144 THE STORY OF GERM LIFE. 



disease after having been cultivated for a short 

 time in ordinary culture media in the laboratory. 

 This is easily understood upon the suggestion 

 that it is a parasitic bacillus and does not thrive 

 except under parasitic conditions. Its patho- 

 genic powers can sometimes be restored by pass- 

 ing it again through some susceptible animal. 

 One of the most violent pathogenic bacteria is 

 that which produces anthrax, but this loses its 

 pathogenic powers if it is cultivated for a con- 

 siderable period at a high temperature. The 

 micrococcus which causes fowl cholera loses its 

 power if it be cultivated in common cultur^jnedia, 

 care being taken to allow several days to elapse 

 between the successive inoculations into new 

 culture flasks. Most pathogenic bacteria can 

 in some way be so treated as to surfer a dimi- 

 nution or complete loss of their powers of pro- 

 ducing a fatal disease. On the other hand, other 

 conditions will cause an increase in the virulence 

 of a pathogenic germ. The virus which produces 

 hydrophobia is increased in violence if it is 

 inoculated into a rabbit and subsequently taken 

 from the rabbit for further inoculation. The 

 fowl cholera micrococcus, which has been weak- 

 ened as just mentioned, may be restored to its 

 original violence by inoculating it into a small 

 bird, like a sparrow, and inoculating a second 

 bird from this. A few such inoculations will 

 make it as active as ever. These variations 

 doubtless exist among the species in Nature as 

 well as in artificial cultures. The bacteria 

 which produce the various wound infections and 

 abscesses, etc., appear to vary under normal con- 

 ditions from a type capable of producing violent 

 and fatal blood poisoning to a type producing 



