The Biology of Senescence 



individual to die without actually killing it, the statistical defini- 

 tion of senescence, although it reflects a real process in indivi- 

 duals, can only be tested upon a population. 



It is evident that in applied biology, and especially in 

 medicine, it is desirable to be able to infer not only the existence 

 of senescence in a species but the degree of senile change in a 

 given individual. This estimate must be based on secondary 

 criteria, and can be made with accuracy only in forms whose 

 life-cycle, like that of man or of Drosophila, has been subject to 

 intensive study. The importance of the statistical definition of 

 senescence is that it implies an obligatory recourse to adequate 

 population studies. An over-common practice has been to keep 

 a single specimen, a bird or a bullfrog, for ten or twenty years, 

 and, when it is found dead, having been so for hours or possibly 

 days, to describe histological appearances in its tissues in a note 

 entitled 'Senile change in the nervous system of Passer (or Bufo)\ 

 While senescence cannot be inferred from every life- table in which 

 the force of mortality rises, neither can descriptions of 'senile* 

 changes be properly based on single observations upon sup- 

 posedly ageing organisms belonging to groups whose life-cycle, 

 in relation to senescence, is not fully known. 



In practice, other criteria than the life-table can be applied 

 to organisms whose life-cycle is familiar, as secondary indices of 

 senescence; these are distinct from mere measures of chrono- 

 logical age, based upon the morphology of scales, teeth or 

 otoliths. Certain factors which are, in effect, direct measures of 

 vigour or of vulnerability, such as the mortality from burns, 

 (Ball and Squire, 1949), or even the annual absenteeism from 

 sickness (Schlomka and Kersten, 1952) follow the general force 

 of mortality in man. The supposed decline in the rate of wound 

 healing proposed as a measure of senescence by du Nouy (1932) 

 was based on grossly inadequate clinical material and is not 

 supported by later work (Bourliere 1950). Less general criteria 

 such as skin elasticity in man (Evans, Cowdry and Nielson, 

 1943; Kirk and Kvorning, 1949), organ weight and relative 

 organ hyporplasia in rats (Korenchevsky, 1942; 1949), heart 

 rate in Cladocerans (Ingle, Wood and Banta, 1937, Fig. 39, 

 p. 146), milk yields in cattle (Brody, Ragsdale and Turner, 

 1923), tgg production in fowls (Clark, 1940, Fig. 14, p. 63), 



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