826 



SCIENCE. 



[N. S, Vol. XX. No. 520. 



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 pas- 

 sages. Under certain conditions it invades 

 the lungs, pleural and pericardial, more 

 rarely the peritoneal cavity, producing 

 pneumonia and extensive exudates on the 

 serous membranes, and causing death. The 

 disease of the thoracic organs evidently fol- 

 lows 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 vir- 

 ulence 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 differ- 

 ent from that produced by inoculation with 

 highly virulent races. These multiply rap- 

 idly 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 inocu- 

 lating a second, and so the virulent race 

 perishes. 



We can understand, furthermore, why 

 the bacteria associated with definite dis- 

 eases in animals produce a diseased condi- 

 tion with difficulty after inoculation. The 

 virulence of the specifically adapted mi- 

 crobe is of a relatively low order-, and in 

 the production of epizootics various condi- 

 tions !iiust be realized which assist the mi- 

 eroora'ani'^m. The careful analysis of these 



conditions will form one of the great prob- 

 lems of pathology in the immediate future. 



The phenomenon of the elimination of 

 the most virulent races and the establish- 

 ment of parasitic races of less invasive 

 power I have portrayed in the simplest 

 terms. But it is probably much more com- 

 plex. The parasite, to be successful, must 

 also stand in a definite relation to the tissue 

 through which it enters. It is quite prob- 

 able that the race of rabbit septicemia ba- 

 cilli of high virulence would not be able 

 to maintain itself in the mucus of the 

 upper air passages. This ability to multi- 

 ply in certain places is evidently an acqui- 

 sition which gives the particular race its 

 specific character. Without doubt the bo- 

 vine tubercle bacillus, though of great vir- 

 ulence, does not possess the specific power 

 of entering the human body, or it may be 

 of maintaining itself after entry in certain 

 tissues, such as the lymph-nodes, except 

 under certain accidental, favoring condi- 

 tions not yet understood. Perhaps the 

 process of cultivating vaccine virus in the 

 skin has deprived it of the capacity for 

 entering through the respiratory tract, and 

 has converted it into a purely inoculable 

 virus. 



In the study of pathogenic bacteria the 

 relative ease with which pure cultures may 

 be obtained from the blood and other or- 

 gans only accessible by way of the blood 

 has made this a favorite way of obtaining 

 such cultures. But it may be asked 

 whether we are not in this way obtaining 

 bacteria of maximum virulence. May not 

 the non-agglutinability of some typhoid 

 bacilli immediately after isolation be ac- 

 counted for in this way? In general, the 

 bacteria thus obtained can differ but little 

 from the type, as they are all recently 

 descended from a single bacillus or a very 

 few which caused the infection. It is dif- 

 ferent in the so-called passages through se- 

 ries of animals in which the usual portals 



