230 MICRO-ORGANISMS AND DISEASE. [CHAP. 



the body of a suitable animal it produces the highly infectious 

 fatal malady known as anthrax. The micrococcus of ery- 

 sipelas is now well known through the admirable researches 

 of Fehleisen to be capable of existence and multiplication 

 outside the animal body ; it grows well in artificial cultures, 

 so does the tubercle bacillus of Koch, so does the bacillus 

 which I described of swine-plague, mentioned in a former 

 chapter, and so do other micro-organisms. Davaine's 

 septicaemia in rabbits, Koch's septicsemia in mice, &c., &c., 

 cannot be produced by every putrid blood or putrid organic 

 fluid, only by some, only now and then, i.e., when the 

 particular micro-organism capable of inducing the disease 

 is present in those substances, and then only when it finds 

 access into a suitable animal. Davaine's septicaemia of 

 rabbits cannot be induced in guinea-pigs, Koch's septicaemia 

 of mice cannot be induced in guinea-pigs, anthrax bacilli 

 cannot induce the disease in dogs, and so with the other 

 micro-organisms. 



We conclude then from this that some definite micro- 

 organisms, although as a rule existing and growing in various 

 substances of the outside world, have the power when finding 

 access into the body of a suitable animal to grow and thrive 

 here also, and to induce a definite pathological condition. 

 But this power they have ab initio. Those that do not 

 possess this power cannot acquire it by any means whatever. 

 Just as there are species of plants .which act as poisons to 

 the animal body, and other species of plants which, although 

 belonging to the same group and family, and although very 

 much alike to the others, have no such power and cannot 

 acquire such a power by any means, so there are micro- 

 organisms which are pathogenic while others are quite 

 harmless. The latter remain so no matter under what 

 conditions and for how long they grow. 



