260 BACTERIOLOGY. 
A study of the above table will show that it makes 
changes in the genus of some of the most common 
bacteria, as in the restoration of the old genus bacte- 
rium and the assigning to it of all non-motile, rod- 
shaped organisms, thus altering the genus of some of 
the most common pathogenic bacteria from bacillus to 
bacterium. Other changes are seen in the spirilla. 
Any such scheme is at times very arbitrary in placing 
some varieties under one generic division and others 
_ closely allied in another. It has also the objection, 
already noted, that it is only one of several classifi- 
cations already in use, and until some authoritative 
body agrees on some one it is almost useless in such 
a volume as this to change the usually employed names 
for others which are, perhaps, intrinsically somewhat 
better. Another important reason for waiting is that 
with the increase of our knowledge we are constantly 
changing the position of different bacteria. Thus such a 
well-known germ as the tubercle bacillus is now found 
to produce, under certain conditions, long thread-like 
branching forms, so that it ceases to be under the clas- 
sification of Migula a bacterium. We will, therefore, 
simply use the usual nomenclature, and consider to- 
gether, in so far as is practicable, certain groups of 
bacteria whose members are closely allied to each other 
in some one or more important directions. 
The Permanence of Bacterial Species. When we come 
to study special varieties or groups of bacteria, such as 
the bacilli which produce typhoid fever, diphtheria, and 
tuberculosis, it is of great importance for us to deter- 
mine, if possible, to what extent the peculiar character- 
istics which each of these groups of bacteria possess are 
permanent in the generations which develop from them. 
