CHAPTER I 



STRUCTURE AND CLASSIFICATION OF THE 

 MICRO-ORGANISMS 



BACTERIA 



WHEN Leeuwenhoek with his improved microscope discovered 

 the new world of micro-organisms, he supposed them, on account 

 of the active movements they manifested, to be small animals, and 

 described them as animalculae. The early systematic writers, 

 Ehrenberg and Dujardin, fell into the same error, and it was many 

 years before biologists had arrived at even approximate accuracy in 

 arranging them. Indeed, for a long time a great number baffled 

 systematic writers, and no less an authority than Haeckel, in 1878, 

 suggested that they form a group by themselves to be known as 

 Protista. Such a grouping, however, was unsatisfactory alike to 

 botanists and zoologists, and, therefore, was used by few. 



It was evident that structure could not be looked upon as a 

 satisfactory differential character, for between the protozoa, or 

 most simple animals, and the protophyta, or most simple plants, 

 the structural differences were too minute to prevent overlapping. 

 Motion and locomotion had to be abandoned, since it was common 

 to both groups. Reproduction was likewise an unreliable means 

 when taken by itself, for much the same means of. multiplication 

 were found to obtain in both groups. One great physiologic and 

 metabolic difference was, however, noted: plants possess the power 

 of nourishing themselves upon purely inorganic compounds, while 

 animals are unable to do so and cannot live except upon complex 

 molecular combinations synthesized by the plants. In this metab- 

 olic difference we find the present criterion for the separation of the 

 living organisms into the two main groups. But this does not dis- 

 pose of all of the difficulties, for there are certain small groups to 

 which it does not apply. Thus, for example, the fungi which, when 

 judged by other criteria, are undoubted plants, lack the power of 

 inorganic synthesis, and so resemble animals. 



Fortunately, the question is a purely academic one. Though 

 seemingly at first sight a most fundamental one, it is, in reality, of 

 trifling importance, for after a limited experience the student un- 

 hesitatingly assigns most of the known organisms to one or the 

 other groups, and that occasional mistakes may be made, and 

 organisms, like the spirochaeta, appear sometimes in the group of 

 plants among the bacteria, and in other writings in the group of ani- 

 mals among the protozoa, is a matter of small consequence so long 



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