2 



THE DISTRIBUTION OF SENESCENCE 



2-1 Character of the Evidence 



To find out which animals exhibit an increasing mortality with 

 increasing age, we should ideally keep large numbers of each 

 species, or of representative species, from birth to death, under 

 optimal conditions of captivity. In point of fact, apart from the 

 impracticability of keeping any significant number of species in 

 this way, the results would be both artificial and potentially 

 misleading. It is possible to invent about animal senescence a 

 paradox rather analogous to the principle of physical uncer- 

 tainty: it is Virtually unknowable' or, in other words, mean- 

 ingless to ask, whether certain organisms are 'susceptible to 

 senescence', because the organism is biologically dependent on 

 its environment: in the wild state these forms never normally 

 live long enough to reach senescence, while domestication or 

 protective interference with the environment brings about 

 changes in physiology and behaviour which produce effectively 

 a different organism. The object of the paradox is to point out 

 the fruitlessness of argument over 'potential' behaviour which 

 is practically unrealizable. Almost all our detailed knowledge 

 of senescence comes either from the observation of man, or of 

 domestication-artefacts such as the laboratory mouse or the 

 laboratory strains of Drosophila. In the wild state it is most 

 unlikely that any species of Mus or of Drosophila reaches old age 

 with sufficient regularity to be subject to study. In most cases 

 we are creating for study a state which has no part in the life 

 cycle as it has been shaped by evolution, but is at most a 

 potentiality. This must be taken into account on every occasion 

 when theories of the evolution of senescence are being based on 

 the appearance of senescence in domestic animals. 



42 



