LOUIS PASTEUR 313 



and irregular at the beginning of the series of successive passages, but 

 it soon enough fixes itself at a minimum of five days. The maximum 

 virulence in guinea-pigs is reached after seven or eight passages only. 

 It is worth noting that the number of passages required before reach- 

 ing the maximum virulence, both in guinea-pigs and in rabbits, varies 

 with the origin of the first virus with which the series is begun. 



If now this rabies with maximum virulence be transferred again 

 into the dog from guinea-pig or rabbit, there is produced a dog-virus 

 which in point of virulence goes far beyond that of ordinary canine 

 madness. 



But, a natural query — of what use can be that discovery as to the 

 existence and artificial production of diverse varieties of rabies, every 

 one of them more violent and more rapidly fatal than the habitual 

 madness of the dog? The man of science is thankful for the smallest 

 find he can make in the field of pure science, but the many, terrified 

 at the very name of hydrophobia, claim something more than mere 

 scientific curiosities. How much more interesting it would be to be- 

 come acquainted with a set of rabies viruses which should, on the 

 contrary, be possessed of attenuated degrees of virulence! Then, 

 indeed, might there be some hope of creating a number of vaccinal 

 rabies viruses such as we have done for the virus of fowl-cholera, of 

 the microbe of saliva, of the red evil of swine (swine-plague), and 

 even of acute septicaemia. Unfortunately, however, the methods 

 which had served for those diflFerent viruses showed themselves to be 

 either inapplicable or inefficient in the case of rabies. It therefore 

 became necessary to find out new and independent methods, such, for 

 example, as the cultivation in vitro of the mortal rabies virus. 



Jenner was the first to introduce into current science the opinion 

 that the virus which he called the grease of the horse, and which we 

 call now more exactly horse-pox, probably softened its virulence, so 

 to speak, in passing through the cow and before it could be trans- 

 ferred to man without danger. It was therefore natural to think 

 of a possible diminution of the virulence of rabies by a number of 

 passages through the organisms of some animal or other, and the ex- 

 periment was worth trying. A large number of attempts were made, 

 but the majority of the animal species experimented on exalted the 

 virulence after the manner of rabbits and guinea-pigs ; fortunately, 

 however, it was not so with monkey. 



