1 8 SENESCENCE AND REJUVENESCENCE 



of new molecules from the nutritive substances. In other words, 

 the living substance once produced is self-perpetuating, at least 

 within a very wide range of external conditions. But if the ability 

 to perpetuate itself in this way is a property of the living substance, 

 then it is in this respect also very different from any other sub- 

 stance with which we are acquainted. 



It appears then that when we analyze this hypothesis of a labile 

 proteid substance which gives rise to the manifestations of life by 

 its chemical transformations we find that it does not help us to 

 any great extent in bridging the gap between the organism and the 

 inorganic world. The self -perpetuating substance or substances 

 which constitute the basis of life remain unique in character. 

 They are highly labile, yet persist under a great variety of con- 

 ditions, and "die" in most cases without the liberation of any very 

 great amount of energy. During life they regulate their own 

 chemical changes in some way, they determine the formation of 

 new molecules like themselves, and they are responsible somehow 

 for an orderly sequence of differentiation of parts of the organism. 

 Evidently they are very different from other chemical substances, 

 even highly labile ones, with which we are familiar. 



The numerous difficulties which arise in connection with hy- 

 potheses of this character must at least raise the question whether 

 the point of view on which they are based is fundamentally correct. 

 Is life at bottom simply a complex of chemical reactions or is there 

 some other factor involved which the hypothesis of a labile mole- 

 cule as the basis of life fails to take into account ? In the following 

 sections an attempt is made to answer this question. 



PHYSICO-CHEMICAL THEORY 



A few years ago the existence of a living substance as a more or 

 less definite chemical compound was very generally accepted, and 

 only rarely were criticisms and questionings heard. 1 



1 See for example A. P. Mathews, '99, '05; Driesch, '01 (pp. 140-52). Mathews 

 pointed out that living matter must be a mixture of many substances among which 

 various chemical reactions occur. Driesch denies very positively the existence of a 

 definite living substance, but for him this is merely one point in the argument for the 

 autonomy of vital processes. 



