THE VITAL PHENOMENA OF BACTERIA. 139 



living organisms, or to produce disease, is, as 

 a matter of fact, variable. It can dimmish, 

 but it can also under some conditions increase. 



Bacteria depend for their provision of food 

 and energy upon the conditions of nutrition. 

 If these conditions remain constant the action 

 of the bacteria does not vary. If the condi- 

 tions of existence do not remain constant, one 

 of three things comes to pass. Either the 

 bacteria change in form and action and adjust 

 themselves to the new conditions, or they form 

 spores which preserve the species until better 

 times return, or they fail to adapt themselves 

 at all and so perish. The various kinds of pig- 

 ment, ferment, and disease-producing bac- 

 teria are therefore in fact not to be considered 

 as species in the natural history sense, but as 

 nutritional modifications. 



For this reason we recognize the existence 

 of complete "cycles of activity." According 

 to Fitz the butyric acid germ forms butyric acid 

 out of the saccharoses as the chief product, and 

 butyl alcohol as a by-product ; while out of 

 glycerin it forms, besides butyric acid, much 

 normal butyl alcohol, and as by-products propyl 

 glycol and lactic acid. The bacteria of glanders 

 form a brown pigment on potato, and their 

 growth is accompanied by decrease of virulence ; 

 the yellow pus-coccus (Staphylococcus pyogenes 



