PHYSIOLOGY OF MICRO-ORGANISMS 1 75 



narrow limits in respect to a great many characters, fluctuating 

 about a mean type which is that best adapted to the environment. 

 With a change in surrounding conditions, this mean or normal 

 type may no longer be best adapted, but a variation slightly 

 removed in respect to certain characters may flourish better and 

 become the mean type about which the fluctuating variants group 

 themselves. Thus the pure culture seems to respond to environ- 

 mental change. Whether the fluctuating variations are due to 

 small differences in the immediate surroundings of the individual 

 microbes, or whether they arise as a result of a property of varia- 

 bility inherent in protoplasm, may be disputed, but the latter 

 view is more commonly held by biologists. 



, The Products or Microbic Growth 



The effects resulting from the growth of a micro-organism 

 depend on the one hand upon the nature of the organism and on 

 the other upon the environment, more especially the medium in 

 which it grows and the conditions of temperature and oxygen 

 supply. Apparently slight variations in the latter may influence 

 the results to a marked degree. 



Physical Effects.- — Heat is evolved by many actively growing 

 bacterial cultures and is especially evident in the fermentation 

 of such substances as ensilage and manure. Perhaps some of 

 the heat may result directly from microbic activity, but the most 

 of it appears to arise from secondary chemical reactions in which 

 the microbic products som,e times play a part. Microbes which 

 produce heat are designated as thermogenic. Light is also emitted 

 by some microbic cultures. Here it seems certain that the light 

 is produced by the oxidation of a bacterial product and not emitted 

 directly by the micro-organisms. These phosphorescent or photo- 

 genic organisms occur in salt water and on fish and they have 

 rarely been found in other places. 



Chemical Effects. — These are the most important results of 

 microbic growth. As we have just seen, the production of 

 heat and light is probably due to a secondary reaction entered 



