Ageing and Death 153 



occurring at the end of the reproductive period. 



If this is correct, it would seem that the life span is determined by 

 the interplay of two effects— the necessity of living long enough to 

 start off the new generation and, having performed this task, the fact 

 that a further life time is unnecessary and, in many respects, harm- 

 ful to the well-being and development of the species. It is quite pos- 

 sible that mechanisms exist in organisms which bring about this 

 limitation of the life period, when the biologically useful period is 

 over, but we do not know what these mechanisms are. 



We can conclude that most organisms have not only a mechanism 

 for reproduction and growth, but also an ageing mechanism which 

 automatically brings life to a gradual end when the useful period 

 is over. We must regard this ageing mechanism as 'built in' to the 

 cells as an essential feature of their construction, a kind of bio- 

 logical clock with a time scale which is characteristic of each species. 

 We do not know where the clock is located. It has often been thought 

 that ageing is brought about by the failure of one or other of the 

 endocrine glands and attempts have been made to produce rejuvena- 

 tion by grafting glands or injecting glandular extracts, e.g. from the 

 testicles of juvenile animals. These treatments have not had more 

 than a very limited success. Although they are capable of supple- 

 menting to some extent the operation of failing glands, they do not 

 prevent the ageing process as a whole. 



It might be said that these considerations do riot apply to mankind, 

 which is no longer subject to evolutionary change. While it is true that 

 over the historic period no definite evolutionary trends seem to have 

 been noted, we cannot be certain that evolution is not actually taking 

 place or that selection of certain types is not actively occurring. How- 

 ever, in human communities, factors other than simple reproduction 

 and survival assume a much greater importance than in animal com- 

 munities. It can be argued that survival of the community de- 

 pends on qualities such as 'wisdom', i.e. the correct interpretation of 

 experience, in which age is a definite advantage. It can be argued and 

 was argued for example by Bernard Shaw that human beings usually 

 die or become senile at an age when their experience would be of 

 great value to the community and also before they have made use of 

 their full potentiality. However, Shaw's picture of life among the 

 ancients is not' a very attractive one and not many people would re- 

 gard it as particularly desirable to Hve to 150 just to sit and contem- 

 plate. 



Life in an aged population would really be very static, unless 

 human nature changed a good deal. Even now most of the initiative 

 and new ideas come from the comparatively young. It would seem 



