THEORIES OF IMMUNITY. 



the bacteria may be supposed to elaborate a ma- 

 terial that is hurtful to themselves, and this sub- 

 stance is retained in the tissue cells or fluids for 

 some varying length of time, during which time 

 a fresh invasion of bacteria would be followed by 

 no results. To explain such cases as those of 

 the Algerian sheep by this theory — in which 

 the sheep are not susceptible to attack by small 

 doses of anthrax bacilli, but are susceptible to 

 overwhelming doses — it would be necessary to 

 suppose the existence of a fixed amount of the 

 opposing substance which would be more than 

 neutralized by the excessive doses of the anthrax 

 bacilli, after which infection occurred. Although 

 Chauveau objected to the exhaustion theory, that 

 it did not explain natural immunity, his own 

 supposition did not do so any better. ]STor did 

 it serve to explain more than a very small part of 

 the experimental facts observed. Pasteur him- 

 self furnished one instance — that of chickens 

 that are not susceptible to anthrax at their 

 normal temperature, but become so when this 

 temperature is lowered. His comment upon 

 this fact and its bearing upon Chauveau's 

 theory was that it could not be supposed that a 



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