1904 - 5 .] Study of the Lakes of Scotland and Denmark. 429 
with their shallow and, in summer, warm lakes. On the other 
hand, from Ekman’s explorations in the northern alpine lakes in 
the Sarek, we know that the seasonal variations are by no means 
so conspicuous there as in the more southerly parts of Sweden. 
Brehm (1902) arrives at a similar result as regards the Daphnids 
in the Achensee, North Tyrol. From my own observations in the 
Icelandic lakes (which will he published shortly), I know that the 
seasonal variations are there extremely inconspicuous, and now 
the investigation of the Scottish lakes has given the same result. 
From these facts, and in accordance with the observations of 
Ostwald and myself, we may conclude that the seasonal variations 
are of slight importance in arctic and cold alpine lakes, while, as 
might have been expected, they are conspicuous in the lakes of 
the Central European plain, characterised by the great annual 
variations in the temperature of the water. In this connection it 
will be seen how interesting a thorough exploration of the great 
tropical lakes would prove to be. 
According to the published papers by the investigators of the 
alpine lakes and the lakes of the European plains, it may be con- 
sidered as a general rule that many animals always display more 
vivid colours in the cold alpine lakes than in the warm lakes of 
the plains, and that the animals retain their bright colours in the 
alpine lakes throughout the year, whereas in the lakes of the 
plains the vivid colouring is only observed in winter when the 
temperature is low. Brehm supposes that the red colouring of 
alpine organisms is a means of protection against the cold, and 
gives good reasons for this supposition. The examination of the 
plankton in the Scottish lakes has now shown that the Crustacea, 
for instance Daphnia hyalina, Diaptomus gracilis , Cyclops strenuus , 
as in other alpine lakes, are frequently in summer of a deep red 
or deep blue colour ; in my own country I have only seen these 
vivid colours in winter, and never in the summer months. 
With regard to the vertical distribution of the plankton-organisms 
in the Scottish lakes we know very little, and further observations 
on this point are necessary. It is an interesting fact that what 
is known of the vertical movements of the plankton shows that 
these movements are very conspicuous in the alpine lakes, but 
inconspicuous, and often hardly traceable, in the lakes of the 
