- ESSAYS ON BACTERIOLOGY. 49 



if haply it survives the first, is endowed, with an im- 

 munity from subsequent infection by the same germ. 



Here, is a phenomenon quite different from self- 

 limitation, and reqiiiring, perhaps, a different explana- 

 tion, for they are by no means commensurate, nor 

 even co-existent in the history of a disease. The one 

 may be present, the other absent; one may be strongly 

 marked, the other moderate, or but very slight. Ap- 

 parently, therefore, they do not necessarily depend 

 upon one or the same cause; indeed, there is reason to 

 believe that, in some diseases, at least, they result 

 from quite different causes, and are brought about in 

 different ways. 



Can we explain immunity, or, if not, can we dis- 

 cover and imitate the process by which it is estab- 

 lished? Here, too, theory has run riot in the field 

 where knowledge was wanting, but a satisfactory con- 

 clusion has not yet been reached. Of these theories 

 the following are worthy of mention: First, that of 

 'exhaustion; that the growth of the germs exhausts the 

 nutritive substances necessary for their life, thus leav- 

 ing the body incapable of supporting the germs in the 

 future. This explanation is now believed to be un- 

 tenable, for reasons previously stated. Second, the 

 theory of retention; that some peculiar substance is 

 left in the body which antagonizes the germs should 

 they subsequently enter. Of this Frsenkel says it is 

 possible, though not probable, and may be retained a-3 

 a working hypothesis. Third, the phagocytosis the- 



