THE LABOK4TOEY OF THE ECOLE NOKMALE. 287 



' The Academy remembers, without doubt,' said 

 Pasteur in a recent communication, * that, some time 

 ago, we discovered a microbe virus in the saliva of 

 hydrophobia. This microbe, though very virulent for 

 rabbits, is shown to be harmless for adult guinea-pigs, 

 but it kills rapidly guinea-pigs only some hours or 

 days old. In following out this inoculation from 

 young guinea-pigs, we have seen the virulence increase, 

 and easily arrive at the point of causing death to 

 older guinea-pigs. There was even at last a marked 

 difference in the lesions. The increase of virulence, 

 by successive passages through individuals of one 

 race, was clearly shown. 



' But the new and unexpected result that I wish to 

 point out to the Academy consists in this : that the 

 microbe, after having increased its virulence by 

 successive passages through the bodies of guinea-pigs, 

 shows itself to be less virulent in relation to rabbits 

 than it was before, 



' In these new conditions, it gives to the rabbit a 

 disease which is spontaneously curable ; and, moreover, 

 having once gone through the malady, the animal be- 

 comes refractory in regard to the microbe which is 

 deadly to rabbits. From this arises the all-important 

 consequence, that the habit of living in one species 

 (the guinea-pig) at a definite corresponding degree of 

 virulence, can change this virulence in relation to 

 another species (the rabbit), so much diminishing its 



