HUMAN LONGEVITY 73 



rapid the pace of living is, the shorter the time that life endures. 

 This relationship has been shown to exist for a variety of forms, 

 including plants, various lower animals, insects, and men. It is 

 a relation that is obviously in some degree within the power 

 of individual, personal control. 



The study of longevity is not ended. In the view of the biolo- 

 gist it has only just got well under way. A great deal more 

 will be learned, and just possibly we may find out how to 

 lengthen significantly and at will the span of human life, in- 

 stead of merely increasing its average duration. But if and 

 when this happens there will immediately become evident some 

 of the difiiculties inherent in another set of problems of social 

 biology, different in kind from those of longevity fer se. Let 

 us now give some consideration to these other problems. 



VI 



Up to this point the discussion has been of the total duration 

 of life of man considered as a whole. But in actual fact the life 

 span of man from birth to death falls into three natural bio- 

 logical divisions. The fact itself that there Is such a natural di- 

 vision, and the nature of the parts themselves into which the 

 total span divides itself, are phenomena of great significance, 

 and particularly so in relation to their social consequences, both 

 superficial and fundamental. These three divisions of the life 

 span may be conveniently and justly labelled as (I) the m- 

 fantile fre-refroductive feriody extending from birth up to fif- 

 teen years of age, as a round figure j (II) the reproductive 

 feriody extending from fifteen years up to about fifty years; 

 and (III) the senescent fost-refroductive feriody extending 

 from about 50 years to the end of the life span. The first two 

 of these periods are obviously of first-rate importance in the 

 business of human living, philosophically viewed. For if the 

 vital fires are to be kept perpetually alight the young must be 

 nurtured until they are ready to reproduce. Also the reproducers 

 must keep busy about their breeding as long as they retain the 

 capacity, if the continuance of the race is to be safely assured. 



