4°4 



THE QUARTERLY REVIEW OF BIOLOGY 



sistent and accordant results will be ob- 

 tained if inherent vitality, the manifesta- 

 tions of which are dynamic expressions 

 of the biological organization of the in- 

 dividual, be measured by observation 

 either of growth or of duration of life. 



The negative sign of the correlation 

 between growth rate during the growth 

 period and total duration of life means 

 that the faster the rate of growth the 

 shorter the duration of life, and vice versa. 

 This result confirms experimentally the 

 point of view expressed some years ago 

 in my book The Biology of Death, where it 



Entirely independent experimental con- 

 firmation of this result is furnished by 

 the work of MacArthur and Baillie on the 

 mortality of Daphnia magna. They find 

 that 



The relative longevity of the sexes may be ex- 

 pressed as varying inversely with their average rates 

 of metabolic activity. The average age at death for 

 males was 37.8 and for females 43.53 days, the dura- 

 tion of life for females exceeding that for males by 

 14.6 per cent. Now in metabolism, as estimated by 

 heart rate, the males surpass the females by nearly 

 the same percentage, and the product: heart beats 

 per second (weighted average through life) X 

 average age in days at death = a constant, e.g. 



Mean of Whole Experiment* 100% 





Seedlings 



that 



lived I* 



days 



Seedlings 



that 



lived/5 



days 



Seedlings 



that 



lived is 



days 



Total duration of life 



95%*^ 



-^J02%^ 



^»I09% 



CO* rate per day of growth 

 during growth period only 



104% * 



^^^^^^ 



^-^ 81% 



Dry matter metabolized 



I05%«^\ 



//<#7%\ 



^^ 87% 



Water metabolized 



KH%r 



100% 



>• 88% 



Fig. io. Chart Showing Results of Seedling Metabolism Experiment 



was suggested that duration of life depends 

 fundamentally upon two variables: 



a. The constitution (organization or 

 pattern) of the individual, genetically de- 

 termined. 



b. The average rate of metabolism or 

 rate of energy expenditure during life. 



On this view, if the average rate of 

 metabolism, or of energy expenditure, is 

 high the duration of life will tend to be 

 shortened, while if, for any reason, the 

 average metabolic rate or rate of energy 

 expenditure is low the total duration of 

 life will tend to be prolonged. 



Males: 4.3 beats X 37.8 days = 161.54 

 Females: 3.7 beats X 43.8 days = i6i.o6 

 The experimental data thus tend to support the 

 view that a definite endowment of vital energy, po- 

 tential in the protoplasm of the species and line, 

 may be transformed and expended rapidly, as in the 

 short-lived males, or more slowly, as in the longer- 

 lived females. The evidence so far available indi- 

 cates also that length of life in Daphnia magna has 

 the usual temperature coefficient for a chemical 

 reaction. Other factors being equal, it would seem 

 that, at least in this species, longevity is a function of 

 metabolic rate. 



SEEDLING METABOLISM 



During the past year we have been 

 carrying on some experiments with cante- 



