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 algae and the lower fungi, but they 

 have also some points of resemblance with certain of the pro- 

 tozoa. On account of their extreme smallness it is impossible 

 to analyze the structure of the individual bacteria 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 

 probably not perfectly trustworthy. The agglutination of a 

 species of bacteria by blood-serum specific for the species (see 

 Chapter VII., Part II.) has been used for purposes of identifica- 

 tion. It is likely that forms which are now considered as dif- 

 ferent 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 



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