CHEMICAL AND HUMORAL THEORIES OF IMMUNITY 313 



virulent bacteridium, others with the attenuated bacteri- 

 dium. The first develops and kills the sheep. The 

 second, after a period of growth made with more or less 

 difficulty and causing a transitory illness of the sheep, 

 abandons the struggle and leaves the animal more or 

 less vaccinated. This is one method of studying the 

 influence of the bacteridium alone. 



Now let us take a normal sheep and a vaccinated 

 sheep, into which we inoculate a very virulent strain of 

 the anthrax bacteridium. It kills the first and has no 

 effect on the second. Here we have a way to study the 

 influence of immunity acquired by a former vaccination. 



Let us take now a French sheep and an Algerian sheep ; 

 let us inoculate both of them with a light dose of a viru- 

 lent strain of the bacteridium. The first will die, the 

 second will resist, after an illness in general benign. In 

 this we have the influence of race or of natural immunity. 



The French sheep has a natural immunity for the 

 attenuated bacteridium; the Algerian sheep, a natural 

 immunity against the virulent disease; the vaccinated 

 sheep an acquired immunity, more or less marked; the 

 dog, a natural and absolute immunity. In all the cases, 

 the natural or acquired immunity, when it is complete, is 

 correlative with the non-development of the bacteridium, 

 which instead of invading the tissues, remains confined 

 to the point of inoculation or its vicinity, and finally 

 perishes there. 



What is the cause of this non-development of a 

 living cell which has been sown? This question is what 

 the inquiry led to! We see that it was precise. It was 

 already a conquest only to be able to state it thus. Until 

 that time it had been necessary to bow down without 

 seeking to penetrate the mystery. What reply, in fact, 

 can be given to this general question: Why is the sheep 

 sensitive to anthrax, and the dog not sensitive? Why 



