74 THE EVOLUTION OF DEATH 



of potential capacity to grow and to divide is presumably 

 fixed by the organization of each cell, but there occur in the 

 body inhibiting influences, perhaps also exciting. Thus it 

 may happen that a cell potentially capable of division cannot 

 divide, or that a cell which has long remained inactive may 

 be excited to division by special newly arisen influences. The 

 phenomena are by no means simple. 



The theory of senescence which I have expounded to you 

 was proposed, as you have heard, by myself. All achieve- 

 ments of science originate in this way. They are at first 

 purely personal. Afterward when they have been tested they 

 acquire general validity. And so with regard to my theory, 

 until the discussion is concluded we must wait in order to 

 decide whether this theory or some other which may be 

 brought forward is to be finally adopted. 



Some of the theories of senescence we may now discuss 

 briefly. That of Conklin has been previously mentioned. I 

 have already indicated to you the reasons which lead me to 

 designate these theories as insufficient. There are besides a 

 number of theories which have been conceived from a purely 

 medical point of view, and which are little adapted to satisfy 

 a biologist. First of all must be named the theory of Mets- 

 chnikoff, of which probably all cultivated men have heard. 

 The Russian investigator, who has been working for many 

 years in the Pasteur Institute in Paris, published in the year 

 1903 a peculiar book with the title, "La nature de Fhomme." 

 With the views of life presented therein, we have at present 

 nothing to do. We restrict ourselves to the discussion of the 

 theory of disharmonies presented in this book. According to 

 Metschnikoff, a disharmony arises whenever the structure 

 of an organ is incompletely adapted to the needs of the body. 

 The disharmonies he mentioned do not seem to me very 



