Growth and Senescence 



tained, has not been carried out, although there is some evi- 

 dence that it may be experimentally feasible. We cannot yet 

 identify any single process which, by its failure, produces the 

 senile decline of homoeostasis in mammals. It is however pos- 

 sible to treat the developmental sequence leading to senescence, 

 in its relation to growth and to differentiation, as an 'integrating 

 system', of the type employed in various calculating and timing 

 devices. 



The most familiar example of such a system, functioning as a 

 calculating device, is the taxi-meter. This machine records time 

 when the taxi is stationary, and distance, or time and distance, 

 when it is moving. The real taxi-meter does so upon an 'open- 

 ended' scale, the amount of the fare which can be rung up on the 

 the dial being theoretically unlimited, since the dials after 

 reaching £99 \9s. \\\d. return to zero. For the purpose of our 

 argument, the biological taxi-meter has been adapted by an 

 anarchist to produce an undesirable result when a particular 

 fare is reached— say £10; or, more correctly, an increasing prob- 

 ability of this result as £10 is approached and passed: an increas- 

 ing impairment of the brakes and steering would be a suitable 

 device. The meter records one shilling per minute, so long as 

 the taxi is stationary, and half a crown per mile plus one shilling 

 per minute so long as it is moving. In this case, if the journey 

 never begins, the impairment will take place eventually, though 

 not for a very long time. For an extended biological analogy it 

 is probably better to take the case in which the conditions of the 

 impairment reaching a disastrous stage are, first, that the fare 

 shall reach £10, and second that the taxi shall have travelled 

 at least a short distance from its starting point. 



The question we have to ask is this: does mammalian sen- 

 escence effectively resemble such an integrating system, in 

 which differentiation is the higher-scoring and the essential 

 component, but in which retardation leading to continuance 

 of growth directly or indirectly delays the point at which sen- 

 escence appears; or does cessation of growth itself, whether it 

 arises from some active mechanism of size limitation or through 

 the attainment of an equilibrium state, directly cause the senile 

 deterioration? The crude application of the calculating-machine 

 or the time-fuse analogy has many objections, the chief of them 



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