THE PEESENT EVOLUTION OF MAN — PHYSICAL 217 



But when the treatment is begun the infected indi- 

 vidual has already within him the disease germs and 

 toxins ; by adding to these germs and toxins we might 

 expect on d priori grounds that the rapidity of the 

 disease would be increased, and the fatal result rendered 

 more certain, since the phagocytes would have less time 

 wherein to vary fitly — to acquire the power of destroy- 

 ing the microbes of rabies in the presence of their toxins. 

 The contrary, however, is the case, for under such treat- 

 ment the infected individual usually recovers. This, 

 then, is the difficulty which must be surmounted before 

 the phagocytic theory of immunity can be accepted. 



When under fit conditions yeast is introduced into a 

 solution of grape sugar, that micro-organism multiplies 

 rapidly. Concurrently with its multiplication, and in 

 proportion to it, the sugar disappears, and is replaced 

 by alcohol. .Whence we may fairly conclude, that the 

 sugar is used as food by the micro-organism, and that 

 the alcohol is an excretion, an effete waste product, 

 comparable t6 the waste products present in the 

 excretions — e.g. urine — of higher animals. Like those 

 waste products, it is more or less poisonous to the 

 organism that produces it, as is proved by the fact 

 that, when the alcohol reaches a percentage of fourteen 

 in the solution, the yeast ceases to multiply and perishes. 

 It was at one time thought the behaviour of yeast in a 

 solution of sugar afforded an explanation and an illus- 

 tration of the way in which immunity against microbic 

 disease is acquired. The micro-organisms of disease 

 were believed to produce waste products, poisonous to 

 themselves, which were retained as in a bottle by the 

 host, and which, therefore, when they attained a certain 

 strength, wrought the destruction of the micro-organisms 

 that produced them, and rendered the infected individual 

 immune for the future, or at least for so long as the 

 waste products were not excreted and were retained 



