ROBERTKOCH 381 



cited is more probable, an inexact and inaccurate observation. When, 

 therefore, several species of bacteria occur together in any morbid 

 process, before definite conclusions are drawn as to the relations of the 

 disease in question to the organisms, either proof must be furnished 

 that they are all concerned in the morbid process, or an attempt must 

 be made to isolate them and to obtain a true pure cultivation. Other- 

 wise we cannot avoid the objection that the cultivation was not pure, 

 and therefore not conclusive. I shall only briefly refer to a further 

 necessary consequence of the admission of the existence of different 

 species of pathogenic bacteria. The number of the species of these 

 bacteria is limited; for, of the numerous diverse forms present in 

 putrid fluids, one or but few can in the most favorable cases develop 

 in the animal body. Those which disappear are, for that species of 

 animal at least, not pathogenic bacteria. If, however, as follows from 

 the foregoing, there exist hurtful and harmless bacteria, experiments 

 performed on animals with the latter, e. g., with bacterium termo, 

 prove absolutely nothing for or against the behavior of the former — 

 the pathogenic — forms. But almost all the experiments of this nature 

 have been carried out with the first mixture of different species of 

 bacteria which came to hand without there being any certainty that 

 pathogenic bacteria were in reality present in the mixture. It is there- 

 fore evident that none of these experiments can be regarded as fur- 

 nishing evidence of any value for or against the parasitic nature of 

 infective diseases. 



In all my experiments, not only have the form and size of the bac- 

 teria been constant, but the greatest uniformity in their actions on the 

 animal organisms has been observed, though no increase of virulence, 

 as described by Coze and Feltz, Davaine, and others. This leads me to 

 make some remarks on the supposed law of the increasing virulence of 

 blood when transmitted through successive animals, discovered or con- 

 firmed by the investigators just named. 



The discovery of this law has, as is well known, been received with 

 great enthusiasm, and it has excited no little interest owing to its inti- 

 mate bearing on the doctrine of natural selection (Anpassung and 

 Vererbung). Some investigators, who are in other things very exact, 

 have allowed themselves to be blinded by the seductive theory that the 

 insignificant action of a single putrefactive bacterium may, by con- 

 tinued natural selection in passing from animal to animal, be increased 



