of parasitic organisms. In support of this theory there 

 is certainly strong presumptive evidence : the stage of 

 incubation ; the unlimited reproductive power of the 

 virus ; the cyclical course and self-limitation of the dis- 

 ease. The stage of incubation can be explained by the 

 assumption of no unorganized virus ; all mere chemical 

 compounds with which we are acquainted, even the fer- 

 ments ptyalin and pepsin, begin to manifest the charac- 

 teristic effects as soon as absorption has occurred. 



Panum found that even boiled putrid materials, i.e., 

 the products of bacterial activity, though inducing the 

 other features of septicaemia, failed to exhibit this charac- 

 teristic incubation. By assuming an organism as the in- 

 fecting agent this phenomenon becomes intelligible : the 

 stage of incubation is then the period during which the 

 inducted organisms are multiplying. In this way, too, 

 the various durations of the incubative stage character- 

 istic of the different diseases become intelligible. We 

 cannot conceive that one chemical poison should require 

 two to four days for the manifestation of its constitu- 

 tional effects, as in scarlet fever, another forty days, as in 

 syphilis ; but we know that different micro-organisms 

 multiply with different degrees of rapidity. A micro- 

 coccus may produce a second in thirty minutes ; the ba- 

 cillus anthracis may accomplish its entire vital cycle in 

 twenty-four hours ; the bacillus tuberculosis seems to 

 require days. The unlimited reproductive power of the 

 virus, characteristic of many infectious diseases, cannot 

 be attributed to an unorganized poison or even organic 

 ferment. A drop of blood from an animal poisoned with 

 opium or strychnine exhibits only the power of the di- 

 luted poison. A drop of vaccine lymph, of variolous or 

 gonorrhoeal pus induces, in successive generations, un- 

 limited quantities of identical materials. This effect 

 cannot be justly ascribed even to any physiological unor- 

 ganized ferment a favorite refuge of those who are 

 determined to deny to bacteria any influence whatever. 

 Ptyalin can, it is true, convert into grape-sugar many 

 times its bulk of starch, and the ptyalin is not thereby 



