126 George A. Sacher 



200 kilocalories per gram. The evidence adduced by 

 Rubner was essentially that in Table I. The lifespans he 

 attributed to some species in the table are out of line with 

 currently accepted values, and introduction of the more 

 accurate values would considerably weaken his evidence. 



Although Rubner' s own evidence is hardly adequate to 

 support his thesis, the results of the present analysis indicate 

 that his conjecture nevertheless has considerable merit. His 

 hypothesis may be reformulated to say that the lifespan of a 

 species varies inversely as its basal metabolic rate or, in the 

 notation employed above, 



X = — 1-OOm + constant (11) 



Equation (10) shows, however, that the empirical relation 

 between these variables is numerically 



X = — 0'75m + constant (12) 



The significance of the difference between the theoretical and 

 observed coefficients cannot be tested rigorously, so we cannot 

 say whether this difference is in fact significant. However, 

 Rubner's hypothesis that lifespan varies inversely as the first 

 power of the metabolic rate is in any event subject to quantita- 

 tive revision, for he considered only the basal metabolic 

 energy. There is no good reason to distinguish between the 

 resting and active energy expenditure with respect to their 

 effects on length of life. Furthermore, the relation of active 

 energy expenditure to body size is not accurately known. 

 Hence, it can only be concluded at present that the empirical 

 findings are in accord with the general hypothesis that the 

 attainable length of life of a mammalian species is dependent 

 in part on its rate of energy dissipation. 



Rubner's original discussion of the energetic theory called 

 attention to the fact that the lifetime energy expenditure for 

 man is seriously out of line with the values calculated for 

 domestic animals (Table I). This discrepancy was in fact one 

 of the considerations that inspired me to undertake this 



