10 p. B. Medawar 



widest sense) and that senescence is the process of becoming 

 ever more accident-prone. In other words, everyone is 

 assumed to die of something, no-one to die what is most 

 inaptly called a "natural" death. Most pathologists beheve 

 this to be true (Aschoff among them), and all are obliged 

 to do so by the laws of certification. The approach to natural 

 death is therefore asymptotic. The lethal threshold falls 

 until, in theory, the most trivial accident imaginable is a 

 cause of death. 



(3) The measurement is corrupted by the existence of 

 causes of death which are not random in their incidence, i.e. 

 to which the entire population is not equally at risk. The 

 barnacles of Hatton's (1938) life table* may well be equally 

 exposed to hazard, but human beings are obviously not; 

 special ages and occupations have special risks. Evidently 

 the measurement must be corrected for causes of death having 

 any kind of specific frequency of incidence. 



(4) A life table set up by tracing the life histories of a 

 cohort of newborns from birth to death will certainly be 

 corrupted by secular changes in the causes of mortality. 

 Men aged fifty-four today were born in 1900, when the nature 

 of the causes of death and their pattern of incidence were 

 very different from what they are today. Animals reared in 

 laboratories may not be so affected; nor need it be assumed 

 that secular errors of this kind affect wild populations of 

 animals with comparatively short lives. 



(5) If the force of mortahty is to be a reasonably efficient 

 measurement of the degree of senescence, there should be no 

 gross inherited differences of susceptibility between members 

 of the population at risk. For let it be supposed that the 

 population with which the life table begins is genetically 

 subdivided into stalwarts and weaklings. Selection will then 

 occur in the course of life, and the population which the later 

 years of the life table deals with will be a quite unrepresenta- 

 tive sample of that with which it began. In the laboratory, 



*For the literature of life tables for wild animals, see Deevey (1947), AlleC 

 Emerson, Park, Park and Schmidt (1949), and Haldane (1953). 



