198 ELEMENTS OF WATER BACTERIOLOGY 



coroba and B. cloacae (Clemesha's resistant types) 

 are less abundant in the filtered and stored than in the 

 raw water. Houston's lactose-fermenting forms clas- 

 sified in MacConkey's four great groups show the rela- 

 tions indicated in the table below, which are almost 

 the reverse of what should be expected if the dulcite- 

 fermenting forms (Groups II and III) were indicative 

 of recent pollution. 



DISTRIBUTION OF MACCONKEY'S GROUPS IN RAW, 

 STORED, AND FILTERED WATER AT LONDON 



Statistical Classification of the Colon Group. From 

 a biological standpoint, there is a twofold difficulty 

 with such a classification as that of MacConkey and 

 Jackson. In the first place it is enormously complex, 

 or soon becomes so, as new investigators add new 

 diagnostic tests. In the second place, it is entirely 

 arbitrary in its choice of the order in which particular 

 tests are to be used in splitting up the group. Closely 

 related forms may be widely separated if they chance 

 to differ in the one respect first chosen for dichotomic 

 division. 



The best basis for a classification following natural bio- 

 logical lines seems to us to be the statistical method first 



