THE MECHANISTIC CONCEPTION 17 



more complex one; Kastle and Loewenhart caused 

 a fat-splitting enzyme to form the ethyl ester of 

 butyric acid from butyric acid and ethyl alcohol; 

 Taylor succeeded in making a protein-splitting 

 enzyme (a special trypsin) build up a simple protein 

 from its cleavage products. That the same enzymes 

 which break down sugars, fats, and proteins may, 

 under suitable conditions, reverse their action so 

 as to construct the more complex molecules from 

 simpler ones, is a conception of the very first impor- 

 tance for biology. Obviously it gives a substantial 

 clew to the way in which living protoplasm is built 

 up in the course of ordinary metabolism and during 

 the process of growth. 



The breakdown of protoplasm and foodstuffs in 

 normal metabolism is accomplished in part through 

 the agency of oxidation, which may properly be 

 regarded as typifying the most fundamental kind 

 of chemical processes in the living cells. It has 

 been indicated that there is a resemblance between 

 the combustions in an engine of human make and 

 the combustion of food by living animals. Let us 

 discuss this resemblance further. For if it can be 

 shown that the resemblance is essentially superficial, 

 this must be admitted to be inimical to the mechan- 

 istic hypothesis ; whilst the detection of an essential 

 identity of action in the two cases must be regarded 

 as a significant support to this view. The serious 

 discussion of this question is hardly possible with- 

 out reference to some technical details of a chemical 

 o 



