THEIR PL A CE IN NA TURE. 33 



To such adaptable species the designation " facultative " 

 is given, and, when employed, signifies that the species 

 in question has the faculty of adapting itself to en- 

 vironments other than those in which it is usually en- 

 countered. In this sense all of the disease-producing 

 bacteria that can be cultivated artificially are manifestly 

 facultative metatrophs or saprophytes. 



The life-processes of bacteria are so rapid, complex, 

 and energetic that they result in the most profound 

 alterations in the structure and composition of the 

 materials in and upon which they are developing. 



Decomposition, putrefaction, and fermentation result 

 from the activities of the metatrophic bacteria ; while 

 the changes brought about in the tissues of their living 

 host by the purely parasitic forms find expression in 

 disease-processes, and not infrequently in complete death. 



THEIR PLACE IN NATURE. The role played in 

 nature by the metatrophs is a very important one. 

 Through their functional activities the highly compli- 

 cated tissues of dead animals and vegetables are resolved 

 into the simpler compounds, carbonic acid, water, and 

 ammonia, in which form they may be taken up and ap- 

 propriated as nourishment by the more highly organized 

 members of the vegetable kingdom. It is through this 

 ultimate production of carbonic acid, ammonia, and 

 water by bacteria, as end-products in the processes of 

 decomposition and fermentation of dead animal and 

 vegetable tissues, that the demands of growing vegeta- 

 tion for these compounds are supplied. 



The chlorophyll plants do not possess the power of 

 obtaining their carbon and nitrogen from such highly 

 organized and complicated substances as serve for the 

 nutrition of bacteria, and as the production of the simpler 

 compounds, carbon dioxide and ammonia, by the animal 



