CLASSIFICATION OF THE COLON-CLOACAE GROUP 255 



Kligler (1915) suggests that salicin be substituted for dulcitol, 

 in subdividing coli-like bacteria, pointing out that salicin fermen- 

 tation correlates better with the Voges-Proskauer reaction than 

 does dulcitol decomposition. He thus recognizes a sucrose neg- 

 ative, salicin negative group (B. acid-lactiti) ; sucrose negative, 

 salicin positive group (B. communis) ; sucrose positive, salicin 

 negative group (B. communior) and sucrose positive, salicin posi- 

 tive (B. aerogenes) . B. cloacae is differentiated from B. aerogenes 

 by its inability to ferment glycerol. 



The characterization of B. communior as salicin negative is 

 probably untenable. The term B. coli-communior was first em- 

 ployed by Durham to describe a variety of B. coli which fer- 

 mented sucrose and which was motile. Later Ford recognized it 

 as a species B. communior. Such organisms usually ferment 

 salicin as will be shown later in this paper. 



Where the principle of correlation has been employed the best 

 correlated character has apparently been picked out by inspection 

 of the data. Inspection is a tedious and difficult procedure, 

 entirely inapplicable where the number of characters considered is 

 large, and it does not permit of a concise statement of the degree 

 of correlation which exists between different reactions. Consid- 

 erable information in an abstract, concise, and workable, 

 form may however be obtained from a study of the coefficients 

 of correlation. 



THE COEFFICIENT OF CORRELATION 



Where we are concerned merely with the presence or absence 

 of characters the coefficient of correlation between any two char- 

 acters may be easily determined. Suppose that it is desired to 

 know if the characters X and Y are correlated and that a study 

 of a number of organisms showed that 'a 7 cultures are positive 

 for both X and Y ; ' b ' organisms positive for X but negative for 

 Y, V cultures are negative for X and positive for Y; and 'd' 

 strains are negative for both X and Y. The distribution of the 

 organisms is first tabulated as shown below. 



