110 APPLIED BACTERIOLOGY 



1. The exhaustion or pabulum hypothesis. 



2. The retention hypothesis. 



3. The hypothesis of acquired tolerance, or acclimatisa- 

 tion. 



4. The phagocytosis hypothesis. 



1. Exhaustion or Pabulum Hypothesis. — The supporters of 

 this theory hold that the bacilli by their action abstract 

 from the blood some chemical compound necessary for their 

 growth, the consequence being that, once this pabulum is 

 exhausted, it must be re-formed before a second attack is 

 possible. On turning to the higher branches of the vege- 

 table kingdom, we have no difficulty in finding analogies. 

 It is a matter of experience that, if a crop of wheat, for 

 example, is grown on the same soil year after year, it will 

 abstract the particular elements required by the wheat-plant 

 to such an extent that wheat will not grow so satisfactorily 

 till the land has rested or the abstracted elements have in 

 some way been restored. To carry the analogy still further, 

 we find that, though the land may be exhausted in respect 

 of wheat, it still can produce a full crop of potatoes or 

 roots ; while as regards the human body, after an attack 

 of small-pox there is a very strong protection against a 

 second attack of that disease, but none at all against 

 influenza. 



In order to admit the truth of this hypothesis, we must 

 believe that each disease organism requires a special 

 pabulum which is present, in the blood of every susceptible 

 animal at birth, and that the varying degrees of immunity 

 that are produced are due to the ease or difficulty with 

 which the pabulum for that particular disease is repro- 

 duced. 



The Antidote or Retention Hypothesis.— This hypothesis 

 assumes that after the first attack the micro-organisms leave 

 behind them some product of metabolism that is inimical to 



