150 Lectures on Bacteria. [J xiii. 



sively in a particular disease, and therefore have high diagnostic 

 value, without being the contagium which causes the disease. 

 To make sure of this it is absolutely necessary to experiment with 

 pure material and to obtain a clear positive result ; there must 

 therefore be a pure separation of the parasite to be examined 

 from all admixtures, a pure infection of the proper subject for 

 experiment with pure matter, and the strictest control and 

 criticism of the result. The example of anthrax described above 

 will illustrate these rules. Without successful experimentation 

 there is always a gap in the proof which cannot be filled up by 

 other arguments, however well adapted they may be to serve as 

 the foundation of a personal conviction. It is true that the latter 

 may persist in spite of defective experimental proof. A parasite, 

 as has been before explained, does not thrive, or does not thrive 

 equally well in every host-species; it can attack and cause 

 disease in one and not in another. The experiment therefore in 

 the case in question may give no positive result, because the 

 right, that is the susceptible, species of warm-blooded animal was 

 not employed. This point must be specially attended to in the 

 case of infectious diseases which attack human beings chiefly. 

 We cannot or must not experiment freely with human beings, 

 but must trust in the main to experiment on other warm-blooded 

 animals, and this may be the only reason why that in certain 

 cases, some of which will be noticed further on, the results of 

 experiment have as yet remained doubtful or negative. 



Enough has been said to give inexperienced persons an idea 

 beforehand of the reasons why we may have to speak here of 

 doubtful cases and doubtful statements. 



We will now proceed to a consideration of the facts. Our 

 object, as was said before, is to bring forward whatever is most 

 important in connection with the Bacteria which are the exciting 

 causes of disease. A minute discussion of the diseases them- 

 selves does not fall within the plan of these lectures, and must 

 be sought in medical publications. 



Whatever peculiarity there may be in each individual case, we 



