194 PROFESSOR LISTER. 



amounts to this, that wherever there was a fermentative particle 

 there was a bacterium, and wherever there was a bacterium there 

 was a fermentative particle. But, suppose you admitted that — 

 that there were exactly as many of the Bacterium lactis as there 

 were of the hypothetical true fermentative particles — suppose 

 you admitted that inconceivable thing, I say it would be again 

 inconceivable that, if mutually independent, they should accom- 

 pany one another in pairs, that invariably where there was 

 Bacterium lactis there should be a ferment particle, and where 

 there was no Bacteriiim lactis no ferment particle. That would 

 be a thing as inconceivable as the other. Therefore, we have 

 two inconceivables, one of which would have been sufficient to 

 show that we cannot admit any other hypothesis than that 

 Bacterium lactis is the cause of the lactic acid fermentation. 



But the experiment tends to even more than this. Where we 

 find the effect so exactly ])roportioned, as regards the number of 

 glasses affected with fermentation, to the adult bacteria that we 

 count, we are led to infer that this particular bacterium, at all 

 events, has not any spores — that there are no spores existing in 

 addition to the bacteria. People seem often to assume that 

 bacteria must necessarily have spores or germs. It seems to me 

 an unlikely thing that they should. They are, as it were, a 

 generative apparatus per se, they are constantly multiplying ; 

 why should they have spores ? I do not say that bacteria may 

 not have spores. There are very different kinds of bacteria ; 

 some may have spores, and some may not ; but this sort of result 

 seems to indicate that this particular bacterium has no spores ; at 

 least, in the condition in which it exists in souring milk ; be- 

 cause, if we had, besides the bacteria that we can count, spores 

 of bacteria disseminated through the liquid also, we should have 

 the effect more than in proportion to the bacteria that we have 

 counted. The only fallacy here is that it may be that the 

 bacterium has not been diff'used uniformly through the milk. 

 Therefore, I do not say that in this case it is absolutely proved. 

 But, at all events, this experiment gives us a line of inquiry, by 

 means of which we may probably settle that point with regard 

 to any individual case of bacterium. This, however, is a point I 

 do not desire now to insist on ; but, what I do venture to urge upon 

 you, is that you will seriously ponder over the facts which I have 

 had the honour of bringing before you to-day ; and, if you do 

 so, I believe you will agree with me that we have absolute evidence 

 that the Bacterium lactis is the cause of the lactic acid fermen- 

 tation. And thus I venture to believe that we have taken one 

 sure step in tlie way of removing this important but most 

 difficult question from the region of vague speculation and loose 

 statement into the domain of precise and definite knowledge. 



