CH. xi] THE ENZYME THEORY OF DISEASE 169 



and their arrival be greeted with fireworks. A couple of in- 

 nocent looking steamers might be engaged in piracy and open 

 a destructive fire from their guns. The tall barque might con- 

 ceivably land a cargo of fish. In other words each type of 

 vessel might give rise to a train of events rightly regarded as 

 characteristic of an altogether different type, for the effects 

 they produce depend not on the activities of the ships them- 

 selves, which are merely carriers, but on those of their occu- 

 pants. 



At the same time the function of each different type of 

 vessel, though dependent upon its occupants, is also to some 

 extent governed by its structure and the equipment it carries 

 (comparable to the structure and metabolism of a micro- 

 organism). A mere exchange of crews would not necessarily 

 effect an exchange of function. For example, a party of 

 fishermen sent to sea in an ironclad would be as unlikely to 

 land a catch of fish as a force of naval officers and seamen 

 embarked in fishing smacks would be to bombard a town. 



In one respect our analogy fails. Hitherto we have spoken 

 of the enzyme as something grafted on to the micro-organism, 

 in the nature of a parasite, but there is much to suggest in 

 the evidence we have quoted that it is, in reality, a body ela- 

 borated by the organism itself, comparable to one of Ehrlich's 

 *' side-chains." Such a conception of its nature would go far 

 towards explaining the apparent dependence of the "enzymes" 

 of a particular disease upon a particular organism. But every 

 argument in favour of such a supposition in the case of the 

 enzymes which cause disease applies equally to our conception 

 of the nature of those which ferment carbohydrates. The pur- 

 pose of the arguments here presented has not been to explain 

 the precise nature of these ultra-microscopic bodies but merely 

 to show that the lesions and symptoms of disease may with 

 some confidence be attributed to the action of the same class 

 of body as that to which we unhesitatingly attribute the fer- 

 mentation of sugars. 



115 



