110 MICROBES AND TOXINS 



and to the ox, to birds, reptiles, frogs, and fishes, but no one 

 has ever succeeded in producing a tubercle bacillus of the 

 human type from the tubercle bacilli (or acid fast bacilli) of 

 the frog. Nocard's experiments in which he transformed the 

 human bacillus into the bird bacillus by repeated passages 

 have not been confirmed. The researches of recent years 

 confirm the idea of Th. Smith and R. Koch, that the human 

 bacillus and the bovine bacillus cannot be transformed one 

 into the other, at least under the time conditions of our 

 experiments ; the fixity of these acquired characters has even 

 raised the hope that it might be possible to vaccinate cattle 

 against bovine tuberculosis by means of bacilli of the human 

 type and vice-versd, perhaps, men with the bovine bacillus or 

 products derived from it. 



The Darwinian conception of evolution in pathogenic mi- 

 crobes is nevertheless true, although direct proofs are lacking. 

 Similarly the simian origin of man would be quite as certain, 

 although some of the proofs were lacking, and even although 

 we might not be able to demonstrate forms intermediate 

 between man and monkey. 



Specificity. It is necessary to distinguish between the 

 specificity of the microbe and that of the disease. 



Typhoid fever is caused by the bacillus of Eberth and by 

 it alone specificity of the disease. The typhoid bacillus 

 remains the typhoid bacillus in cultures and in the intestine ; 

 it does not tend towards either the dysentery bacillus or the 

 coli bacillus specificity of the microbe. These two ideas hang 

 together ; the same causes must produce the same effects if 

 there is to be any science at all, but to produce the same effect 

 it is absolutely necessary that the cause remain the same. 



A disease appears with certain symptoms and anatomical 

 lesions due to a definite microbe, but these symptoms and 

 these lesions may be produced by others ; for example, the 

 tubercle is the lesion par excellence produced by the tubercle 

 bacillus of Koch, but tubercles are also produced by the 

 bacillus of glanders. There are not so many possibilities of 

 reaction in the body as there are bacteria; fever, effusions, 



