908 
belonging to the so easily cultivable species Chlorella vulgaris, lose, 
when out of the Hydra body, howsoever fed, as well the power of 
photosynthesis as that of growth, so that it is very difficult to 
cultivate them. So, here is a case where change of food causes the 
loss as well of the function of photosynthesis as of that of growth. 
CONCLUSION. 
Some of the common denitrifying bacteria, such as B, denitrificans 
and B. stutzeri (these names taken in a broad sense), and probably 
some other species, may occur under two physiologically different 
modifications, which are hereditarily constant, when their feeding 
conditions remain unchanged. One form, the autotropic, is adapted to 
the anorganic medium (sulfur- or thiosulfate-chalk-nitrate) and shows 
chemosynthesis; the other, the heterotrophic form, requires organic 
food. They may be compared to the oligotrophic and the polytro- 
phic condition of the nitrate ferment. Intermediate forms, feebly 
denitrifying in the anorganie medium, also occur, hence the 
autotrophy may be lost gradually. 
The heterotrophic forms preserve the power of denitrification with 
organic food. 
The nitrite ferments of the ammonium salts are also related to 
hereditary modifications with the character of saprophytes, living on 
organic food and unable to oxidise ammonium salts. 
Great changes in the nature of the food may thus be the cause 
of hereditary modifications of certain factors, and this seems to throw 
some light on the causes which underlie ontogony. 
