36 THE PHILOSOPHY OF BIOLOGY 



of immunisation : immunity has been conferred on it. 

 Now can we compare the two operations, that of the 

 solution and precipitation of the metal and that of 

 the immunisation of the animal ? We can to some 

 extent, but the analogy soon fails, and indeed we should 

 not attempt to formulate a theory of immunity on a 

 physico-chemical basis if we did not start with the 

 assumption that the series of operations was one in 

 which only physico-chemical reactions were involved, 

 that is to say, there is nothing in the phenomena of 

 immunisation that suggests that what occurs in the 

 animal body is similar to what we can cause to occur 

 in inorganic materials outside the tissues of the living 

 organism. We start with the assumption that the 

 administration of the toxin causes the formation of an 

 antitoxin in very much the same sort of way as the 

 administration of hydrochloric acid to a solution of 

 nitrate of silver causes the formation of chloride of 

 silver. This antitoxin then neutralises the dose of 

 toxin which may be administered after the process of 

 immunisation has been effected, very much in the same 

 sort of way as a certain amount of some acid can be 

 neutralised by an equivalent amount of some base with 

 which the acid can combine. If the reader will analyse 

 any of the theories of immunisation current at the 

 present day he will find that these are the physical 

 ideas that are involved in it.^ But physiological 

 science has the much more formidable task of explain- 

 ing the persistence of the immunity. The animal 

 rendered immune to the toxins produced by certain 

 species of bacteria may remain so for many years, that 

 is, for a very long time after the antitoxins origin- 

 ally produced by the reaction of the tissues to the 



^ Except that, of course, the reactions that are supposed to occur are 

 very complex ones. 



