The Distribution of Senescence 



learned by the collection of actuarial data for guppies subjected 

 to different programmes of growth: this work is in hand, but it 

 is in the nature of the subject that life-table making cannot be 

 hurried, and life-tables under various conditions of growth were 

 not ready for inclusion here. The growth curves shown in 

 Fig. 18b are instructive, however, when we compare them with 

 the closely similar growth-behaviour and pseudospecific size of 

 wild trout. 



It seems probable that there is as much variation in 'sen- 

 escence' as in growth-patterns among teleosts. Some forms 

 apparently resemble monocarpic plants, mortality being linked 

 to reproduction. Some, in captivity, have a life-span determined 

 by senescence, their mortality increasing with age on a curve 

 closely similar to that of mammals. Some forms, however, may 

 conceivably have an effectively indeterminate life-span, and are 

 not at present known to undergo any form of senescence, repro- 

 ductive or general, other than the accumulation of injuries, 

 though this may well mean only that their 'determinate' maxi- 

 mum, as in wild birds, comes so late in relation to mortality 

 as never to be reached in practice. The effect of variation in the 

 growth-rate upon the development of senescence has yet to be 

 determined. 



2-4-2 REPTILES 



There are no published reptilian life-tables, but a number of 

 careful studies of reptilian growth have been made (Sergeev, 

 1937; Townsend, 1931, 1937; Cagle, 1946). By collating these 

 with maximum age records, a good deal of significant informa- 

 tion can be obtained. Sergeev found that while, in all reptiles, 

 early growth depends on environmental conditions, being some- 

 times very rapid, and growth rate declines with increasing age, 

 there are a number of forms where both sexes have an effective 

 specific size which is reached early in life, and after the attain- 

 ment of which no further growth occurs. The cessation of 

 growth in these forms is apparently as definitive as that in 

 mammals, and its timing does not appear to depend on the 

 arrival of sexual maturity. There appears to be no close correla- 

 tion between either of these two patterns of growth and the 

 length of the life-span. 



77 



