2 NATURAL SCIENCE [July 
against the colon bacillus possesses no agglutinating power upon the 
typhoid bacillus, and vice versd. This test is probably of more value 
in distinguishing between the two species than any other one test, 
except, perhaps, the number of cilia. Yet even here the distinction 
is not absolute. Durham has shown that there is a bacillus, the 
B. enteritidis of Gartner (intermediate in character between the 
typhoid bacillus and B. coli communis, though probably to be 
regarded as a variety of the latter), which is feebly agglutinated by 
typhoid serum, although not in such high degrees of dilution as is 
the typhoid bacillus itself. 
There are some species of bacteria which are sharply marked 
off: thus the tetanus bacillus is one which both morphologically 
and in its pathogenic powers is a distinct and definite species. The 
typhoid bacillus and B. coli communis are members of a group in 
which the reverse is the case. The epidemiology of typhoid fever 
can leave no doubt on the mind that B. typhosus is a distinct and 
fixed species; yet apart from the disease it produces, its certain 
recognition is, as above indicated, not always so easy a matter. 
The colon bacillus occurs in countless varieties, and has been 
described under many different names. Its specific characters are 
ill-marked, and it is probably a rapidly varying dominant species, 
the characters of which are not yet fixed. 
The same may be said of the group of Streptococci. In the field 
of pathogenesis they are as it were a ‘dominant’ group, with 
varying and ill-defined specific characters, and with equally varying 
and ill-defined pathogenic effects. Unlike B. typhosus, which causes 
one distinct and definite disease, it seems probable that a single 
species of Streptococcus may give rise to suppuration, erysipelas, 
malignant endocarditis, puerperal fever, septic peritonitis, and half 
a dozen other diseases which clinically are distinct enough. And 
there are probably a number of different species of Streptococci, any 
one of which may cause a number of different diseases according to 
its grade of virulence and seat of infection, while clinically similar 
diseases may be due at different times to different species of 
Streptococcus. Specificity, if such there be, is here at its very 
vaguest. 
Compared with tetanus and diphtheria, two well-marked species 
causing definite disease, streptococcus infection is a most complex 
and ill-defined condition, and the task of the bacteriologist in pro- 
viding an antistreptococcus serum is proportionately difficult. No 
one can venture to affirm with confidence how many pathogenic 
species of Streptococcus exist, nor whether a given case of disease is 
due to one or other of the supposed species which are recognised, 
Marmorek, in preparing his antistreptococcus serum, employed a 
Streptococcus which he obtained from the fauces, and the virulence of 
