90 THE PHILOSOPHY OF BIOLOGY 



(2) Katabolic changes in the animal body corre- 

 spond in their frequency of occurrence to the anabolic 

 changes of the plant organism. In them complex 

 chemical substances undergo transformation into 

 relatively simple substances, and the contained energy 

 at the same time undergoes a parallel transformation, 

 passing into the form of heat and mechanical energy, 

 while a fraction becomes dissipated. Food-stuffs taken 

 into the alimentary canal break down in this way, but 

 to a very limited extent. Proteids undergo dissoc- 

 iation or decomposition into amido-substances, while 

 fats are dissociated into fatty acids and glycerine. 

 Doubtless energy is dissipated in these processes, 

 serving no other purpose but to heat the contents of 

 the alimentary canal, but this energy-transformation 

 has not been worked out very completely and it is a 

 question whether, given a healthy animal and perfect 

 food-stuffs, any energy would necessarily be lost 

 during the digestive processes. The reactions involved 

 in the latter do not belong to the category of chemical 

 changes proceeding from the complex to the simple, 

 with a liberation of energy ; but appear to involve 

 rather a rearrangement of the constituents of a com- 

 plex molecule, a process in which the contained energy 

 need not undergo change in quantity. These processes 

 involve the action of enzymes. 



Enzymes play a great part in modern physiological 

 theory and we must consider them in detail. Let us 

 attach a concrete meaning to the general notion of 

 enzyme-activity by considering the phenomena known 

 as catalysis. The metal platinum can be brought 

 into a very fine stage of division when it is known as 

 platinum black. In this condition it brings about 

 reactions in chemical mixtures or substances which 

 would not otherwise occur : a mixture of oxygen and 



