430 Proceedings of Poyal Society of Edinburgh. [sess. 
plains. Seeing, however, that no thorough investigations have as 
yet been carried out on this point in the lakes of the plains, or 
the facts have not been sufficiently elucidated, I consider any 
discussion on this subject as rather premature. Mr James Murray 
has told me that at night a very great accumulation of plankton 
takes place in the surface waters of the Highland lakes, and we 
may therefore conclude that very conspicuous movements occur 
at different times of the day and night ; in this particular also the 
plankton of the Highland lakes agrees with that of other alpine lakes. 
Before leaving the plankton of the Scottish lakes I wish to draw 
attention to a very peculiar feature. The singular abundance of 
Desmids has been already mentioned, and needs an explanation. 
To suppose that the Scottish lakes should be the only known home 
of an entire plankton-flora of Desmids seems to me, at first sight, 
from my knowledge of fresh- water planktons, on the whole an odd 
idea. I presume that the occurrence of the Desmids in the 
plankton must be regarded in connection with the appearance of 
a good many other organisms in the Pelagic region of the lakes ; 
for instance, Polyphemus pediculus, Sida crystallina , Chydorus 
sphcericus, Clathrulina , several Eotifers, and very many Diatoms 
of the sub-orders Naviculoideae and Surirelloideae. All these 
organisms may be considered as littoral forms, washed out by the 
waves from the precipitous hillsides, blown out by the wind from 
the few shallow bays, and carried out into the deeper part of the 
lakes by rivers and currents. Knowing that the original home of 
the Desmids is in peat-moors, and that the sloping sides of the 
hills in Scotland are almost everywhere covered with mosses, 
which are quite moist for the greater part of the year, and in many 
places all the year round, the thought immediately struck me that 
the plankton Desmids must have been originally derived from the 
hillsides, or from tarns and moors on the hilltops, and, associated 
with the littoral species above named, have been carried by the 
rivers out into the centre of the lakes. Later on, when I read 
the most interesting paper of Messrs West, I observed that, 
according to these gentlemen, the plankton Desmids of the 
Scottish lakes “are also known to us from the bogs and rocky 
pools of north-west Scotland and the Outer Hebrides ” (1904, 
p. 553). Further, the authors report the very interesting fact 
