434 Proceedings of Royal Society of Edinburgh. [sess. 
Once more calling to mind the mist-wrapped, moss-covered 
Scottish hills, with their peaty moors and precipitous sides, I 
think we must seek the main cause of the general extreme poverty 
' of animal and vegetable life in the Highland lakes in the general 
geographical conditions of the country itself. 
From this sketch of the organic life in the Danish and Scottish 
lakes it will appear that the differences are extremely great. I 
suppose that what has been said with regard to the life in the 
Danish lakes will hold good also as to the lakes of the northern 
part of the Central European plain. On the other hand, the very 
imperfect sketch I have given of the Highland lakes can by no 
means be taken as applicable also to alpine lakes in general. It 
would indeed have been fortunate could we have drawn a com- 
parison between the Highland lakes of Scotland, their nature and 
their organic life, and the Norwegian alpine lakes, many of which 
are similar in some respects ; but this is impossible, since the 
Norwegian lakes have been very insufficiently explored, and we 
can only compare the Scottish lakes with the southern alpine 
lakes, especially the well-explored Swiss lakes. I may refer to the 
admirable works of Forel (1892-1902), Zschokke (1900), and 
others, relating to the fauna and flora of the Swiss lakes. Any- 
one who has read these, and knows something of the life in the 
Scottish lakes, will be aware that in every respect life is much 
richer in the Swiss lakes than in the Scottish lakes. 
III. 
The Influence of the Organic Life upon the Lakes 
THEMSELVES AND THEIR SURROUNDINGS. 
A. The Danish Lakes. 
It stands to reason that the organic life will always exercise the 
greatest influence upon the surrounding medium where the 
organisms are in excess, both as regards the number of species 
and the number of individuals. When we remember that 
Denmark is built up of friable soil, while Scotland, on the other 
hand, consists for the greater part of hard rocks, it will be 
evident that the influence of organic life is far more intense, and 
