Process or Power Changes 27 



that blood reaction. And a man who is vacci- 

 nated builds up his powers of resisting smallpox 

 by beginning his exercise on a weak form of it, 

 with the result that he becomes able to resist fully 

 virulent smallpox when it later puts in an appear- 

 ance. But after he is vaccinated he ceases to 

 exercise his powers of fighting smallpox virus 

 because there is none present to fight, and the 

 result is that these powers gradually decline. If 

 he is to continue to be able to resist smallpox he 

 must again exercise the powers he has by again 

 being vaccinated. 



Here we have a case of one kind of power 

 being built up in the protoplasm of a germ by 

 exercising that power, and simultaneously another 

 power in the same protoplasm was permitted to 

 decay by reason of a failure to exercise it. Also, 

 we have the case of resisting powers being built 

 up in the bodily cells of the man by exercising 

 such powers as he had at the time, and subse- 

 quently the decline of the same powers in the 

 same person by a failure to exercise them for 

 some years. 



When disease-producing bacteria are raised on 

 some artificial food, as bouillon or agar, they do 



