478 SYSTEMATIC. 



The belief is nevertheless rapidly gaming ground that the lowest 

 forms of vegetable life cannot be divided by a hard-and-fast line 

 into a series with chlorophyll (Algse), and a series without it (Fungi), 

 and the tendency now is to solve the difference of opinion between 

 Cohn and Nageli by following the example of Sachs, and amalga- 

 mating the two series into one group, the Thallophytes. 



Researches by competent observers have more recently clearly 

 demonstrated that several micro-organisms in their life cycle exhibit 

 successively the shapes characteristic of the orders of Cohn. 



This doctrine of pleomorphism, now widely accepted, was dis- 

 tinctly foreshadowed in a publication by Lister in 1873, though 

 this memoir contained certain conclusions which have since been 

 abandoned. Lister described forms of cocci, bacteria, bacilli, and 

 streptothrix in milk, which he regarded as phases of the same 

 micro-organism, Bacterium lactis. As a result of his observations, 

 Lister remarks that " any classification of bacteria hitherto made 

 from that of Ehrenberg to that of Cohn based upon absolute mor- 

 phological characters is entirely untrustworthy." To Lankester, 

 however, belongs the credit of having definitely and precisely formu- 

 lated this doctrine. In a paper, also published in 1873, Lankester 

 observed that the series of form-phases which he had discovered in 

 the case of a 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, in that they all pos- 

 sessed 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. 



Cienkowski and Neelsen worked out the different forms assumed 

 by the bacillus of blue milk ; Zopf has in a similar manner investi- 

 gated Cladothrix, Beggiatoa, and Crenothrix, and traced out various 

 forms (Fig. 196) ; Yan Tieghem has investigated Bacillus amylobacter 

 with a similar result; Hauser has described bacillar, spirillar, 

 spirulinar, and various other forms in the Proteus mirabilis and 

 Proteus vulgaris. These facts obviously shake the very foundation 

 of Cohn's classification, and we are left without possessing a sound 



