METHODS 



115 



definite tissue has come to the term of its differential evolu- 

 tion. There is no longer any variation possible in the 

 protoplasmic nature of each element, but only quantitative 

 augmentations or diminutions without functional variation. 

 Now, in an adult, the destructive reactions of the tissues 

 counterbalance their constructive reactions, since the 

 animal remains the same, like to itself ; and for each tissue 

 which no longer varies we have to speak of assimilation 

 pure and simple, and not of adaptive assimilation. The 

 only question is What are the conditions determining 

 in the tissues now assimilation and now destruction ? 



The same element M, which can work only according to 

 its nature M, is successively a part of different organs 

 for a man or a cat never does twice in succession the same 

 thing. According to the successive functions into which 

 the history of the vital activity of a man or a cat is decom- 

 posed, the element M exercises its proper function M a 

 great deal, or little or not at all. If it destroyed itself 

 in accomplishing its function, it would be the frequent 

 repetition of the function which tended to make it disappear 

 contrary to the law of habit. The smithing qualities 

 of a blacksmith are developed in proportion as he works. 

 On the contrary, an idle man's muscles are destroyed as 

 muscle and loaded up with fatty reserves which, as we have 

 seen, are products of destruction. 



Consequently when, from the descriptive point of view, 

 we are uncertain as to the peculiar function of any tissue 

 in an organism, we have to study the conditions in which 

 the tissue develops quantitatively while still remaining 

 like to itself. 



We are now able to express in all its generality the law 

 of functional assimilation the fundamental law of all 

 biology. Relying on Claude Bernard, most physiologists 



