ON SPECIFIC THERAPEUTICS. 79 



we are dealing in these questions with a highly 

 complex field of research, in which many possi- 

 bilities are present, the most important being 

 this difference of avidities. 



We have previously seen that the trypano- 

 somes may alter their avidity for arsenic, and thus 

 acquire an apparent immunity. The contrary 

 may occur in the case where the substance in 

 question is not a poison, but a nutritive body. 

 Here we should be perfectly justified in sup- 

 posing the avidity of the receptor .to have 

 become increased. The same thing may take 

 place with such micro-organisms which can 

 physiologically diminish or increase their avidi- 

 ties. Thus in all these cases the struggle lies 

 between the adaptability of the parasite and 

 that of the host. The one whose adaptability is 

 the highest will remain the victor. 



Of course this struggle is to a great extent 

 influenced by indirect actions, consisting in the 

 secretion by each antagonist of dissolved sub- 

 stances hostile to the vitality and receptivity of 

 the other organism. On the part of the bac- 

 teria, these substances are the toxins and the 

 dissolved intracellular substances ; on the part 

 of the body, the anti-substances. 



A further role is played by actions of a pro- 



