48 MAN THE ANIMAL 



sible. For the will to live is the most deeply rooted and per- 

 sistent of the biological characteristics of protoplasm organized 

 into individuals. At the beginning of each persons's life this 

 urge to survival is wholly unconscious, just a part of living 

 like digestion or respiration j later on this underlying proto- 

 plasmic will to live, the vital momentum, will be supplemented 

 in the individual by a conscious effort for longevity. The great 

 part of today's babies who manage to survive until they are 

 somewhere in the twenties, will then begin to think a little about 

 what they should do to preserve their health so that they may 

 keep on living longer. Virtually all of them who live until they 

 are seventy years of age or upwards, will think about little else 

 from that time on. For it is an odd but profoundly true generali- 

 zation of human biology that the longer a human being has 

 lived, the more anxious and personally concerned he is, by and 

 large, to keep on living still longer. 



II 



The duration of the journey through life that so many 

 young hopefuls have started upon today will vary greatly 

 amongst them. Some of the lot will end it tomorrow, so incom- 

 plete is their vital resource and so fragile their design for living. 

 Others, a very few others, of the lot will be living a hundred 

 years from now. 



The pattern of these varying life journeys, and the changes 

 in that pattern in quite recent years, are matters of considerable 

 interest and worth looking into on their own account as well 

 as to give us a solider ground for the further discussion of human 

 longevity. The "order of dying" of a cohort of individuals all 

 born at the same time is given with great accuracy by a device 

 known as the life table, that combines mathematics and biology 

 in a happy and useful mating. For purposes of the present dis- 

 cussion a certain function called the "survivorship" for two life 

 tables has been put in a graphic form, on the supposition that 

 the life journey of which we have been speaking consists in 

 climbing a long and huge ladder. The first of the two life tables 



