The Distribution of Senescence 



'showed every appearance of being young animals', but other 

 wild specimens taken when full-grown had been kept for 20 years 

 without decline of vitality or reproductive power. Hildebrand 

 placed the maximum life-span for this species at 40 years or 

 more, but evidence from other small terrapins suggests that this 

 may be a considerable underestimate. Although the only aquatic 

 species which is known to have reached an age comparable with 

 that of the land tortoises is Emys orbicularis, it would be very 

 difficult to argue upon the existing evidence that specific size 

 and determinate age are correlated in chelonians. Contrary to 

 Bidder's hypothesis, specific size here seems to be an adaptation 

 to carnivorous life in small pools, while continuing growth is 

 found in land tortoises and marine turtles (Parker, 1926, 1929). 

 Crocodiles have also been credited with indeterminate growth 

 — 'Crocodili perhibentur esse admodum vivaces, atque grande- 

 scendi periodem itidem habere insignem; adeo ut hos solos ex 

 animalibus perpetuo, dum vivunt, grandescere opinio fit. . . . 

 At de aliquo testaceo genere, nihil certi, quod ad vitam ipsorum 

 attinet, reperimus' * (Hist. Vitae et Mortis) . Claims of longevity 

 are based on the exceptional size of some specimens. Large 

 alligators have been observed in captivity to remain for 25 years 

 in a non-growing state, e.g. Alligator sinensis (Dathe, 1935), 

 though the difficulties of accurate length-measurement are 

 evident. 



2-5 Distribution of Senescence in Invertebrates 



Among invertebrates not only is there a demonstrable variety, 

 greater than in vertebrates, in the nature of the preponderant 

 senile process, but we have also the full range, from indeter- 

 minacy to very sharply defined determinacy of life-span. The 

 gaps in our knowledge of life-cycles are so large that we cannot 

 yet picture the distribution of senescence in invertebrate phy- 

 logeny: papers entitled 'The life-history of . . .' only very 

 exceptionally include reference to the senescence of the species 



1 'Crocodiles are held to be very lively, and to have a notable span of 

 growth — so that they alone of beasts, so opinion runs, grow so long as they 

 live. . . . But of any hard-skinned beast, as pertaining to their length of life, 

 we find nothing certain.' 



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