The Mechanisms of Senescence 



in which long-term trends in cell specificity are themselves 

 controlled by the signals involved in the homoeostatic process — 

 as if the system A ^ B were fitted in a vehicle whose move- 

 ment it controlled, but which travelled into a hotter and 

 hotter environment, thereby upsetting the characteristics of A 

 andB. 



These highly complex hormonal homoeostatic systems are of 

 the greatest importance in higher vertebrates, although the 

 problem of 'three-dimensional' homoeostasis, or homoeostasis 

 superimposed on morphogenesis, is general in all developmental 

 physiology. The views of Minot (1908) upon 'cytomorphosis' 

 (differentiation and maturation) as a cause of senescence carry 

 the very important, and at first sight very probable, inference 

 that no complex organism, and certainly no vertebrate, can 

 remain in an indefinitely stable equilibrium. Where growth 

 processes and differentiation are superimposed on homoeostasis 

 they are analogous to 'drift' in a control system — on this basis 

 any system of differential growth must tend to increasing dis- 

 equilibrium, unless the developmental 'drift' itself modifies the 

 homoeostatic system to keep them appropriate to the altered 

 situation. 



6-2-2 GONAD-PITUITARY SYSTEM 



Senescence of the gonad regularly precedes or accompanies 

 senescence of the owner in a number of phyla — so much so that 

 declining reproductive capacity is a token of senescence almost 

 as valuable as the direct measurement of increased force of 

 mortality. The relation of gonadal senescence to somatic sen- 

 escence has clearly much evolutionary interest, since once the 

 first is complete, in an organism, to the point at which that 

 organism's contribution to posterity is no longer statistically 

 significant, any further adverse change in viability is generally 

 speaking inaccessible to the influence of natural selection, except 

 in a very roundabout way. 



In almost all litter-bearing mammals, a decline in litter size 

 is characteristic of senescence. The long post-reproductive period 

 found in women is exceptional in mammals. It probably repre- 

 sents a genuine biological difference, quite apart from the far 

 greater perfection of the techniques for keeping human beings 



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