310 PASTEUR: THE HISTORY OF A MIND 



panning, after a certain number of passages through 

 individuals of the same species. 



Having once attained this stability, which is not its 

 maximum virulence for the race, as we shall see later, 

 the virus preserves this degree of virulence practically 

 unchanged, if the paths of penetration do not vary. 

 This increased virulence may permit it to invade another 

 race or another species. Thus our anthrax bacteridium, 

 invigorated by a passage through the guinea-pig, can 

 infect the sheep. But there may also be produced cases 

 analogous to those in the experiments of MM. Chamber- 

 land and Roux, in which the virulence augmented for 

 one species will be diminished for another, or inversely, 

 and we reach a third possible case, that of the diminution 

 of virulence for one species bj^ passages through another 

 species. 



We shall find an example of this fact, so clear that it is 

 almost diagrammatic, in the work of Pasteur and Thuil- 

 lier, on the erysipelas of the pig. This disease is due to 

 the development in the tissues of the animal of a very 

 short and slender rod. It goes through its stages of 

 evolution very rapidly, and may cause death in some 

 hours. 



It is not confined to swine, but may also be communi- 

 cated to the pigeon and the rabbit. If there is injected 

 into the pectoral muscles of a pigeon the microbe of the 

 erysipelas taken from a diseased pig, or from a culture in 

 veal bouillon, the pigeon dies in from 6 to 8 days, 

 after having shown the external symptoms and the som- 

 nolence of chicken cholera. We might believe that the 

 two diseases are identical if the organism of the erysipelas 

 were not absolutely harmless to the chicken, which is so 

 sensitive to the action of the cholera microbe. 



If the blood of the first pigeon is injected into a second, 

 the blood of the second into a third, and so on, the malady 



