The Biology of Senescence 



being that the senile decline in resistance in mammals is not a 

 sudden process, as it is in the rotifer, but a smooth rise in the 

 force of mortality beginning at an early stage. Mechanical 

 timing devices produce as a rule a single event after a fixed 

 programme, not an increasing probability throughout the pro- 

 gramme, though this objection does not hold good for analogue 

 computing systems: it is relatively simple to devise an electronic 

 taximeter-bomb in which the probability of an explosion in- 

 creases with the increase of time and distance, or a system in 

 which the steering of the taxi becomes increasingly impaired 

 as the 'programme' continues. A far more serious objection is 

 that in ordinary taxi-meters time and distance are not normally 

 interlocked, as growth and differentiation, though experi- 

 mentally separable, are interlocked in the developing animal. 

 It has been suggested that the two processes are in some degree 

 mutually exclusive (Bertalanffy, 1933, 1941) — a conception 

 which goes back to Minot. It seems probable that in most 

 organisms it is the component of differentiation, not that of 

 mere growth, which is responsible for senescence. 



Analogies, in any case, are mostly of use as teaching-illustra- 

 tions. In the final analysis, senescence, even if it never reaches 

 the ideal state of being expressed as a sequence of chemical 

 reactions and equilibria, must presumably be reducible to a 

 series of definite processes — such-and-such a mechanism leads 

 to the loss of dividing-power in such-and-such cells, which then 

 have a life-span limited by the non-renewability of their enzymes 

 to so many chemical operations, after which they deteriorate 

 with the following consequences. We are nowhere near such a 

 picture of any one senile or developmental process in any 

 organism, let alone of mammalian senescence or morphogenesis 

 in general. A certain amount of experimental evidence has, 

 however, accumulated — enough to indicate the directions in 

 which further research might profitably be directed. 



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