328 THE PHILOSOPHY OF BIOLOGY 



to explain the metabolic processes by assuming the 

 intervention of enzymes, until the number of these 

 substances has become legion, and much of the original 

 simplicity of the notion of ferment-activity has been 

 lost. But why do not these enzymes, if they are always 

 present in the tissues, always act ? They must be 

 activated, says modern physiology ; that is, the enzyme 

 really exists in the tissues as a " zymogen '* or a sub- 

 stance which is not, but which may become, an enzyme ; 

 or they exist as " zymoids," that is, substances which 

 appear to be chemically enzymes, but which must 

 be activated by " kinases " before they can become 

 functional. 



Undoubtedly it is along these lines that physiology 

 is making advances, has increased our knowledge of 

 the activities of the animal, and is conferring on the 

 physician greater power of combating disease ; but the 

 hypotheses of the activity of the enzymes is obviously 

 one which has been based on the results of the 

 physico-chemical investigation of inorganic reactions, 

 and it has taken the precise form it has because of the 

 attempted analogy of many metabolic processes with 

 catalytic processes. Why do the inert zymoids become 

 activated by the kinases just when they are required 

 by the general economy of the whole organism ? We 

 do know that kinases are produced by the entrance 

 of digested food into certain parts of the alimentary 

 canal, and that these kinases are carried in the blood 

 stream to other parts where they activate the zymoids 

 already there. But of the nature of the machinery 

 by means of which all this is effected physiology gives 

 us no hint, and it is an assumption that the mechanism 

 involved is a purely physico-chemical one. Suppose we 

 say that the entelechy of the organism possesses the 

 power of suspending the activation of the enzyme, 



