INTERNAL ECONOMY OF THE SEA _ 85 
We may speak of it as the distinction between 
the hard-mouthed and the soft-mouthed, and it is 
radical. But to follow it up would take us too far 
from our present theme. 
The middlemen are Bacteria, which get involved 
in so many different ways in the business of life. 
Salt is well known to be antithetic to them, but it 
has not kept them out of the sea, where they have 
more than one important réle to play. Thus some 
are putrefactive, breaking down the dead bodies 
of animals and plants, and the excreta of animals, 
reducing them to carbon-dioxide, ammonia, 
ammoniates, and the like, which may re-enter the 
field of life by forming food for Algze. Microbes 
of this sort are for ever making a clean thing out 
of an unclean. But there are others which play a 
subtler part, by changing the ammoniacal nitrogen 
into nitrites, and others which carry on this work 
by completing the oxidation into nitrates. And as 
nitrites are more available for the nutrition of plants 
than are ammoniacal compounds, and as nitrates 
are more available than nitrites, we see, as they said 
of old time, how well this world is governed. It is 
not to be forgotten, however, that there are many 
denitrifying bacteria which work the wrong way by 
reducing nitrates to nitrites, nitrites to ammonia, 
and ammonia to free nitrogen. Professor Brandt 
has suggested as a reason for the remarkable fact 
that the cold Polar waters are richer in Plankton 
than tropical seas, that the higher temperature 
favors the action of denitrifying bacteria, which 
