BIOLOGY OF BACTERIA. 57 



them, how is it that we are not constantly subject to 

 disease ? The consideration of this question, together 

 with the closely-related questions why we should be 

 subject to certain diseases only, and to these diseases 

 at certain times only, must be reserved for another chap- 

 ter, where the subjects Immunity and Susceptibility can 

 be taken up at length. 



