246 PASTEUR: THE HISTORY OF A MIND 



to the point of entry, but it did not on that account 

 change its nature, and its entity, its fundamental unity 

 in the midst of the different phases of the disease which 

 it produced, was the foundation of the doctrine. With- 

 out doubt variations in strength, in virulence, had been 

 observed when a virus was transmitted from one spe- 

 cies to another, but this had been observed also and even 

 to a greater degree in the same species; epidemics of 

 smallpox were more or less benign; smallpox produced 

 by inoculation was ordinarily less dangerous than that 

 which had furnished the material for the inoculation. 

 All of these variations in the severity of the disease or 

 of the epidemic seemed beyond the reach of experimen- 

 tation and were attributed to external factors, cold, 

 heat, or meteorological conditions. Such is the state 

 to which one was reduced by the impossibility of ob- 

 serving the virus outside of a living creature. 



If, on the contrary, the bacteridium is a ferment, a 

 parasite, the aspect of the question is changed. We 

 can cultivate the bacillus outside of the organism, study 

 its properties, learn its physiology, and compare its 

 physiological with its pathological r61e to discover what 

 effect its normal functions have on the normal functions 

 of the animal it invades. Disease resulting thus from the 

 physiological conflict between two organisms which can 

 be studied separately, its study took a new direc- 

 tion. It is very clear that Pasteur was not thinking at 

 this tune of variations in virulence among bacteria, nor 

 of vaccinations. But his was an intellect so keen that 

 I would not affirm that this idea was not in the back- 

 ground of his mind and I could cite as an argument the 

 eagerness with which he fell upon the first explicable fact 

 in this class of ideas. We shall see him at this presently. 

 In the first place, the important question to be solved 

 seemed to him to be this: is the essential agent of anthrax 



