by the fact that it is only possible to establish the existence of hereditary differences by extensive investigation of very large populations which are genetically homogeneous. The span of life in higher animals is relatively too great and makes the pursuit of the character controlling the duration of life in several successive generations very difficult, if not impossible, for a single investigator.
These difficulties are largely absent from the researches of R. Pearl (1921–4) on the hereditary differences in the duration of life in line-bred strains of Drosophila melanogaster. With this material (whose genetic homogeneity in different strains is known in an exact manner)...
