CH. X. MORPHOLOGY OF THE BACTERIA. ARTHROSPOROUS BACTERIA. 473 



is used in natural history. The fact is that Cohn in his publication of the year 1872, 

 which laid the foundation for the morphological treatment of the group, distinguished 

 a certain number of genera, Micrococcus, Bacterium, Bacillus, Vibrio, Spirillum, &c., 

 by a series of marks, and especially by the shape of the individual cells and their 

 simplest forms of connection, and gave the name of species to the several forms 

 which recur regularly within each of these genera, and have a characteristic shape, 

 decomposing effect and other qualities. It appears therefore that what Cohn dis- 

 tinguishes is that which we have named above form-genera and form-species. 



The other view goes so far in the opposite direction as to deny the existence of 

 distinct species of Bacteria, and to regard their forms as modifications of one species, 

 or, as it may be expressed in other terms, it supposes that they are modifications 

 which may be transformed into one another by breeding. Earlier allusions to this 

 view are to be found, but it was distinctly opposed to Cohn's classification by 

 Lankester 1 and Lister in 1873 2 , and Billroth in 1874 included all forms of Schizo- 

 mycetes with which he was acquainted in one collective species Coccobacteria septica. 

 It subsequently received support from the views which Nageli expressed in 1877 

 in the words, 'I have in the last ten years examined some thousands of Schizo- 

 mycetes and I could not maintain, except in the case of Sarcina, that there is any 

 necessity for distinguishing them into so many as two specific forms s ; ' he adds 

 however, that he is far from asserting that all the forms do belong to a single species, and 

 that it would be rash to express a decided opinion in a matter in which morphological 

 observation and physiological examination are both so defective. He gave utter- 

 ance to similar sentiments in 1882*. He accepts in fact the principle which led 

 Cohn to establish his form-genera and form-species and the species which he founded 

 on physiological characters, namely the necessity for a provisional arrangement, whilst 

 expressly declining to say whether the forms as distinguished by him do actually 

 correspond to real natural history species. 



Nageli's words quoted above in full contain a pregnant criticism of the whole 

 point in dispute as far as it has at present been explained. Neither side rests on the 

 only sure foundation, an exact observation of the continuity or non-continuity of the 

 development of the supposed forms or species, and this is especially apparent in 

 Billroth's work. Without this observation the question cannot be decided; it is 

 more necessary in this case because the forms in question are small and very like one 

 another, and are often mixed up together and liable therefore, unless very carefully 

 observed, to be mistaken one for another. Lankester made some approach to an 

 exact observation of continuity in one case only, in which the characteristic tints of 

 his Bacterium rubescens (Beggiatoa roseo-persicina) showed the connection between 

 the forms with more than usual distinctness. We have before us at present some 



1 [Professor Ray Lankester in a letter published in Nature, vol. xxxiii. p. 414 (March 4, 1886), 

 pointing out the significance of his observations upon Bacterium rubescens published in 1873 in 

 relation to the pleomorphism of Bacteria and criticising the statement in the text, says, ' I cannot 

 think that he [De BaryJ gives a correct statement of my relation to the conclusion which he finally 

 adopts. The view which I put forward in 1873 is precisely that which Professor De Bary now 

 espouses.' For further particulars the reader is referred to Professor Ray Lankester's letter.] 



2 Both in the Q. J. Micr. Sc., new series, XIII. * Die niederen Pilze &c., p. 20. 

 4 Unters. ii. niedere Pilze, p. 130. 



