PART II. 



CHAPTER I. 



CLASSIFICATION ; GENERAL MORPHOLOGY AND PHYSIOLOGY 

 OF BACTERIA. 



THE relationships existing between bacteria and other 

 kinds of organisms are not perfectly clear. It is quite 

 generally conceded, however, that bacteria are plants. They 

 show affinities with both the lower algse and the lower fungi, 

 and they have even some points of resemblance with certain 

 of the protozoa. On account of their extreme smallness it 

 is impossible to analyze the structure of the individual bac- 

 teria and to contrast the structure of one with that of 

 another. The classification cannot therefore be established 

 on morphological grounds chiefly, as is done with large 

 animals or plants. We are obliged to rely also upon their 

 growth with relation to the presence or absence of oxygen 

 and to temperature, their behavior on culture-media, the 

 appearances of the growths, and the production of certain 

 substances with peculiar chemical reactions, when we wish 

 to establish the points of difference between one species and 

 another all of which is extremely unsatisfactory and prob- 

 ably not perfectly reliable. It is likely that forms which are 

 now considered as different species are not really such in all 

 cases, and also that different species may now be included 

 under one heading as a single species. Notwithstanding the 

 unsatisfactory condition of the classification of bacteria, it 

 must not be supposed that the species of bacteria are not 



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