THE UNIQUENESS OF THE INDIVIDUAL 



lately for the laboratory vole,^ and here too we find the same 

 smooth passage to extinction. *" Voles drop off at all times of 

 life,"* says Elton,^ speaking of this evidence, 



'though not at the same rates. And these are not "ecologicar* 

 deaths; few of them probably are "parasitological" deaths. We 

 hardly know what process is at work, and for want of a better term 

 we may call it "wear and tear". This has the suggestion of an 

 internal breakdown in the physiological organization. We might 

 almost say that the process of senescence begins at birth.' 



This final inference, which I have italicized, is by no means 

 immediate. The actuary's life table is not a mapping of the 

 course of individual life: it is founded on the distribution 

 through life of the ages at which people die. It thus relates to 

 no event in life save one, its end. Even if the sudden flowering 

 of an evil gene caused voles to age and die within a day, the 

 ages of their deaths might well be so pieced out among the 

 population as to yield just that smooth, continuous curve the 

 actuary maps for us. If, however, the population is reasonably 

 uniform, then the life table (or rather, the force of mortality 

 computed from it) does indeed give us what may be called a 

 •"statistical picture** of the course of ageing. For we may define 

 'senescence"* as that which predisposes the individual to death 

 from accidental causes of random incidence; and it follows that 

 the frequency distribution of the ages of death gives us a 



^ P. H. Leshe and R. M. Ranson, Journal of Animal Ecology ^ 9, p. 27, 

 1940. For life tables for invertebrate animals, cf. A. J. Lotka, The Elements 

 of Physical Biology, Baltimore, 1925; W. H. Dowdeswell, R. A. Fisher, and 

 E. B. Ford, Annals of Eugenics, 10, p. 123, 1940; C. H. N. Jackson, ibid., 

 p. 832. Jackson finds that the life table of tsetse flies is biased, during the 

 rainy season only, by an element contributed by senescence. [For further 

 evidence see Principles of Animal Ecology, by W. C. Allee, O. Park, A. E. 

 Emerson, T. Park and K. P. Schmidt (Philadelphia, 1949); E. S. Deevey, 

 Quart. Rev. Biol., 22, p. 283, 1947; The Natural Regulation of Animal 

 Numbers, by David Lack (Oxford, 1954); The Biology of Senescence, by 

 Alex Comfort (London, 1956).] 



2 C. S. Elton, Voles, Mice and Lemmings, pp. 202-5, Oxford, 1942. 



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