Data are presented showing that the reduction in life span of male Drosophila melanogaster produced by a given dose of radiation is smaller for doses given later in life. This suggests the hypothesis that radiation shortens life by accelerating subsequent ageing, so that the reduction in life span for small doses is proportional to the dose and to the expectation of life prior to the dose being given. It is shown that this hypothesis can explain our own data on delayed doses, and can also explain the data of Baxter and Blair (1967, Radiation Res. 30, 48-70, and 31, 287-303) on life span after varying doses given to young flies and on divided doses. There is, therefore, no need to suppose that there is a component of the damage which is proportional to the square of the dose, or that there is a threshold dose below which radiation has no life-shortening effect.

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