ACQUIRED IMMUNITY 79 



cases, by passing through a series of non-susceptible 

 animals. 



As a general rule, pathogenic virulence is increased 

 by successive inoculations in susceptible animals and 

 diminished by cultivating the pathogenic micro-organ- 

 ism in artificial media outside of the animal body, or 

 by passing it through animals having but slight sus- 

 ceptibility to its pathogenic action. As pathogenic 

 virulence depends, to a considerable extent at least, 

 upon the formation of toxic substances during the 

 active development of the pathogenic micro-organism, 

 we infer that diminished virulence is due to a dimin- 

 ished production of these toxic substances. 



An important step was made in the progress of our 

 knowledge in this field of research when it was shown 

 that animals may be made immune against certain 

 infectious diseases by inoculating them with filtered 

 cultures containing the toxic substances just referred 

 to, but free from the living bacteria to which they 

 owe their origin. The first satisfactory experimental 

 evidence of this important fact was obtained by Sal- 

 mon and Smith in 1886. These bacteriologists suc- 

 ceeded in producing an immunity in pigeons against 

 the pathogenic effects of the bacillus of hog cholera, 

 which is very fatal to these birds, by inoculating them 

 with sterilised cultures of the bacillus mentioned. 

 Similar results were reported by Roux in 1888, from 



