f XIIL] Causal connection with infectious disease. 149 



important parasites of contagion in warm-blooded animals 

 hitherto certainly determined are Bacteria. 



We may note as consequences of what has now been said, 

 firstly, that Henle's doctrine has become a widely-accepted 

 dogma. There is no objection to this, if we put in the place of 

 belief that intelligent personal conviction which is distinctly 

 directed toward a particular view, but does not lose sight of the 

 possibility that it may some day be corrected or altered. That 

 the parasite required by the theory has not yet been found is no 

 reason for abandoning it, for the parasite may very easily have 

 been overlooked, owing to its extreme minuteness, or its power 

 of refraction, or because the observer has not learnt the right 

 place or time to look for it. When Henle founded his doctrine 

 in 1840, the Bacillus of anthrax had never been seen; the 

 Trichinae had been seen, but no one suspected that they were 

 the cause of disease. 



The second result is that at present in almost every doubtful 

 or questionable case, it is only for Bacteria that search is made. 

 This is wrong in principle ; it may be practically right to search 

 for such forms as present experience shows are the most likely 

 to be found. But it should be remembered that organisms of 

 another kind may make their appearance unexpectedly, about 

 which we at present perhaps know very little. It is not so long 

 ago, that we knew very little about Bacteria and expected them 

 as little. That this is not idle talk is shown by some surprising 

 experiences recorded in connection with plant-pathology and by 

 the history of pe'brine which will be noticed presently. 



Thirdly, if belief is stronger than the critical faculty, there is 

 great danger of concluding at once from the presence of a Bac- 

 terium that it is the exciting cause of disease for which we may 

 be seeking. From what we learnt in Lecture V on the wide 

 diffusion of Bacteria possessing full powers of development it 

 will at once be seen that Bacteria may be developed in a diseased 

 body before or after death, and that a particular form may be 

 present as a characteristic feature and even constantly and exclu- 



