826 THE POPULAR SCIENCE MONTHLY. 



may note that even those researches which, at the first sight, seem 

 to overthrow established facts, only result in a deeper knowledge 

 of diseases and their modifications. Thus, the recent investiga- 

 tions of MM. Lesage and Macaigne, who have finally succeeded 

 in differentiating the typhoidic bacillus from the Bacterium call 

 a microbe which is constantly met with in our intestines, and 

 only under certain conditions acquires an especial virulence are 

 one of the best examples of how further research deepens our 

 knowledge of microbes ; and Dr. Cunningham's discovery of ten 

 different varieties of the choleraic bacillus * certainly will have 

 the same effect : it will simply widen our knowledge of the differ- 

 ent forms assumed by cholera. 



Things stand, however, quite differently with the means of 

 combating infectious micro-organisms. Most of the specifics 

 which once awakened so many hopes have proved in the long run 

 to be as ineffective against bacilli as the specifics periodically pro- 

 posed by allopaths and homoeopaths are powerless against the dis- 

 eases themselves. And the more the study of bacteria is advanc- 

 ing, the more it is recognized that a healthy body which is 

 capable of itself putting a check on the development of morbid 

 micro-organisms is the best means of combating them ; that sani- 

 tary measures which prevent the very appearance of morbid 

 germs are the surest means against the possibilities and the risks 

 of infection. But what permits a healthy body to resist its inva- 

 sion by morbid organisms ? What gives several animals im- 

 munity against certain special diseases ? Why do rats resist 

 anthrax, and dogs and monkeys resist the tuberculosis of fowls, 

 while the same microbes are fatal to rabbits and guinea-pigs ? 

 And how can immunity against certain diseases be acquired 

 either by vaccination or by previously having suffered the same 

 disease ? We know the microbes ; but what is it that renders 

 them highly offensive in some cases, and quite inoffensive in 

 some others ? 



Several theories have been constructed to explain the phe- 

 nomena of immunity ; and, although none of them has succeeded 

 in dispelling all doubts, it must be recognized that each of them 

 accounts for at least large groups of phenomena. In fact, of the 

 two leading theories, one being purely biological, while the other 

 pays its chief attention to the chemical aspects of the subject, 

 they rather complement than contradict each other. The broad- 

 est and most ingenious of all explanations of immunity is the 

 theory, elaborated in 1883 by Elie Metchnikoff, which represents 

 an extension of the leading principles of struggle for life to the 



* Scientific Memoirs by the Medical Officers of the Army of India, Part VI ; analyzed in 

 Annales de Micrographie, 1892. 



