74 MAN THE ANIMAL 



Persistent living during these two periods has not only bio- 

 logical justification but is absolutely imperative in a statistical 

 sense. For the third period there is no evident biological justifi- 

 cation. The preservation in life of individuals who have passed 

 the reproductive period is a biological and social luxury that 

 we encourage only because civilized man is the kind of animal 

 that likes this particular form of vital extravagance. Among 

 lower animals and primitive men the existence of a post- 

 reproductive period of life is only, and strictly, on a permissive 

 basis rather than one of solicitous promotion as with us. Bio- 

 logically life after reproduction is ended is only the expression 

 of a sort of vital momentum. The machinery of living having 

 got well going in infancy and maturity it keeps on by sheer 

 momentum for various lengths of time after any valid bio- 

 logical reasons for its continuance have ceased to operate. 



The relative or percentage distribution of the life span of 

 present-day civilized man between these three portions is, in 

 round figures, about as follows: roughly 21 per cent of the 

 whole span is spent in the infantile period as here defined, 53 

 per cent in the reproductive period, and 26 per cent in the post- 

 reproductive period. At first glance more than a quarter of the 

 whole of life seems to be an excessively long portion to be spent 

 in the running down of the machinery after the main business 

 of life is finished. It is, however, in no way exceptional or pe- 

 culiar to the human species. As an example, we have been able 

 to show by life table analysis that a moth, the pecan nut case 

 bearer Acrobasis caryae Grote, spends about 23 per cent of its 

 total adult life span in the post-reproductive period. And this is 

 a form whose total adult life span from emergence to death is 

 only from six to eight days long. But even with so short an 

 absolute sojourn for Acrobasis ^ it and civilized man both agree 

 in taking about the same proportionate time to make their 

 exits from their respective nutty worlds after they have finished 

 biologically justifying their existences. 



But the social implications and consequences of the three- 

 fold partition of the life span are very different in Acrobasis 

 and man. What makes them so widely dtfferent is because man's 



