IIT. 
MODIFICATIONS OF BIOLOGICAL CHARACTERS, 
We have already referred to the production of an asporogenous 
variety of the anthrax bacillus. This was effected by Behring by 
cultivation in media containing small amounts of hydrochloric acid, 
caustic soda, methyl violet, malachite green, and various other 
agents. This is only one of many instances of a change in biologi- 
cal characters due to changed conditions of environment. We have 
abundant experimental evidence that growth may occur under ad- 
verse conditions when the species is gradually habituated to these 
conditions. Thus the temperature limitations may be passed by suc- 
cessive cultivations at temperatures approaching these limits, and 
bacteria may grow in the presence of agents which in a given pro- 
portion have a complete restraining influence upon their develop- 
ment. For example, in the experiments of Kossiakoff, published in 
the Annales of the Pasteur Institute (vol. i.), it was found that the 
several species tested all became habituated to the presence of anti- 
septic agents in proportions which at first completely restrained 
their growth. 
This modification of biological characters is well shown in the 
case of the chromogenic bacteria, some of which only form pig- 
ment under exceptionally favorable conditions of growth. It has 
been shown by several observers that non-chromogenic varieties 
of some of the best known chromogenic species may be produced 
by special methods of cultivation. Thus Wasserzug obtained a 
non-chromogenic variety of the bacillus of green pus (Bacillus 
pyocyaneus) by the action of time added to that of antiseptics. He 
says: ‘‘ These two actions combined have permitted me to obtain 
cultures which remained without color in a durable way, and in 
which, consequently, the chromogenic function was abolished by 
heredity.” In the case of a chromogenic bacillus obtained by the 
writer in Havana (my Bacillus Havaniensis), a non-chromogenic vari- 
ety was obtained from a culture on nutrient agar which had been kept 
in a hermetically sealed glass tube for about a year. The variety 
preserved the morphological characters of the original stock, but, al- 
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