378 CLASSICS OF MODERN SCIENCE 



Although Brefeld's demand is undoubtedly theoretically correct 

 it cannot be made a sine qua non in every investigation on pathogenic 

 bacteria. We should otherwise be compelled to cease our investiga- 

 tions into the etiology of infective diseases till botanists have suc- 

 ceeded in finding out the different species of bacteria by cultivation 

 and development from spore to spore. It might then very easily 

 happen that the endless trouble of pure cultivation would be ex- 

 pended on some form of bacterium which would finally turn out to 

 be scarcely worthy of attention. In practice only the opposite method 

 can work. In the first place certain peculiarities of a particular form 

 of bacterium different from those of other forms, and in the second 

 place its constancy, compel us to separate it from other less known 

 and less interesting, and provisionally to regard it as a species. And 

 now, to verify this provisional supposition, the cultivation from 

 spore to spore may be undertaken. If this succeeds under conditions 

 which cut out all sources of fallacy, and if it furnishes a result cor- 

 responding to that obtained by the previous observations, then the 

 conclusions which were drawn from these observations and which led 

 to its being ranked as a distinct species must be regarded as valid. 



On this, which as it seems to me is the only correct practical method, 

 I take my stand, and, till the cultivation of bacteria from spore to 

 spore shows that I am wrong, I shall look on pathogenic bacteria as 

 consisting of different species. 



In order, however, to show that I do not stand alone in this view, 

 I shall here mention the opinion of some botanists who have already 

 come to a similar conclusion. 



Cohn states that, in spite of the fact that many dispute the necessity 

 of separating bacteria into genera or species, he must nevertheless 

 adhere to the method as yet followed by him, and separate bacteria of 

 a different form and fermenting power from each other, so long as 

 complete proof of their identity is not given. 



From his investigations on the effects of different temperatures and 

 of desiccation on the development of bacterium termo, Eidam came to 

 the conclusion that different forms of bacteria require different condi- 

 tions of nutriment, and that they behave differently towards physical 

 and chemical influences. He regards these facts as a further proof of 

 the necessity of dividing organisms into distinct species. 



I shall bring forward another reason to show the necessity of look- 



