December 16, 1904.] 



SCIENCE. 



825 



eral avenues of entry and escape. In some 

 of the protozoa there is but one avenue, 

 and this is highly specialized and is 

 through the body of some insect. Among 

 the bacteria the channels of escape are less 

 highly developed, and there may be sev- 

 eral. As a rule, the mici-.obe adapts itself 

 eventually to a locus more or less in direct 

 contact with the exterior, and in some in- 

 stances the loci of entry, multiplication 

 and exit have coincided. If we think over 

 the various infectious diseases of man and 

 animals, of which we have any definite in- 

 formation, we shall be surprised to find in 

 how many the points of attack are in or- 

 gans or tissues in direct communication 

 with the exterior. In the most common 

 type of tuberculosis, pulmonary consump- 

 tion, the process is almost wholly limited 

 to the respiratory oi'gans. In typhoid 

 fever the process is to a large degree car- 

 ried on in the intestinal wall. In dysentery 

 and cholera it is wholly so. Even in the 

 very protracted disease of leprosy, the skin 

 is the chief seat of the disease, while the 

 discharge of bacilli from the ulcers of the 

 nose is the rule in the tuberculous type. 

 In that exquisitely parasitic, highly special- 

 ized group of microorganisms producing 

 the eruptive diseases the final process is 

 carried on in the skin. In these diseases 

 the mechanism of escape is the most 

 perfect. 



On the other hand, among the spore- 

 bearing pathogenic bacteria the means of 

 escape is uncertain. Thus the anthrax 

 bacillus betrays its saprophytic nature, as 

 pointed out by Koch many years ago, in 

 its inability to produce spores within the 

 body. "Were it not for the accidental dis- 

 charges of blood from the mucous surfaces 

 and the operations of man, the bacillus 

 might not escape at all to sporulate. Sim- 

 ilar conditions obtain for the bacillus of 

 tetanus and of Rauschbrand. Both pro- 

 duce disease probably in an accidental 



manner and not as progressive parasites. 

 Their continued existence is assured by veg- 

 etation and spore formation outside of the 

 body, and it is highly probable that the 

 species would continue to exist if they did 

 not attack animal life, and that their in- 

 cursions into the body are of no use to 

 them. On the other hand, all attempts to 

 demonstrate the production of spores in 

 bacteria whose existence depends largely or 

 wholly upon parasitism have thus far 

 failed. The spore is evidently poorly fitted 

 to parasitism and is replaced by other de- 

 vices of more adaptability. 



The mechanisms of invasion and escape 

 bear a distinct relation to the pathogenic 

 power or virulence. It is safe to assume 

 that those varieties or species, no matter 

 how virulent, will be eventually destroyed 

 if these mechanisms are imperfect. In 

 fact the more virulent the microbe, the 

 more rapid the death as a result of inva- 

 sion, the less the opportunity for escape. 

 Hence there will be a selection in favor of 

 those varieties which vegetate whence they 

 can escape. The surviving varieties would 

 gradually lose their highly virulent inva- 

 sive qualities and adapt themselves more 

 particularly to the conditions surrounding 

 invasion and escape. That some such 

 process of selection has been going on in 

 the past seems the simplest explanation of 

 the relatively low mortality of infectious 

 diseases. These individuals or races of mi- 

 crobes which invaded the host too rapidly 

 and caused death would be destroyed in 

 favor of those which vegetated more slowly 

 and in tissues permitting escape of the mi- 

 crobe after a certain time. 



We may now return to the rabbit sep- 

 ticemia 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 



