THE RETURN TO VIRULENCE. 247 



preventing the aerobic microbe from turning to its 

 own account the oxygen of the blood. The disease 

 does its work and death supervenes. 



After all, there is nothing surprising in the fact 

 that the vital resistance of a newly-born guinea-pig 

 should differ from that of an adult one. But what is 

 very remarkable is, that if an older guinea-pig be 

 inoculated with the blood of one a day old ; if a third, 

 still older, be inoculated with the blood of the second ; 

 and so on ; the virulence of the microbe will be gradu- 

 ally reinforced that is to say, the usual habit of this 

 parasite to develop itself in the body of the animal 

 will be restored. The process may be likened to that 

 of an animal or vegetable species, passing by successive 

 stages and long sojourns, from one region to another 

 very distant one, subjected to quite new conditions of 

 climate, and gradually becoming acclimatised to the 

 last one. How great, then, must be the importance 

 of the medium of cultivation, with regard to the viru- 

 lence of the microbes of communicable diseases ! 

 Cultivating the microbe by passing it from one guinea- 

 pig to another, we soon arrive at a strength capable 

 of killing guinea-pigs of a week, a month, or several 

 years old, until at last the smallest drop of the blood 

 of these guinea-pigs suffices to kill a sheep ; and from 

 the sheep we may pass on to the ox. 



The same is the case with the microbe of fowl 

 cholera. When it has ceased to have any effect upon 

 13 



