906 
all to Bacterium stutzeri, if taking the conception of species in a broad 
sense; superficially there is a great difference between the colonies 
of this group. The usual form, which is very remarkable and 
easily recognisable by the shape of the colonies, has been described 
in these Proceedings by Professor van [TErson *). Even in the smallest 
tlocecules of the sulfur denitrifications some form of B. stutzeri is found, 
although the soft colonies prevail. But besides, other varieties 
of B. stutzeri occur, for example such which slightly liquefy gelatin, 
or such which are light brown or rose-coloured, or whose colonies lack 
the so characteristic structure, and again others with that structure, 
but wanting the denitrifying power. There are, too, intermediate 
forms between the tough and the soft class, and I think it possible 
that they originate from each other by mutation. 
That Bacteriwm stutzeri in the anorganic denitrifications possesses 
autotrophy, follows from the above described experiment with the 
silicic plates. But this may also be proved for colonies of “organic” origin, 
if only the right moment be chosen for experimenting with them. 
In the organic- plate cultures the autotrophy of this species gets 
however rapidly lost. Only with quite fresh colonies, grown on 
thiosulfate-agar plates, and transferred to the anorganic medium, just 
at the time of their becoming visible a feeble but distinct anorganic 
denitrification could be obtained, which continued during several 
days with the same degree of intensity, only much feebler than the 
spontaneous denitrification. So it seems proved that the autotrophy 
does not disappear as an indivisable factor, but may get lost in parts. 
That the autotrophy is really lost in the originally active colonies, 
is corroborated by the fact that not only the single colonies of the 
organic plates, but likewise the combinations of the colonies of the 
different species are quite inactive. Even all the colonies of broth- 
agar plates together, mixed with the undeveloped germs lying between 
them, do not produce any denitrification in the anorganic mixture. 
And this must be true for all the different species which produce 
anorganic denitrifications and evidently possess the power of chemo- 
synthesis in their natural habitat. 
This form of variability is obviously analogous to that of the 
nitrate ferment, which I formerly described’) and as said called 
physiological species-formation. In both cases a new elementary 
species is produced. It is remarkable that a number of species or 
varieties living under the same conditions are subject to this trans- 
1) Ophoopingsproeven met denitrificeerende bakterién. Acad. of sciences. -Amster- 
dam, July 1902. 
2?) Ueber das Nitratferment und über physiologische Artbildung. Folia micro- 
