1904 - 5 .] Study of the Lakes of Scotland and Denmark. 427 
observed the transparency and the colour of the water to be 
influenced by the plankton. Further, I should think it is 
exceptional to find in the Highland lakes a single plankton - 
organism giving the entire plankton the uniform monotonous 
character frequently observed in our lakes due to Melosira , 
Aphanizomenon , and others. And it will be easily understood 
that the marked changes which almost invariably take place in 
our lakes when the great development of Diatoms ceases and the 
maximum development of the Cyanophycea sets in are never so 
conspicuous in the Scottish lakes. Finally, I am inclined to 
think that many of the plankton-organisms in the Scottish lakes 
show a less marked maximum and minimum development than 
is the case in our lakes ; and should further explorations confirm 
this supposition, the fact must be ascribed to the much lesser 
amplitude in the annual variation of temperature in the Highland 
lakes, where the water never attains those very low or very high 
temperatures at which life in an active form, owing to the 
structure of the organisms, becomes impossible ; the organisms 
may therefore not be forced to form resting organs, but may 
remain in the layers of water as free swimmers. 
According to the observations of Mr James Murray and myself, 
the seasonal variations of the plankton-organisms are never so 
conspicuous in the Scottish as in the Danish lakes. I have 
pointed out (1900) that in several very different plankton- 
organisms the longitudinal axis is simultaneously lengthened 
during summer and shortened during winter, and that the 
formation of all the various structures (spines, floating apparatus, 
etc.) considered necessary to enable the organism to float are most 
distinctly visible in summer-forms and summer-individuals. I 
also pointed out that the explanation must be looked for in the 
varying external conditions, which, so to speak, compel the 
organisms to vary regularly in accordance therewith. I ascribed 
these variations mainly to the annual changes in the specific 
gravity of the water, occasioned by the regular annual fluctuations 
in the temperature, starting from the supposition that if the 
velocity of the falling motion of the plankton-organisms be not 
the same at all seasons, the organisms must, in order to exist as 
such during the season when the velocity of the falling motion is 
