THE UNIQUENESS OF THE INDIVIDUAL 



a man who has just celebrated his eightieth birthday anni- 

 versary is no more or less likely to celebrate his eighty-first than 

 is a seven-year-old to celebrate his eighth. In my diagram, the 

 force of mortality, being constant, would appear as a straight 

 line parallel to the axis defining age. 



In real life it is far otherwise. As Fig. 2 shows, the vulner- 

 ability of newborns is, not unexpectedly, very high; not until 

 nearly the seventieth year of life does it become so high again. 

 The curve of the force of mortality falls precipitously to a 

 minimum around age twelve and then climbs upwards, slowly 

 at first and latterly much faster. Age twelve (or thereabouts) 

 is therefore the actuarial prime of life; at twelve one is more 

 likely than at any other age to survive one further year, or 

 month, or minute. But notice the smoothness of the curve that 

 defines the force of mortality in later life. There is no break or 

 singularity to give evidence that at any later age development 

 and maturation are at least completed and that deterioration 

 then sets in. Any complete theory of the origin and evolution of 

 senescence must explain the smoothness and coherence of the 

 curve of increasing vulnerability. It is not quite good enough 

 merely to think up reasons why very elderly animals should 

 die. 



Because there are clearly special reasons why baby animals 

 should be more vulnerable, though no less charged with vitality 

 than their elders, I am proposing to neglect the arc of the curve 

 of the force of mortality that lies before its minimum, but to 

 use its later stretch as a measure of the degree of senility. This 

 is a decision that cries aloud for qualifications and reservations, 

 and it is part of my purpose to reveal what some of these 

 may be. 



You will notice first that although the force of mortality may 

 purport to measure a process that happens in the life of an 

 individual animal (decline of "vitality", or what you will) it is in 

 fact founded upon the age-frequency distribution of a single 



50 



