254 BACTERIOLOGY. 



" specific " activities and characters into dis- 

 ease-producing or pathogenic, fermentation- 

 causing or zymogenic, and pigment-forming 

 or chromogenic, the foregoing examples show 

 that a single bacterial species, a single " spe- 

 cific " minute living thing is capable of exercis- 

 ing all three of the specific activities formerly 

 held to be essentially distinct. The " specific " 

 bacteria are therefore not the true cause ; that 

 lies in the character of the nutrient medium ; 

 the bacteria can elicit only what is preformed in 

 the structure of the medium. I have given the 

 name of " cycle of activity " to this class of phe- 

 nomena. These facts plainly militate against 

 the doctrine of " specific " disease germs held 

 by Cohn and Koch, and they help us to un- 

 derstand one important thing concerning the 

 production of disease, namely that it is not the 

 transferable u essence " that determines the 

 character of the " specificity " of disease, but 

 the similarity and the permanency of the 

 conditions of life. Since disease germs that 

 are presumed to be " specific " are able to 

 cause fermentations and to form pigments, it 

 is clear that a close relation exists between the 

 " parasitic " bacteria occurring in living human 

 beings and the so-called " saprophytic " bac- 

 teria, or bacteria of putrefaction, which are 

 able to live outside of the human body upon 



