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 + and Lister in 1873 %, 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 Nägeli 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*;’ he adds 
however, that he ıs far from asserting that all the forms do belong to a single species, and 
that tt would be rash to express a decided opinion in a malter 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. 
Nägeli’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 Bary] 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. ] 
? Both in the Q. J. Micr. Sc., new series, XIII. 3 Die niederen Pilze &c., p. 20. 
* Unters. ü. niedere Pilze, p. 130. 
