CLASSIFICATION OF DAIRY BACTERIA. 95 
bacteria the conception of word species which zodlogists and 
botanists have developed in the last century. It has been 
recognized by the modern zoologist that the early conception of 
a species, as something sharp and distinct, is bound to be modi- 
fied as variations are recognized. If this is true in animals and 
higher plants, it is more emphatically true of bacteria. Indeed 
we must practically abandon any thought of using among bac- 
teria the term species with a meaning which has any similarity 
to that which is used for the rest of the living world. The 
question as to whether physiological variations are sufficient to 
characterize a new species is one which we cannot now answer, 
but this would be involved in an attempt to determine species 
of bacteria. Into these questions we do not propose to enter at 
all and shall not make any endeavor to arrange bacteria descrip- 
tions in the form of species and genera. The term species, there- 
fore, will not be used at all in the following classification. 
As our data has been increasing, it has become more and more 
evident that the bacteria which we have been studying natur- 
ally fall into series of groups. This has been dimly recognized 
by bacteriologists for some years, the &. coli group, for ex- 
ample, being one that for many years has had a somewhat 
recognized position. As we have collected the dairy bacteria, 
we have been able to see such groups becoming more and 
more apparent. In such a group there is usually a somewhat 
central type, but from this the numerous cultures which may be 
obtained and compared with each other show variations in all 
directions. Inasmuch as our characters are chiefly physio- 
logical, the variations of physiological properties naturally 
produce an immense confusion in the attempt to satisfactorily 
arrange cultures in groups. By these variations our different 
groups are more or less connected with each other, even when 
the groups are founded upon clear, sharp, positive distinctions. 
For example, the power to liquefy gelatin has already been 
recognized as one of the most fundamental characters, and yet 
this power of liquefaction is manifestly subject to wide varia- 
tions. As has been shown several times, organisms which 
have the power of liquefying gelatin may lose this power more 
or less completely, and hence it is perfectly clear that some 
groups which liquefy may perhaps be only physiological modi- 
fications of non-liquefying types. Among the cocci described 
