Definition and Measuremi:nt of Senescence 11 



the use of inbred strains or their F^ liybrids can eliminate this 

 sx)urce of error. 



None of the five objections so far raised challenges the 

 pririciple of using vulnerability as a measure of senescence, 

 though they make it very evident that grave difficulties stand 

 in the way of its practical use. They are difficulties which 

 apply with particular force to human beings — in this respect, 

 as in so many others, a tiresome and anomalous animal, 

 rightly excluded from the syllabus of studies in most depart- 

 ments of zoology. We may now consider two genuine short- 

 comings of principle, one of which is reparable, the other not. 



The first is that senescence may reveal itself by a true and 

 radical deterioration that is not accompanied by any increase 

 in the likelihood of dying, viz: a deterioration of reproductive 

 power. Should not any measurement of senescence take 

 reproductive capacity into account? 



The question is pertinent, because in human females 

 reproduction comes to an end over an epoch of life that is not 

 distinguished by any dependent change in the likelihood of 

 dying. (On the contrary, one cause of death, in childbirth, 

 has been outlived.) It may have less bearing on other animals, 

 in which reproduction, instead of stopping rather suddenly, 

 merely slows down in step w^ith other senescent changes. A 

 more fundamental measure of biological "efficiency" — of an 

 animal's power not merely to maintain itself as a going con- 

 cern, but to perpetuate its kind — should have regard to both 

 mortality and fertility. Several such measures suggest them- 

 selves, of which the most useful might be the total future 

 expectation of offspring per head from a chosen age x onwards, 

 viz: the reproductive value 



1 r* 



— p^b^.dx, 



where b^ is the age-specific birth rate, and p^ (=lxlto) repre- 

 sents the probability of living to age x (or, in effect, the value 

 of /^ when /<, is taken to be unity). 



