254 BACTERIOLOGY. 



Thus, in alcoholic fermentation, the yeast plant is com- 

 monly said to be the cause, but, as pointed out on p. 96, a 

 large number of different species of yeasts are known 

 which have this power. Moreover, many bacteria and 

 moulds possess similar properties. Again, a considerable 

 number of bacteria have been shown to be capable of in- 

 ducing acetic, lactic, butyric acid fermentations, the am- 

 moniacal and hydrogen sulphide fermentations of urine, the 

 phosphorescence of sea-water, etc. 



The most that can be said of a given organism which 

 Induces a certain change, therefore, is that it is the cause 

 in that particular instance. The possibility of other or- 

 ganisms giving rise to the same change, or effect, or to the 

 same chemical products must be conceded, and the demon- 

 stration of the relation of an organism to such a change 

 rests with the proof that it is a cause. 



Just as there are organisms which induce changes in 

 dead animal or vegetable matter, there are others which 

 are capable of inducing similar changes in living animals 

 -and plants, thus living at the expense, and, frequently, to 

 the detriment of the host. The infectious diseases in man, 

 animals and plants, possess as an essential characteristic 

 the property of transmissibility. They are the result: first, 

 of infection that is the entrance of a specific micro-organ- 

 ism; and second, of intoxication due to the poisonous pro- 

 ducts elaborated by that micro-organism. 



The poisonous chemical compounds, which are thus 

 made, may produce the symptoms and the changes observed 

 in the infectious disease. They are the cause of those symp- 

 toms and changes, in other words, of the resulting intoxica- 

 tion. But, they are not the cause of the disease itself, since 

 the symptoms and changes thus obtained, are not transmissi- 

 ble from one individual to another. Chemical substances 

 have no power of multiplication and the effect observed is, 

 therefore, directly proportional to the amount of the chem- 

 ical compound introduced. Micro-organisms, however, 



