TEXT-BOOK OF BACTERIOLOGY. 153 



nection with this one particular disease, we may fairly conclude 

 that it stands in a particular intimate relation to this disease, and 

 the probability of its being also the cause of this disease is so strong 

 that it approaches very near to certainty. 



The last link in the chain of evidence is of course supplied only 

 by a successful transmission, before the overwhelming force of 

 which all opposition must yield. Until that is obtained the objec- 

 tion is always possible that the bacteria may be a regular sequel 

 or accompanying symptom of the disease, in consequence of the 

 fact that certain morbid metamorphoses offer peculiar facilities 

 for the development of certain bacteria. This view of the case 

 has, it is true, an immense weight of probability against it, but it 

 cannot be finally disproved without separating the bacteria from 

 all their natural surroundings and experimenting with the pure 

 culture, to find out whether the specific qualities attributed to them 

 are still there or not. 



As long as we continue to transmit virus obtained directly from 

 a diseased organism, there is a possibility that other substances 

 are inoculated along with the bacteria, and that these other sub- 

 stances contain the disease-causing matter. If, however, we take 

 as our point of departure a culture which has been extended 

 through several generations, the above objection collapses, and by 

 the successful transmission the reproduction of an affection like 

 the original disease the specific character of a given bacterium is 

 indisputably proved. 



Unfortunately, we cannot yet fulfil this requirement in all cases. 

 We have already often referred to the very different degrees of 

 susceptibility possessed by the different species of animals as re- 

 gards the attacks of the pathogenic micro-organisms, and when 

 we reflect on the obstinate tenacity with which more highly - 

 organized parasites often attach themselves exclusively to one 

 species of animal, which they never leave under any circumstances, 

 we will no longer find it astonishing that the lower parasites can- 

 not always be transmitted from one species of animal to another. 



For this reason, from this consideration we may at once draw 

 the conclusion that in our attempts to reproduce a bacterial disease 

 we should, as much as possible, experiment with those animals 

 which are susceptible to it under natural conditions. Now it is the 

 infectious diseases of man which possess by far the most interest 

 for us, and we know that many of them attack human beings exclu- 

 sively. We must not here require impossibilities from our method. 

 Even if the attempt to transmit the disease to animals failed, that 

 is no reason for doubting the specific agency of the bacteria in 



