CHAPTER I. 



STRUCTURE AND CLASSIFICATION OF THE 

 MICRO-ORGANISMS. 



BACTERIA. 



WHEN Leeuwenhoek with his improved microscope dis- 

 covered 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, sug- 

 gested 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 differ- 

 ence 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 metabolic difference we find the present 

 criterion for the separation of the living organisms into the 

 two main groups! But this does not dispose of all of the 

 difficulties, for there are certain small groups to which it 



29 



