28 Process of Power Changes 



not have to fight a hostile blood reaction, and as 

 a consequence they gradually lose their disease- 

 producing powers by failing to exercise them. 

 By putting such weakened bacteria through a 

 course of training in which they are forced to 

 use these powers to a gradually increasing ex- 

 tent, their virulence can be increased to something 

 greater than it was when they were first placed 

 upon artificial food. These facts were first found 

 by Pasteur in working on the anthrax bacillus, 

 and have since been found by other persons work- 

 ing on other germs. 



