482 BACTERIOLOGY. 
on artificial media; but all attempts to separate them 
into various classes, until recently through the use of 
specific serum, have failed, because the differences ob- 
served, though often marked, are not constant, many 
varieties having been found to lose their distinctive 
characteristics, and even to apparently change from one 
class to another. A further objection to any previous 
classification of streptococci, based on the manner of 
growth on artificial culture media, is that it has been 
impossible to make any which would at the same time 
give even an approximate idea of their virulence. Ex- 
periments have proved that the streptococci originally 
virulent may become non-virulent after long cultivation 
on artificial media, and, again, that they may return to 
their original properties after being passed through the 
bodies of susceptible animals. The peculiar type of 
virulence which they may acquire tends to perpetuate 
itself, at least for a considerable time. 
One important fact that experience teaches us is, 
that those streptococci are the most dangerous to any 
animal which have come immediately from septic con- 
ditions in the same species of animal, and the more 
virulent the case the more virulent the streptococci are 
apt to be in other animals of the same species. There 
seems also to be a strong tendency for a streptococcus 
to produce the same inflammation, when inoculated, as 
the one from which it was obtained; for example, strep- 
tococci from erysipelas tend to produce erysipelas, from 
septicemia to produce septicemia, etc. Streptococci, 
however, obtained from different sources (abscesses, 
puerperal fever, sepsis, erysipelas, etc.) are in many 
instances capable, under favorable conditions, of pro- 
ducing erysipelas when inoculated into the ear of a 
