THFAR PLACE IN NATURE. 33 



To such adaptable sjiecies the designation " facultative " 

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

 in question has the faculty of adapting itself to condi- 

 tions other than those in which it is usually encoun- 

 tered. In this sense all of the disease-producing bac- 

 teria that can be cultivated artificially are manifestly 

 facultative metatrophs or sajarophytes. 



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

 and energetic that they result in the moat 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. 



The role played in nature by the metatrophs is a very 

 important one. Through their functional activities the 

 highly complicated tissues of dead animals and vege- 

 tables are resolved into the simpler compounds, carbonic 

 acid, water, and ammonia, in which form they may be 

 taken up and appropriated 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 grow- 

 ing vegetation 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 



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