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THE NATURE AND CRITERIA OF 



SENESCENCE 



1 • 1 Measurement of Senescence 



Senescence is a deteriorative process. What is being measured, 

 when we measure it, is a decrease in viability and an increase 

 in vulnerability. Other definitions are possible, but they tend 

 to ignore the raison d'etre of human and scientific concern with 

 age processes. Senescence shows itself as an increasing prob- 

 ability of death with increasing chronological age: the study of 

 senescence is the study of the group of processes, different in 

 different organisms, which lead to this increase in vulnerability. 



The probability that an individual organism which has sur- 

 vived to time x will die before time x + 1 depends on the rate 

 of mortality (q) per 1000, meaning the number, out of 1000 

 individuals living at time x, who have died by time x + 1 . The 

 force of mortality (fi) is given at any age x by 



dn — d . 

 * = ~ n dx = ~dJ ln - n 

 where n is the number of individuals which have survived to 

 age x. 



In most organisms, the likelihood of dying within a given 

 period undergoes fluctuations, often large, throughout the life 

 cycle. Senescence appears as a progressive increase throughout 

 life, or after a given stadium, in the likelihood that a given 

 individual will die, during the next succeeding unit of time, 

 from randomly-distributed causes; the pressure of the environ- 

 ment, which it has successfully withstood in the past, it now 

 ceases to be able to withstand, even though that pressure is not 

 increased. It is rare that we can determine the vulnerability of 



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