STUDIES ON THE DURATION OF LIFE 215 



upon as a necessary consequence of them. Loeb and 

 Northrop 's final conclusion is: "The observations on the 

 temperature coefficient for the duration of life suggest 

 that this duration is determined by the production of a 

 substance leading to old age and natural death, or by the 

 destruction of a substance or substances, which normally 

 prevent old age and natural death." The view which I 

 have here suggested, completely incorporates this view 

 within itself, if we suppose that the total amount of hypo- 

 thetical "substance or substances which normally prevent 

 old age and natural death" was essentially determined 

 by heredity. 



This view I take to be in no way necessarily or funda- 

 mentally contradictory to that set forth in this work. 

 Whatever the factor which determines specific longev- 

 ity may be; whether a specific chemical substance, as 

 Loeb and Northrop suggest, or more generally, as I have 

 suggested, the kind of material, in the sense of its biologi- 

 cal fitness, composing the multicellular body, and the 

 nature of the organization (in detail) of that material 

 to form the multicellular body; it seems to me that we 

 have now a sufficient mass of critical evidence to say 

 that it is proved that quantitatively the effective magni- 

 tude of this specific longevity factor in each particular 

 case is determined by heredity. This I take to be of 

 greater importance than the precise nature of the specific 

 longevity factor itself, about which we are, admittedly, 

 entirely ignorant. I can see nothing in the available evi- 

 dence which definitely makes Loeb's suggestion inherently 

 more probable than mine. It does, however, seem clear 

 that, by definitely showing the significance of the heredity 

 element in the problem, help has been rendered the prog- 

 ress of future research in the field. 



