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 although Leidy 

 in 1849 looked upon them as algae, and as belonging to the plant 

 kingdom, it was many years before biologists were satisfied as to 

 their true position in nature. Indeed, no less an authority than 

 Haeckel, as late as 1878, suggested that they form a group by them- 

 selves neither animal nor vegetable, but intermediate between the 

 two, to be known as Protista. This, however, was unsatisfactory 

 alike to botanists and zoologistSj and did not become popular. 



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

 factory difierential 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 inorgaiiic compounds, while 

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

 molecular combinations synthesized by the plants. In this meta- 

 bolic 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 



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