The Distribution of Senescence 



continue throughout life to increase tangibly in size, given suit- 

 able conditions, and forms where the maximum size is reached 

 relatively early in life, is fixed for the species, and does not 

 increase further with increasing age even under the most 

 favourable conditions. The chief obstacle to wide generalization 

 about the determinacy or indeterminacy of growth in lower 

 vertebrates, and in other forms such as pelecypods, lies in the 

 fact that arrest of growth at an apparent specific size can be 

 brought about by environmental conditions. In some cases 

 growth can be resumed after such an arrest — in others, appar- 

 ently, it cannot. Even less is known of the effect of these pheno- 

 mena on the life-span than is known of the normal ageing of 

 such forms. Differences are also substantial within each of the 

 main groups of poikilo thermic vertebrates. In many reptiles and 

 small fish, continued growth after a relatively early age is no 

 more evident than in the male rat. In amphibia, 'many species, 

 particularly some tropical forms, seem to have an absolute 

 size, which the males soon attain, but this does not hold for 

 many salamanders, nor for some Northern frogs' (Noble, 1931). 

 In many cases the male has an absolute size and the female 

 has not. If enough data were available, the variety of growth- 

 patterns is more than sufficient to test Bidder's hypothesis — 

 unfortunately, corresponding data upon age/mortality rela- 

 tions are almost entirely lacking. 



The idea of a 'self-maintaining' vertebrate is not impossible 

 ex hypothesis It is in fact what we should expect if growth- 

 cessation is an equilibrium process, if there is no important pro- 

 cess of differential growth at work, and if there is no qualitative 

 change in the regenerative power of cells throughout adult life. 

 It is not self-evident, though it might be true, that an animal 

 should be obliged to increase in size in order to retain the 

 power of carrying out running replacements. It seems reason- 

 able for our purposes to regard an animal of 'indeterminate' 

 growth as one in which the probability of nursing an individual 

 to the point at which increase in somatic size has ceased is 

 infinitely small, and an animal of 'indeterminate' life-span as 

 one in which the survival rate under favourable conditions is 

 substantially independent of age, however long a population 

 of that animal is observed from birth. 

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