7 



CONCLUSION 



We have now briefly examined some of the evidence which 

 requires to be considered, and some of the questions which 

 require to be answered, in attempting to understand animal 

 senescence. We have seen, in particular, that many organisms 

 appear to have been provided by evolutionary selection with a 

 'programme' of development and function which is directional 

 and finite, and that progressive loss of the power to remain in 

 stable function occurs towards the end of that programme. 

 Weismann suggested that senescence is itself a functionally- 

 determined item in the programme: it seems more probable 

 that as the contribution of successive age groups to the next 

 generation of progeny is reduced by natural causes, so the 

 selection-pressure declines, and the efficiency of the homo- 

 eostatic mechanisms with it. The organism ultimately dies of 

 old age because it is now an unstable system which is pro- 

 vided with no further sequence of operational instructions, and 

 in which divergent processes are no longer co-ordinated to 

 maintain function. 



In some cases the system fails suddenly, at a fixed point, after 

 the pattern of the senescence of rotifers or red blood corpuscles. 

 Some such cases apparently depend on the existence of cell 

 constituents renewable only by division. In mammals the 

 decline of resistance and the rise of the force of mortality are 

 gradual and smooth, and agree well with the probable shape 

 of a curve representing the declining efficiency of the evolu- 

 tionary pressure towards survival at different ages. 



Insofar as any general theory of senescence is justified, this 

 seems at present the most plausible. It is probably as unprofit- 

 able to discuss the 'cause' of ageing as to discuss the 'cause' of 



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