PART II. 



CHAPTER I. 



CLASSIFICATION ; GENERAL MORPHOLOGY 

 AND PHYSIOLOGY OF BACTERIA. 



It has been universally agreed to class the bacteria as plants 

 although they show resemblances to both plants and animals. 

 On the one hand, they seem allied to the algas and fungi, 

 and, on the other, to the protozoa. 



The classification of the larger animals and plants is based 

 chiefly upon their morphology. With the bacteria this method 

 of classification is applicable only to a limited extent owing to 

 their extreme minuteness. And in addition it is necessary to 

 resort to the grouping of the individuals as seen under the micro- 

 scope. The difl&culty in classification extends also to the mat- 

 ter of distinguishing one species of bacterium from another. 

 Where no constant difference in morphology exists, resort is 

 made to the grouping of the individuals as seen under the micro- 

 scope, the presence or absence of independent motion, the 

 naked-eye appearance of the growth upon culture-media and 

 their physiological properties in relation to growth under 

 various conditions of temperature, nutrition and relation to 

 free oxygen. The agglutination of a species of bacterium 

 by blood-serum specific for the species (see Chapter VII., 

 Part II.) has also been used for purposes of identification. 



These means of differentiation are not entirely satisfactory, 

 and it is likely that forms which are now considered as different 

 species are not really such in all cases. Notwithstanding 

 the unsatisfactory condition of the classification of bacteria, 



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