THE PROBLEM OF IMMUNITY 303 



brutal strife where the only possible means of interven- 

 tion consisted in the suppression of one of the adversaries, 

 but a gentle strife which one might attempt to direct by 

 augmenting or diminishing the forces of one of the con- 

 testants. It was only a question of finding the ground 

 and the object of the strife, and, for that purpose, he had 

 the experimental method: it was possible, working with 

 a single species subject to anthrax, to study bacteridia 

 of different degrees of virulence; it was possible, with 

 the same bacteridium, to study different species, or 

 animals of the same species unequally vaccinated, which 

 made them, to a certain degree, different animals. We 

 see what a field of labor opened before him. It is 

 characteristic of certain discoveries that they suddenly 

 reveal vast horizons. Pasteur had climbed little by 

 little to one of those mountain heights from which 

 a whole new country is visible. He plunges into 

 it with delight. Let us accompany him. We can 

 no longer follow him closely and must abandon the his- 

 torical order. In the first place, we have reached the 

 latter part of his life and his later conceptions. In the 

 second place, what interests us is the plan of the edifice, 

 and not the order in which its different parts have been 

 erected. If we wish to know which part belongs to 

 Pasteur himself, which part he has built, we must take 

 it in the condition in which Pasteur left it, with its 

 finished parts, with its stones yet unplaced, and with a 

 brief indication of what the progress of science has 

 contributed to it. 



