COHN'S CLASSIFICATION. 89 



67. Cohn's Classification. 



The first point was to bring the confusion of forms into order. 

 What characteristic should be taken as a guide thereto 1 ? Were 

 there several at disposal on which one could rely 1 These ques- 

 tions COHN (I.) may well have asked himself when, in 1872, he 

 felt himself impelled to attempt a classification of the bacteria, 

 and finally thought his object attained by the following system : 



I. Sphcerobacteria, globule bacteria. 



Genus I : Micrococcus. 



II. Microbacteria, short rod bacteria. 



Genus 2 : Bacterium. 



III. Desmobactcria, thread (long rod) bacteria. 



Genus 3 : Bacillus. 

 Genus 4 : Vibrio. 



IV. Spirobacteria, spiral bacteria. 



Genus 5 : Spirillum. 

 Genus 6 : Spirochcete. 



The sarcina organisms have no place in this system, because 

 Cohn did not consider them as belonging to the fission fungi. 



As may be seen, the basis of classification employed was the 

 form of the cells, i.e. their form of growth. However, since 

 methods of pure culture were then undiscovered, the diagnosis of 

 the individual species was as yet impracticable, and the question 

 whether the form of the cells in each species is definite and un- 

 changeable was, in particular, still unsolved. The answer to this 

 question is, nevertheless, of vital importance to the Cohn system, 

 and, if negative, causes it to break down (as was subsequently the 

 case). The weakness of the system was recognised by Cohn him- 

 self, and he particularly stated that his classification was only a 

 provisional one. A number of over-zealous disciples, however, 

 overlooked this reservation, and, by degrees, expounded the system 

 as meaning that each separate species has a single well-defined 

 and invariable cell form ; the one species appearing only as short 

 rods, the second only as cocci, and so on. This constitutes the 

 theory of constant form, also known as Monomorphism. 



68. Billroth's Coeeobaeteria Septiea. 



The exaggeration resulting from the misapprehension of Cohn's 

 attempt at classification soon brought about a corresponding 

 reaction. In proportion as assiduous microscopic research re- 

 vealed the certainty that bacteria do undergo changes of form, 

 so the hasty assumption of monomorphism of species had to be 

 given up. In 1852, PERTY (I.) had already observed a short-rod 

 bacterium, which, on account of its faculty of changing into the 

 thread form, he named Metallacter. Twenty-one years later 

 LANKBSTER (I.) studied a species of red-coloured bacterium, named 



