1904 - 5 .] Study of the Lakes of Scotland and Denmark. 415 
With regard to the fresh-water plankton of the world, two re- 
markable characteristics should be noted. Firstly, that generally 
speaking it seems to he very homogeneous from pole to pole. The 
plankton of the Greenland lakes is similar to that of the North 
African lakes, only certain groups of plankton-Algae being apparently 
rare, or perhaps entirely absent, near the pole. From this general 
rule we know only a few exceptions, especially as regards some 
Crustacea. Yery many species are common to the fresh-waters of 
Iceland and those of North Italy. Secondly, that the central 
domain for the full development of all fresh-water plankton is 
apparently in the temperate zone, and riot in the tropics. If these 
characteristics hold good, the fresh-water plankton differs essentially 
in both these respects from all other associations of organisms in 
the sea or on the land. These two points cannot, however, be 
held as proved until the tropical fresh-water plankton has been 
fully explored ; and I consider it extremely desirable that one of 
the great nations having possessions in the tropics should despatch 
an expedition with the main object of investigating the tropical 
fresh- water plankton. 
The plankton of our lakes does not differ, on the whole, from 
that to be found in any of the larger lakes in the northern parts 
of the Central European plateau, but, as Forel justly remarks, all 
these lakes scarcely merit the name. In most of these compara- 
tively shallow lakes the plankton is characterised by a great 
development of Melosira and blue-green Algae, by the presence of 
Bosmina coregoni, and perhaps by the occurrence of the only two 
common species of the Copepod genus Diaptomus , D. gracilis and 
gradloid.es. The Cyclotella and Oscillatoria , so characteristic of 
alpine lakes, are usually rare, and often entirely absent, while 
certain species of Diaptomns and some peculiar species of 
Chlorophycea, common in southern alpine lakes, have never been 
found in the Central European plateau. 
The plankton of the Danish lakes differs somewhat perhaps 
from that of the lakes in the surrounding lowland countries in the 
rich development of the Diatom genus Stephanodiscus, of the blue- 
green Alga genus Lyngbya , and of the Conferva Tribonema 
bombycinum. As our lakes are usually shallow and the littoral 
zone very extensive, it will be readily understood that many 
