1060 The American Naturalist. [December, 
genic species like Bac. prodigiosus, the germ that causes 
“ bleeding bread” and Bac. cyanogenus, that which provokes 
the disturbance known as “blue milk,” varieties have been 
produced by constant selection and cultivation of cultures in 
which the color producing quality of the germs entirely dis- 
appeared. Laurent modified the chromogenic function of the 
Kiel water bacillus by exposing it to direct sunlight for a lim- 
ited time. Thesuppression of this peculiarity was transmitted 
from generation to generation so that a perfect albinotic 
variety was formed. The color property was also lost when 
cultivated at blood heat and was not regained when continued 
cultivation was carried on at lower temperatures. These 
examples indicate the plasticity of this physiological function 
of color production and show the influence that is exerted 
upon the germ by a continued subjection to certain experi- 
mental conditions. 
The objection may be raised that these cases that show a 
change in the various'vital functions do so because their vital- 
ity is impaired and that the variety so produced is merely a 
degenerated and weakened type. While this may be true in 
certain cases, it does not detract from the value of such experi- 
ments as throwing more light upon the question of environ- 
mental influence. Besides, the rule is by no means general 
that loss and abatement of physiological function is correlated 
with degeneration. We have numerous instances among the 
pathogenic forms whereigreater luxuriance in growth is to be 
noted in connection with the mitigation of the powers of vir- 
ulence as for instancein tuberculosis where cultivation on 
media containing glycerin-agar diminishes the virulence of 
the form while it increases the powers of growth. This can be 
explained as a case of partial reversion of the species, special- 
ized in the direction of the pathogenic property to an ances- 
tral saprophytic mode of existence 
e zymogenic, or fermentative funetion of bacteria has 
also been experimentally modified. The cultivation of the 
lactic acid bacillus, the germ causing the souring of milk, for 
a time in fon-fermentable solutions, entirely destroys the 
eee of decomposing the sugar in the milk and converting 
