58 Proceedings of the Royal Physical Society. 



different regions, which we find in the plankton of the west 

 and north of Scotland, offers a problem in distribution, the 

 solution of which may be difficult. The conditions which 

 combine to produce this remarkable admixture of forms 

 are no doubt sufficiently complex, and I shall here only 

 touch upon some suggestions that have been put forward 

 in explanation of some of the facts. 



Dr Lund (6), in discussing the abundance of Desmids 

 in the Scottish plankton, suggests that they are in great 

 part directly derived from the surrounding bogs by being 

 washed out into rivers, and so carried into the lochs, that 

 some of them may have become adapted to a pelagic life, 

 and modified in accordance with it, and that the peaty 

 water of the Scottish lochs, being favourable to Desmid 

 life, may account for their greater abundance in Scotland 

 than elsewhere. These are no doubt explanations of some 

 of the facts ; peat-bogs are the headquarters of the Desmids, 

 and a large number of the Desmids which we find in the 

 plankton can be recognised as having come from that source ; 

 but we have a very large number of truly pelagic species, 

 some of which are rare in the bogs but common in the 

 plankton ; and we have also the numerous species unknown 

 in Europe, except in this narrow littoral strip of Britain 

 and Scandinavia. There must surely be somewhere else 

 in Europe extensive sphagnum swamps, and lakes deriving 

 peaty water from them, but it is only here that the rich 

 Desmid-flora is found. The species of Desmids so abundant 

 in our plankton are not even in the peat-bogs of the rest of 

 Europe. Some of our clearest and least peaty lochs. Loch 

 Morar for instance, are very rich in Desmids; and some 

 very peaty lochs are poor, Loch Ness for instance. 



Another theory is that the lochs which are richest in 

 Desmids are only found in the older geological formations, 

 but this does not accord with the facts, as I find that such 

 lochs occur in all the formations from the Lewisian to the 

 Tertiary at least ; and it will, I think, be found that some 

 of these lochs lie entirely in glacial deposits. 



Sir John Murray points out that when the area occupied 

 by the rich western flora of Desmids is plotted out on a map. 



