If an outbred population of adult Drosophila is kept from the time of emergence in a uniform and favourable environment there is a fairly protracted initial period during which very few individuals die, followed by a period during which the force of mortality rises rapidly until all individuals are dead. Similar life tables can be obtained for most animal species, provided that the environment is favourable and the population is neither genetically very diverse nor excessively inbred. Such results show that progressive changes take place in individuals, starting at the time of emergence, and that these changes ultimately result in death or render individuals increasingly susceptible to various extrinsic causes of death. As would be expected, in poikilotherms such changes proceed more rapidly at higher temperatures, as is shown by the decrease in the expectation of life with increasing temperature. It was the purpose of the present investigation to discover how far the processes responsible for death in D. subobscura are the same at different temperatures, differing only in the rate at which they proceed, and how far different processes are concerned at different temperatures. The results obtained strongly suggest that different processes are responsible for ageing at different temperatures; they also indicate a connexion between the rate of egg-laying and the rate of ageing, and this possibility has been confirmed by a study of ageing in virgin females and in females lacking ovaries. Reproduced by permission. J. Maynard Smith, The Effects of Temperature and of Egg-Laying on the Longevity of Drosophila subobscura. J. Exp. Biol. 35 , 832-842 (1958).