14 P. B. Medawar 



are not in themselves mortal may build up a debt which is due 

 to age, because it depends upon the passage of time, but 

 which is not due, in the colloquial sense, to "ageing". 



The two classes of agencies which contribute to increase of 

 vulnerability need not be difficult to distinguish by experi- 

 mental means, but in the actual records of mortality the two 

 are inextricably combined. If w^e wish to use an actuarial 

 measurement of senescence, this is a real shortcoming of 

 principle. Yet this confounding of two causes of increase of 

 vulnerability does not in any way detract from the biological 

 value of the use of the force of mortality as a measure of 

 senescence, for both are real agencies that govern the popu- 

 lation dynamics of real communities of animals. It is up to 

 the physiologist and pathologist to distinguish and keep 

 apart the contribution to vulnerability that is made b}^ a 

 true innate decline. 



In summary, the actuarial definition of senescence amounts 

 simply to this: that senescence is a change which accompanies 

 ageing and which makes an individual progressively more 

 liable to die. (Such a definition may be challenged on many 

 grounds, but its simplicity should not be one of them.) Senes- 

 cence may be measured by the force of mortality estimated 

 from the age frequency distribution of the incidence of 

 death, but the measurement is valid only of a genetically 

 uniform population subjected to a constant pattern of hazards 

 impingeing randomly upon it, and is imperfect in proportion 

 as these conditions fall short of being fulfilled. Increase 

 of vulnerability is caused partly by an innate deterioration of 

 the body and partly by the accumulation of an actuarial 

 debt as a result of recurrent exposure to injuries that leave a 

 persistent mark. The separation of the latter contribution 

 from the former should depend upon the context in which the 

 phenomenon of senescence is being studied. 



REFERENCES 



Allee, W. C, Emerson, A. E., Park, O., Park, T., and Schmidt, K. P. 

 (1949). Principles of Animal Ecology. Philadelphia: W. B. Saun- 

 ders Co. 



