286 THE FRESH-WATER LOCHS OF SCOTLAND 
anything like equal degree, by the plankton of the Irish lakes. The 
next feature of importance is the conspicuous Arctic element in the 
plankton Crustacea. A third element is the absence or comparative 
rarity of some of the species commonest in the general European 
plankton. Another peculiarity is the very local distribution of some 
of the Crustacea and many of the Desmids. 
The Desmid Flora. — Some 200 species of Desmids have been 
found in the Scottish lochs. Making the greatest allowance for 
casual species, there still remain 50 or more true plankton species. 
Sometimes as many as 50 species, including casuals, are found together 
in one loch. There is nothing like this elsewhere in European 
lakes, where, as a rule, there are no Desmids, or only a few species. 
The nearest approach to the condition of matters which obtains in 
Scotland is found in Scandinavia and Finland, where a number of 
the Scottish species occur. In a recent report on the Swiss Alpine 
lakes, Tanner-Fl'illemann ^ states that the Schonenbodensee is 
relatively rich in Desmids, 19 species being recorded. 
Various attempts have been made to account for the great richness 
of our Desmid flora. Dr Wesenberg-Lund attributes it to the peaty 
character of the water. Messrs West likewise attribute it to the 
absence of lime and presence of huraic acid, but they further consider 
that the richest Desmid flora is found in lochs lying in the Older 
Palaeozoic rocks. A connection has been suggested between the 
heavy rainfall and the abundance of Desmids. There can be no 
doubt, as pointed out by Messrs West, that the abundance is not due 
to species washed out from peat-bogs. There are many well-marked 
pelagic species which appear to have been long established in the 
lochs, and are rare in the peat-bogs. 
No doubt the peaty water is favourable to the increase of the 
Desmids, but neither of these explanations sufficiently accounts for 
the restriction of this Desmid flora to the British Isles. There are 
Older Palaeozoic rocks and peat-bogs enough elsewhere in Europe, 
and no doubt lochs with peaty water in moorland districts. The 
lochs possessing the Desmids are not exclusively among the Older 
Palaeozoic rocks ; many of them lie partly or wholly among drift, 
and this over rocks of various ages. The means of dispersal from 
loch to loch are easy, and might permit of their distribution over the 
whole country, and into other countries, as rapidly as is known to be 
the case with Crustacea and Rotifers. There are also many peaty 
lochs among the Older Palaeozoic rocks which have not the character- 
istic Desmids. 
In view of all these facts, it seems to me that the prime factor in 
restricting these Desmids to certain regions must be climatic. 
^ Bull. Herb. Boiss., ser. 2, t. vii,, 1907. 
