CHARACTERISTICS OF THE COLON-AEROGENES GROUP 77 



part of the exceptionally fundamental and searching chemical 

 investigations on which the bacteriological work has been 

 based. While intended originally as a study of some of the 

 gas-forming bacteria of milk, it has developed into an in- 

 vestigation of the characteristics of the coloii-aerogenes group 

 in general and of the more intimate processes by which some 

 of the familiar end products are produced. In this way it 

 has been possible to bring about some semblance of an orderly 

 arrangement of the large number of cultures studied and to 

 correlate the types so produced wLh certain definite habitats 

 Li nature. 



There has been a tendency to arrange bacteria according to 

 th? effect that they have oil our bodies or on the food we eat. 

 This has certain advantages, but is as illogical from a taxo- 

 nomic standpoint as a classification of men on the basis of their 

 occupation, an arrangement which would be useful for in- 

 dustrial purposes, but which would separate members of the 

 ham^ family, and bring together distinct ethnological groups. 

 In the same way bacterial classifications of convenience fre- 

 quently separate closely related bacteria and bring together 

 types agreeing- only in superficial characters. On the other 

 hand, there has developed in some quarters a tendency to 

 follow the logical method of classifying bacteria by deter- 

 mining actual relationships and lines of descent. This raises 

 the difficult question of the method of ascertaining how the 

 evolution of a bacterial species or genus may be traced; in 

 other words, which characters will be considered as funda- 

 mental and showing broad relationships, and which are super- 

 ficial and thus of secondary importance. The record of the 

 struggle to secure food and opportunity for reproduction is 

 found especially in morphology in animals and the higher 

 plants and in physiological characters in the bacteria and 

 similar microorganisms. 



The colon-aerogenes family is particularly distinguished 

 by its ability to bring about a gaseous fermentation of carbo- 

 hydrates, alcohols and glycerines, and in attempting to sep- 

 arate this extensive family into species, bacteriologists have 

 usually relied on this fermentative activity as a basis of 



