230 BACTERIOLOGY 



which vegetated more slowly and in tissues permitting escape of 

 the microbe after a certain time. 



We may now return to the rabbit septicemia bacillus. The reason 

 why the most virulent type of this group does not pass to rabbits is 

 due to the fact that there is no satisfactory mechanism of entry and 

 escape. This presupposes a lesion, a wound as a place of entry, and 

 the excretion and transfer into a wound in another animal. In the 

 rabbit this difficulty is worked out in the way usual with this bacillus. 

 The microbe adapts itself to vegetate upon the mucous membrane 

 of the upper air-passages. Under certain conditions it invades the 

 lungs, pleural and pericardial, more rarely the peritoneal cavity, 

 producing pneumonia and extensive exudates on the serous mem- 

 branes, and causing death. The disease of the thoracic organs evi- 

 dently follows some predisposing cause, which enables the bacillus 

 to make a temporary invasion from the mucous membrane. This 

 incursion into the body is not essential to the life of the race. In fact, 

 a little reflection will show that the bacteria which invaded are not 

 likely to be transmitted, whereas those on the mucosa are readily 

 handed down from old to young. The virulence of the bacillus is thus 

 kept on a low level, so low that subcutaneous inoculation of pure 

 cultures produces merely a local lesion. This type of disease is quite 

 different from that produced by inoculation with highly virulent 

 races. These multiply rapidly in the blood throughout the body. 



We can now appreciate Pasteur's failure to exterminate the rabbits 

 of Australia. He believed that with races of this bacillus on "hand 

 which destroy life very quickly, all that is necessary is to start the 

 disease among rabbits, and it will tend to spread. The stricken rabbit 

 with its blood full of germs does not offer the means for inoculating 

 a second, and so the virulent race perishes. 



We can understand, furthermore, why the bacteria associated with 

 definite diseases in animals produce a diseased condition with diffi- 

 culty after inoculation. The virulence of the specifically adapted 

 microbe is of a relatively low order, and in the production of epizootics 

 various conditions must be realized which assist the micro-organism. 

 The careful analysis of these conditions will form one of the great 

 problems of pathology in the immediate future. 



The phenomenon of the elimination of the most virulent races and 

 the establishment of parasitic races of less invasive power I have por- 

 trayed in the simplest terms. But it is probably much more complex. 

 The parasite, to be successful, must also stand in a definite relation 

 to the tissue through which it enters. It is quite probable that the 

 race of rabbit septicemia bacilli of high virulence would not be able to 

 maintain itself in the mucus of the upper air-passages. This ability 

 to multiply in certain places is evidently an acquisition which gives 

 the particular race its specific character. Without doubt the bovine 



