XX.] VACCINATION AND IMMUNITY. 255 



growing and multiplying in the body during the first attack 

 produce, directly or indirectly, some substance which acts as 

 a sort of poison against a second immigration of the same 

 organism. I am inclined to think that this theory is in har- 

 mony with the facts. There is nothing known, from the 

 observations before us, which would negative the possibility 

 of the correctness of this theory ; nay, I would almost say 

 all our knowledge of the life of the micro-organisms points 

 to the conclusion that the different species are associated 

 with different kinds of chemical processes, and that as a 

 result of the activity we find different chemical substances 

 produced. 



The different fermentations connected with the different 

 species of fungi afford striking illustrations of this view. 

 According to this theory, we can well understand that just 

 as in the case of an animal, say a pig, unsusceptible to 

 anthrax the unsusceptibility being due to the presence in 

 the blood and tissues of a particular chemical substance 

 inimical to the growth of the bacillus anthracis so also in 

 the case of a sheep or ox that has once passed through 

 anthrax there is now present in the blood and tissues a 

 chemical substance inimical to the growth and multiplica- 

 tion of the bacillus anthracis whereby these animals become 

 possessed of immunity against a second attack of anthrax. 



Whether this chemical substance has been elaborated 

 directly by the bacilli, or whether it is a result of the chemical 

 processes induced in the body by the bacilli during the first 

 illness, matters not at all ; it is only necessary to assume 

 that the blood and tissues of the living animal contain this 

 chemical substance. 



Some observers (Grawitz, &c.) are not satisfied with this 

 theory, but assume that owing to the first attack the cells of 

 the tissues so change their nature that they become capable 



