CLASSIFICATION OF BACTERIA. 1 83 



peach-coloured bacterium led him to suppose that 

 the natural species of these plants were "within the 

 proper limits protean, and that the existence of true 

 species of bacteria must be characterised, not by 

 the simple form-features used by Cohn, but by the 

 ensemble of their morphological and physiological 

 properties as exhibited in their complete life- 

 histories.' ' Lankester inferred that these phase- 

 forms were genetically connected from their all 

 possessing the common characteristic of a special 

 pigment bacterio-purpurin. These conclusions 

 were vigorously opposed by Cohn, and doubt still 

 remains in the minds of some as to whether the 

 different forms are really only stages in the life- 

 history of a single species. Nevertheless the 

 theory of pleomorphism has steadily gained ground 

 ever since. 



Among the recent observers Cienkowski and 

 Neelsen have worked out the different forms as- 

 sumed by the bacillus of blue milk ; Zopf has in a 

 similar manner investigated Cladothrix,) Beggiatoa, 

 and Crenothrix, and traced out various forms (Fig. 

 66) ; Van Tieghem has investigated B&cillus amylo- 

 bacter with a similar result ; Hauser has quite 

 recently described bacillar, spirillar, and spirulinar, 

 and various other forms in the Proteus mitabilis and 

 Proteus vulgaris. These facts obviously shake the 

 very foundation of Cohn's classification, and we are 

 left without possessing a sound basis for classifica- 

 tion into genera or species. The mode of repro- 



