766 LIFE : OUTLINES OF GENERAL BIOLOGY 



illumination have very diverse effects, sometimes retarding and 

 sometimes accelerating growth. 



{b) Functional. — ^The habits of the animal may influence the 

 life-curve, and this finds a familiar parallel in the way man ages 

 or keeps young according to his tasks and ploys. A strenuous roving 

 creature like the otter, with its several homes and frequent journeys 

 between, remains singularly young, and even playful, throughout 

 its adult life. Perhaps the youthfulness and joyousness of most 

 birds may be in part correlated with the migratory habit, with its 

 stimulating changes and its double summer in the year. When adult 

 life in insects is entirely devoted to reproduction — and some could 

 not open their mouth even if they would — the period of maturity is 

 bound to be short. 



(c) Organismal. — ^The quality of being long-lived is sometimes 

 demonstrably hereditary in human lineage, just like the similarly 

 subtle quality of fecundity. So the shape of life's trajectory may be 

 correlated with the inborn constitution, and the lengthening out 

 (or the shortening down) of different arcs in the curve may be 

 correlated with time- variations in the activity of the endocrinal 

 glands — though these time- variations have also to be accounted for 

 in due course. As the hormone-output of the supra-renal gland is 

 intimately correlated with emotional disturbances, such as anger 

 and fear, it is not so far-fetched as it may seem to ask whether 

 psychological, as well as physiological, factors do not operate in 

 altering the life-curve. As was wisely said of old time, "A merry heart 

 is a continual feast", and "A merry heart is the life of the flesh". 



It is truistic to say that the form of the life-curve will tend to be 

 congruent with the organic constitution, but it is possible to give 

 the idea some concreteness. Thus a very steady growing, without the 

 usual limit of growth, is seen in many fishes and reptiles, which 

 live a markedly anabolic life with abundant nutrition, with resting 

 habits, and with rather cold-blooded reproductivity. The most 

 prolonged maturity known among living creatures is that of the 

 "Big Trees" or Sequoias, some of which have lived for 3,000 years, 

 and they are obviously quietlj^-living, often resting, highly anabolic 

 types, with an enormous income in relation to their expenditure. 

 This holds for some centenarian animals as well, such as elephants 

 and giant tortoises, but we are by no means denying that the same 

 result may be attained along other lines, as the strenuous prolonged 

 maturity of the eagle well shows. 



But while we regard this physiological reading of the life-curve 

 as primary, even though it be still tentative, it requires to be supple- 

 mented by an attempt to interpret the diverse forms as each and all 

 adaptive to particular circumstances. There is an ecological signifi- 

 cance — and, more generally, a survival value in all the differences 

 of time and tune. 



