OLD AGE AND NATURAL DEATH 



reach a moderate though certifiable degree of seniHty. ^As a 

 matter of fact, the contribution that senescence makes to 

 accidental death can be deduced with reasonable accuracy 

 from the mathematical character of the actuary's life table. 

 For if the '"force of mortality** were constant and independent 

 of age; if, that is to say, the chances of dying were the same in 

 the age interval 100-101 years as in the interval 10-11 years; 

 then the curve defined by the life table would be of the familiar 

 die-away type that describes, for example, the loss of heat from 

 a cooling body. But no life table has yet been made for a 

 mammalian species in the wild. All that can be said so far, in 

 the spirit of Lankester'*s generalization, is that some mammals 

 do not appear to live that long. Hinton''s studies^ on fossil and 

 recent voles of the genus Arvicola showed that ""not only are the 

 molars still in vigorous growth, but the epiphyses of the limb 

 bones are still unfused with their shafts. Apparently, that is as 

 far as actual observation goes, voles of this genus are animals 

 that never stop growing and never grow old. But no doubt, if 

 one could keep the vole alive in natural conditions, but secure 

 from the fatal stroke of accident, a time would come when . . . 

 the animal would become senile and die in the normal manner.' 

 Burt's study^ of mice of the genus Peromyscus led to a similar 

 conclusion; but there, so far as I know, the matter stands. The 

 difficulties of constructing life tables for animals in the wild are 

 technically formidable, but they must be solved.* 



From the standpoint of evolutionary biology an animal's 

 expectation of life in its natural surroundings is much more 

 significant than the degree of decrepitude to which it may be 

 nursed in laboratory or zoo. It is a fair guess that much of what 



1 M. A. C. Hinton, Monograph of the Voles and Lemmings, Vol. 1, p. 48, 

 British Museum, London, 1926. 



2 W. H. Burt, No. 48 in Miscellaneous Publications of the Michigan 

 University Museum of Zoology, May, 1940. I must thank Mr D. Chitty 

 for this reference. 



* [As they are beginning to be: see the literature cited in note 1, p. 22.] 



c 33 



