4 A. Comfort 



A great many theories of senescence were devised in the 

 past on the assumption that the loss of vigour with age must 

 involve a single "fundamental" or "inherent" cellular, 

 chemical, or mystical process common to all multicellular 

 animals, contrasting with the extremely ill-named "immortal- 

 ity" of protozoa, and possibly generalizable to include in- 

 organic substances, species in phylogeny, and whole human 

 societies as well. The study of ageing has suffered very heavily 

 from superficial and abstract analogies of this kind, most of 

 them, like the analogy to mechanical wear, grossly misleading 

 in their application. In fact, while some of the assumptions of 

 Weismann and his contemporaries about senescence in 

 metazoa may eventually prove correct, few if any can be taken 

 for granted in comparative work on ageing. It now seems 

 likely that there are multicellular animals which are capable 

 of indefinite somatic cell replacement; in unicellular animals 

 such as suctorians, where mother and daughter cells can be 

 identified at fission, the age-status of the products is not 

 identical, and individuals have a definable lifespan which can 

 be altered experimentally (Rudzinska, 1952, 1955). This 

 might be the case in other acellular animals. It is not possible 

 to assume that all vertebrates undergo senescence — one of 

 the most interesting comparative problems is to find out 

 whether they do so or not. It is not possible to assume that 

 the underlying pattern of senescence is the same in all mam- 

 mals. I draw attention to these assumptions because papers 

 which make them are continually published; many of them 

 are in substance the research problems which comparative 

 studies could properly be directed to settle. It may well be 

 that broad generalizations about the nature of senile change 

 and its relation to growth and nutrition can be based on the 

 behaviour of rotifers, or the effects of vitamins on the lifespan 

 of insects, but it may equally well prove to be otherwise, in 

 which case a great deal of effort could be wasted. 



If we list the apparent causes of decline in the vigour of 

 animals with increasing age, the processes included in that 

 definition are evidently multiple. There is a theoretical 



