33 



for a period somewhat less than six weeks that mice seem to 

 be impervious to its attacks. But how it is that this attenuation 

 takes place — whether it is due to the effect of heat alone or to the 

 effect of oxygen — bacteriologists are by no means agreed. Further 

 it has been shown that when the attenuation of the bacillus has 

 been brought about in a certain way (by the addition for instance 

 of carbolic acid or potassium bichromate to the cultivation) its 

 virulence is not regained when cultivated in fresh material at the 

 ordinary temperature. But, attenuated in a different fashion, it 

 does regain its virulence under those conditions. Again the viru- 

 lence of a parasitic organism is materially altered according to the 

 animal through which it has been transmitted. "In the course of 

 Pasteur's interesting researches on swine plague he found that 

 pigeons inoculated with the virus of swine plague died in six or 

 eight days after suffering in the first instance from symptoms like 

 those of fowl cholera. If the disease is transmitted from pigeon 

 to pigeon, the organism after a time acts more violently, and the 

 animal dies sooner. If, now, pigs are inoculated from these 

 pigeons, death occurs more quickly than when inoculated from a 

 pig, the organism having become more virulent. With the rabbit 

 the converse is the case. The virus kills rabbits, but if it is passed 

 through a sei-ies of rabbits it is no longer able to kill pigs" 

 (Watson Cheyne). 



Now, if this is the case, if the virulence of parasitic or 

 j)athogenic organisms can be thus modified, it seems incredible 

 that organisms of similar structure, though apparently harmless 

 saprophytes, cannot be cultivated in some way or other so as 

 to become pathogenic. And when the bacteriologists tell us that 

 it is impossible, that only means, I suppose, that they have not 

 yet discovered the way. Specific diseases and pathogenic bacteria 

 L. 3 



