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formation, and that between the principal form and the one that has 
completely lost its original character, some feebly denitrifying inter- 
mediate forms are found, which may be compared to subspecies. 
Taking B. denitrijicans as an example we can speak of B. dent- 
trijicans autotrophus and of B. denitrificans heterotrophus, the change 
being possible only in one direction, at least with our present 
knowledge. 
This change is not a mutation in the accepted sense, as thereby 
the primitive stock continues to exist with the mutant under the same 
conditions under which the latter was formed. Here on the contrary 
all germs change simultaneously, so that in this case we have to do 
with a hereditarily constant modification, comparable to the pleomorphy 
of many Fungi, and to a certain extent, to alternation of generation. 
Comparable also to the production of somatic cells from germ cells 
during the ontogony of higher animals and plants, a fact certainly 
of general physiological signification. But modification and mutation 
are conceptions not sharply distinguishable and gradually related. 
Another case of variability, similar to the loss of chemosynthesis by 
feeding with organic substances, | observed in various lower Algae 
respecting photosynthesis. For a long time I have been cultivating the 
gonidia of the lichen Xanthorea parietina, which are identical with the 
Protococeacee Cystococcus humicola. The first isolation was made by 
streaking off the said lichen, rubbed to a mash, on pure agar with 
salts and cultivating it in light. The thus obtained green, pure colonies, 
develop very readily as well in the light as in the dark on maltextract- 
agar and form large green masses, which, however, in course 
of time completely lose the power of photosynthesis, so that neither 
on agar with salts, nor in anorganie liquid media any growth takes 
place. Microscopically no difference is to be seen between the inac- 
tive chloroplasts of these cells and the active ones of normal Cysto 
coccus cells. 
The very same Il observed in cultures of Pleurococcus vulgaris, 
isolated from the bark of trees and long cultivated on maltextract- 
gelatin, on which it grows vigorously in the dark without losing the 
green colour. Hence it is clear that for photosynthesis the presence 
of chlorophyll in the living protoplasm is not sufficient, but the process 
requires still another factor, which may get lost through cultivation 
with organic food. 
The greater part of the chlorella’s of Hydra viridis, undoubtedly 
biologiea, 3e Jahrg. Heft. 2, Pag. 1, 1914. Recently J found that the ferment 
which produces nitrous acid from ammonium salts behaves in the same manner 
and changes, when fed with organic food into a saprophytous non-nitrifying form 
