THE THEORY OF IMMUNITY 133 



and destroyed the organism in which they found them- 

 selves. Others were harmful as they stood, but by pro- 

 ducing a reaction body they were broken up and rendered 

 harmless. Others, again, were not only rendered harmless 

 but actually useful : i.e. they became foods because 

 they provoked a definite catalyst which hydrolyzed them. 

 Like and dislike of foods in the highly developed organism 

 are thus conditioned protective reflexes which defend 

 the body from all but foods selected through evolution. 

 Nutrition thus clearly falls under the head of immunity. 

 J. B. Farmer remarks that they are evidently closely 

 related. The sole real difference is that, in what is now 

 called immunization, the substrate is probably not em- 

 ployed usefully, though it remains possible that in some 

 cases destroyed pathogenic bacteria may actually be 

 used by the body they attack. In any case it is a possi- 

 bility of evolution for bacteria to become gradually a 

 factor of further growth. Such phenomena may have 

 occurred already, as it is at least possible that the colon 

 is partly a function of the bacteria that inhabit it, and 

 that some dead bacteria may be converted into food after 

 their parasitic free existence. 



Nutrition being then a case of immunity, we must 

 infer that food itself originally produces ad hoc catalysts, 

 of an order similar to those produced by toxins. Enzymes 

 very rarely exist without the presence of their substrate. 

 The organism has a specialized method of producing them 

 on definite stimulation. Thus trypsin only appears on 

 trypsinogen coming into contact with entcrokinaze, 

 though it would be better to say that when certain 

 colloidal reactions of pancreatic origin take place in the 

 presence of the latter trypsin is formed. Lactase cannot 

 be found in a meat-fed dog till some days after it is given 



