The Mature and Criteria of Senescence 



as a result of some factor which also causes that decline. The 

 decline is not a measure of senescence in its actuarial sense, 

 since it does not run parallel with the force of mortality, and 

 it would not be even an obligatory precursor of senescence if 

 mortality only increased with age in those animals whose 

 capacity for renewal or growth in some or all tissues has fallen 

 to zero. It is with actuarial, deteriorative senescence that we are 

 here concerned — if senility implied only the decline of growth 

 rate in man, it would cause little public concern. 



It is possible that future work will produce a workable and 

 justifiable 'direct measure' of senescence in individuals, based 

 on the time-lag in cell-division of tissue explants derived from 

 old animals (Gohn and Murray, 1925; Suzuki, 1926; Medawar, 

 1940). No practical test of this kind has yet been developed, 

 however — meanwhile the critical observations on the distribu- 

 tion of senescence in vertebrates have almost all to be made 

 upon organisms (large fish, crocodiles, tortoises) where life- 

 table studies are out of the question. In these forms it is rela- 

 tively easy to observe histologically or by a mating test, the 

 degree of reproductive power persisting in individuals of known 

 age. Inferences based on 'reproductive' senescence are there- 

 fore easy to draw, compared with the insuperable difficulties 

 involved in measuring the force of mortality in such cases. 



Reproductive decline is a very general feature of those vertebrates 

 which undergo senescence as judged by the increasing force 

 of mortality: its evidences in various forms include gonadal 

 changes, loss of secondary sexual characters, cessation of ovarian 

 cycles, and a fall in sperm production, fertilizing power, hatch- 

 ability, litter size and viability. These changes follow a time 

 scale which is different from that of the increase in force of 

 mortality, however, and which bears no constant relation to 

 that increase in different species. The gonad often appears to 

 behave as an 'organism' having its own determinate life-span, 

 but this is equally true of other structures, such as the thymus. 

 The limited life of the gonad is in a special category only 

 because, in terms of evolutionary teleology, the gonad is the 

 significant part of the organism. Ageing of the whole organism 

 after a prolonged post-reproductive period is a process which is 

 realized only by human interference, at least so far as most 



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