BACTERIA IN INFECTIOUS DISEASES. 253 



furnish the favorable conditions required. In this 

 way we may, perhaps, explain the origin of epi- 

 demics of pestilential diseases, such as yellow fever 

 and cholera. If these diseases do not, at the pres- 

 ent day, originate in the manner indicated, they 

 at all events have their permanent abiding place 

 in tropical countries. Although the specific germs 

 of these diseases have not been demonstrated, 

 there is strong reason for believing that they re- 

 sult from the direct or indirect action of living 

 ferments. For there is abundant evidence to 

 prove that the specific poisons to which they are 

 due may multiply indefinitely external to the bodies of 

 the sick. Such multiplication is a property of liv- 

 ing matter only. Moreover, the conditions which 

 favor this multiplication an elevated tempera- 

 ture and the presence of decomposing organic 

 material are exactly the conditions required for 

 the development of low organisms. 



The experimental transformation of the harm- 

 less hay-bacillus (B. subtilis) into the deadly Ba- 

 cillus anthracis has been claimed by Buchner and 

 by Nageli ; and Prof. Greenfield claims to have 

 transformed, by a series of culture experiments, 

 the anthrax bacillus into a harmless form not dis- 

 tinguishable from the hay bacillus. Koch insists, 

 however, that these are distinct species, and the 

 weight of evidence seems to be in favor of this 

 view. However this may be, it is beyond ques- 

 tion that the anthrax bacillus may undergo a re- 

 markable modification as regards virulence; and 



