president’s address. 
169 
The former devised a scheme of colours by mixing solutions 
of copper sulphate and potassium chromate in varying 
proportions, which give a bright blue at one end of the 
scale, merging into a green, and finally at the other end of 
the scale showing a yellow. 
By means of this scale Delebecque has classified all the 
French lakes according to their colour. 
In big lakes the fauna and flora may be divided by their 
habitat into three classes : whether they live along the shores 
(littoral), or in the central regions and surface (pelagic), or 
in the very deep water (abyssal) ; and again, they may be 
divided by their habit into plankton or floating forms, and 
benthos or fixed forms. 
In shallow waters there are no regional distinctions of 
this kind, and no well-marked association of forms, which 
can be called true plankton. 
The abyssal forms have, perhaps, excited the most interest, 
as the conditions under which they live are the most peculiar. 
This region is taken to include all parts of a lake below 
a depth of 30 m., and it is, therefore, very feebly lighted 
with a low temperature, which varies only very slightly with 
the seasons, and is out of the influence of waves and currents. 
As one would expect, life is not very abundant under these 
circumstances. 
On the bottom of the Lake of Geneva, at a depth of 80 m., 
Forel found what he terms an organic felt, composed of 
Oscillaria, Palmella, Zooglea, Pleurococcus and diatoms of 
various species ; the latter he supposes to be erratic and not 
to belong permanently to this association of forms. 
It is this “ felt ” which covers the surface of the mud, 
which makes higher animal life possible in deep water, as 
the microphytes of which it is composed give off oxygen 
gas, which otherwise, owing to the stillness of the water so 
far below the surface, would be absent. 
