EXALTATION OF VIRULENCE 35 



example which, by the constancy of the results obtained, is 

 infinitely superior to his sportive production of a salmon-coloured 

 " Coral king " Primula. It is the classical example adduced long 

 years ago by Pasteur and his lieutenants, Roux and Chamber- 

 land — a bacteriological commonplace, since it is the basis of 

 Pasteur's method of vaccination against anthrax, which, com- 

 mercially speaking, has saved France hundreds of thousands 

 of sheep and cattle and many millions of francs. By exposing 

 cultures of anthrax bacilli to a heat a degree or two below that 

 which will kill them, or again by the action of minute quanti- 

 ties of certain antiseptics added to the medium of growth, races 

 of anthrax bacilli are obtainable which have wholly lost the 

 power of spore production. These races may be grown for 

 months and years — for thousands and, indeed, hundreds of 

 thousands of generations — upon the ordinary media of the 

 laboratory without regaining this striking and characteristic 

 property. Were a culture of this strain given to a systematic 

 botanist without information as to its origin, he most assuredly 

 could classify it as a distinct species. Here again is no matter 

 of chance variation in many directions — all the individuals of 

 the colony subjected to the particular temperature become 

 asporogenous. 



On Exaltation of Virulence 



Now if these things be true regarding other properties, they 

 must be largely true regarding the virulence and pathogenic 

 powers of bacteria. We know in the first place that virulence, 

 when it is the property of any bacterial species, is capable of 

 great modification. This, we observe, is not a matter of chance. 

 We know methods by which we can surely exalt or depress 

 that virulence. We know that by " passage " of a virus through 

 a succession of animals of one species we can rapidly intensify 

 the virulence for animals of that species, while simultaneously 

 we may by this procedure reduce its virulence for animals of 

 another species. We can, with Marmorek, take a culture of 

 streptococci so weak that only the most susceptible animals are 

 influenced by it, and then only by the exhibition of relatively 

 enormous amounts of the culture to the new-born (which have 

 least powers of resistance) ; but recovering the cocci from these 



