FORM AND STRUCTURE OF MICROBES 61 



upon the partisans of Cohn and Koch declared that though the 

 saprophytic species might lack stability, the pathogenic species 

 were stable. In their eyes the bacillus of anthrax was always a 

 bacillus. But it had to be recognised later that pleomorphism 

 existed even among pathogenic species, for example the cholera 

 vibrio and the bacterium of fowl-cholera. 



Finally the stability school maintained the necessity of 

 distinguishing between the constancy of the mere shape and 

 that of the species — a cholera vibrio may modify its form and 

 yet remain always the vibrio of cholera — the tadpole of the 

 green frog does not resemble an adult green frog, yet it belongs 

 without any doubt to the species green frog. But it is certain 

 that it is not only the form, but also the species qua 

 species which is variable amongst the bacteria, and this 

 variability is even more characteristic of their physiological 

 properties than of their shape. The bacillus of tuberculosis, 

 amongst others, is susceptible of very divergent adaptation. 



Bacterial species exist, but they are all of the kind called in 

 the language of the science of classification '■'■ill-defined." 

 "That the methodical classifier should be frequently embarrassed 

 in his attempts to set up his artificial barriers, is very far indeed 

 from being surprising. The world was not created for the 

 special pleasure of descriptive botanists, and it is equally inter- 

 esting to science to possess the demonstration that a classification 

 is impossible as to have established a classification, had such 

 been possible" (E. Duclaux). 



In any case their faculty of adaptation and their power of 

 variation sufficiently explain the history of bacteria. The 

 pathogenic species must have come from saprophytic species 

 by adapting themselves to certain animal bodies. The same 

 virus, adapting itself in the course of ages to the human species 

 and to the cow, has produced the two diseases small-pox and 

 cow-pox, and this adaptation has become the principle of a 

 wonderful prophylactic procedure. It was also by causing 

 variations in the virus by physical and chemical action that 

 Pasteur first prepared vaccines. 



More recently search has been made among bacteria for 



