SOME DISTINCTIVE CHARACTERS IN THE 
FRESH - WATER PLANKTON FROM 
VARIOUS ISLANDS OFF THE NORTH 
AND WEST COASTS OF SCOTLAND 
By JOHN HEWITT, B.A. 
It is now fairly well established that there is a great uniformity in 
the fresh-water plankton from all parts of the world ; the means of 
dispersal of the numerous organisms constituting this assembly have 
been so efficient, and their adaptability to various environmental 
conditions, such as temperature, light, and chemical composition of 
the water, so great, that plankton from the Arctic and from the 
tropical regions have a great number of species in common. Neverthe- 
less, there is a decided though somewhat inconspicuous differentiation 
according to climate, and, for example, it has been shown ^ that there 
is a certain association of zooplankton which belongs particularly to 
the Arctic and subalpine lakes ; and moreover it has been known 
for a long time that in one family, the Diaptomidae, the delimitation 
of the species is quite sharp, the English species of Diaptonms being 
unknown in the lakes of the New World ; and when in addition to 
such considerations we take into account the relative abundance and 
variation of cosmopolitan forms which have predilection for certain 
environmental conditions, it becomes possible to roughly divide the 
world into several zoological regions which coincide with areas of 
different climatic conditions. 
It has been pointed out by Mr James Murray^ that Scotland 
occupies a more or less intermediate position between two such 
regions, and that northwards the plankton contains the Arctic and 
subalpine association, whilst in the lowlands the plankton is more 
closely related to that from the great European plain. 
1 Sven Ekman, "Die Phyllopoden, Cladoceren, etc., der nordscliwedischen 
Hochgebirgen," Zool. Jahrh. Aht Syst. Geogr. Biol, Bd. xxi., 1904. 
2 James Murray, " Distribution of the Pelagic Organisms in Scottish Lakes," 
Proc. Roy. Phys. Soc. Edin., vol. xvi. p. 51, 1905. 
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