STUDIES ON THE DURATION OF LIFE 211 



environment equals diversity. Environment constajit 

 plus varying hereditary constitution also equals diversity. 

 Can we penetrate no farther than this into the matter? 

 I think in the present case we can. In Loeb and Northrop 's 

 experiments, temperature and duration of life were not 

 the only two things that varied. The different tempera- 

 ture groups also differed from each other — because of the 

 temperature differences, to he sure, but not less really — 

 in respect of general metabolic activity, expressed in 

 muscular movement and every other way. In the gene- 

 tic experiments metabolic activity was substantially equal 

 in all the hereditarily different lines. The idea suggests 

 itself, both on a priori grounds and also upon the basis 

 of certain experimental data presently to be in part re- 

 viewed, that possibly duration of life may be an implicit 

 function of only the two variables 



a. Genetic constitution 



b. Kate of metabolic activity. 



The functional relations of metabolic activity with 

 temperature, food, light and other environmental fac- 

 tors are all well known. For present purposes we do 

 not need to go into the question of their exact form. The 

 essential point is that all these environmental factors 

 stand in definite functional relations to rate of metabolic 

 activity, and do not so stand in relation to genetic consti- 

 tution. Genetic constitution is not a function of the 

 environment, but is, for any individual, a constant, and 

 only varies between individuals. 



This may be thought merely to be an involved way of 

 saying what one knows a priori: namely, that duration 

 of life, in general and in particular, depends only upon 

 heredity and environment. So in one sense it is. But 

 the essential point I would make here is that the manner 



