1893.] Bacteriology in its General Relations. 1059 
ing and variable. It is only under rigid experimental con- 
trol that we can obtain positive proof on this question. 
The most noteworthy changes are those that show a decided 
modification of the physiological function of virulence. 
Nearly every species belonging to the pathogenie class is sub- 
ject to greater or less variation in this respect. Anthrax fever, 
the classic example in bacteriology, has been shown by the 
experiments of Chauveau to vary in virulence to such an 
extent that from the original virulent culture, varieties or 
races have been grown that possess every shade of virulence 
from the deadliest type to that which is perfectly innocuous. 
These attenuated types maintain their newly acquired func- 
tion in a perfectly constant manner, so that we have races of 
the germs that are christened “ mouse anthrax,” because they 
are pathogenic for mice only; others less attenuated are able to 
kill guinea pigs or rabbits, while still others are virulent for 
all classes of susceptible animals. 
With many germs the variation of this functional property 
occurs quite readily in cultures under ordinary conditions, as 
in the case of pneumonia and hog cholera, where all degrees 
of virulence may be found. Sometimes abnormal conditions 
of environment seem to be necessary to produce the variation. 
These modified forms may persist only so long as the artificial 
conditions are maintained, reverting to the original type when 
restored to their normal environment, or in some species the 
constitutional characters of the germ are so changed that they 
are perpetuated although the conditions favorable to atavism 
are present. What is true concerning the variation of the 
pathogenic function is likewise true in regard to other physio- 
logical characteristics. 
The chromogenic property of certain forms has usually 
been considered of diagnostic value, but in some instances 
spontaneous sports occur, as in Bac. pyocyaneus a and £, where 
the only observable difference is that the pigment produced in 
one is a bluish green, while in the other it is a fluorescent 
green, and quite distinct from the first. 'This species has been 
modified artificially so that the color producing power of the 
organism has been permanently abolished. From other chromo- 
