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 colon-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 wi.h certain definite habitats 
ia nature. 
There has been a tendency to arrange bacteria according to 
the effect that they have on 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 
tame family, and bring tcgether 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 msrphology 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 
