6 



bacteria seem to possess no differentiation of structure 

 nor localization of function ; nutrition and assimilation are 

 processes of osmosis. This simplicity of structure and 

 function has given rise to discussion as to whether they 

 should be regarded as animals or vegetables ; the question 

 is, of course, merely a technical one of classification; 

 since the features which distinguish the higher animals 

 from the higher plants disappear as we descend the scale 

 of organic life until few or none remain ; yet because 

 almost all of the simplest organisms hitherto called ani- 

 mals, the flagellata, possess a rudimentary mouth and are 

 capable of absorbing solid food, while the simplest plants 

 are not so characterized, the bacteria have been assigned 

 to the vegetable kingdom. Whether they should be called 

 algae or fungi is a question for botanists to decide ; the 

 power of independent motion exhibited by some varieties 

 suggests affinity with the algae ; but the absence of chlo- 

 rophyll is generally considered to require their classifica- 

 tion among the fungi. 



/ The necessities of their existence are as simple as those 

 ' of the mould fungi ; indeed, so nearly identical as to re- 

 quire no discussion. As to the chemical reactions inci- 

 dent to their vital activity, our present knowledge is very 

 scanty ; one variety is known to induce the transformation 

 of grape and milk sugar into lactic acid ; another the de- 

 composition of glucose or lactic acid with formation of 

 butyric acid ; another the change of urea into carbonate 

 of ammonia ; some produce pigments, blue, red, yellow ; 

 of many we know only that they transform a solid sub- 

 stance gelatine, for example into a liquid ; but one of 

 the most important facts in regard to them is the proof 

 that putrefaction of albuminous substances is a phenom- 

 enon incident to the vital activity of certain varieties 

 the bacterium termo and probably others as must be 

 admitted by every one familiar with the work of Pasteur, 

 Tyndall, and their pupils. Until the chemistry of their 

 vital processes is ascertained, it will be impossible to as- 

 sert how they can be injurious to a living tissue, whether 

 by simple mechanical irritation, by the appropriation of 



