400 THE FRESH-WATER LOCHS OF SCOTLAND 
remembered that by far the greater part of our knowledge of the 
Desmid flora, and especially of the tropical, is based upon Messrs Wesfs 
own excellent investigations. Other times may come and other in- 
vestigators whose detailed views of species may perhaps not be the 
same as Messrs West's ; it is thus quite possible that the boundaries 
between the regions given by Messrs West will be abandoned. 
Against my opinion it can further be said that in the high arctic 
zone some types are apparently absent, that the great African lakes 
ai'e characterised by their remarkable Diatom plankton, and that some 
few genera and species {Sida liimietica^ Limnosida frontosa) are re- 
stricted to rather limited areas ; only the Diaptomidas, according to 
our present classification and knowledge, seem to have a distribution 
which is fairly sharply delimited for each species. 
It must be emphasised that the fresh-water plankton connnunities, 
in contrast to all other communities on land or in water, everywhere 
contain the same types, nearly everywhere the same species. The 
arctic or North European zone and the tropical zone have a very 
large number of species in common. This applies especially to the 
Diatoms^ C^anophi/cea?, Chlorophi/cea?, and Flagellata; further, amongst 
the Rotifera, Amerada acideata, Polyarthra platyptera^ AsplancJma 
BiighticeUi^ TriaHhra longiseta, species of the genera Branchionus^ 
Pedalion ; amongst the Cladocera, Bosynina longh^ostris and B. coregon% 
Ceriodaplmia cornuta^ Daplmia hyalina, Chydorus spli^ricus ; amongst 
the Copepoda, Cyclops serrulatiis, C. Leuckarti, C. oithonoides, etc. In 
no other community is so great a number of species common to the 
whole world : only very few new types are found on comparing the 
plankton of northern latitudes with that of southern. Considering to 
what a degree the different plant and animal communities, terrestrial 
as well as marine, change from the pole to the equator, and how no end 
of new types appear for every degree of latitude as we proceed to the 
south, the cosmopolitanism of the fresh- water plankton must first and 
chiefly be emphasised as its greatest peculiarity and one of its greatest 
puzzles, which we are at present unable to solve with certainty. The 
phenomenon is confirmed by every new research ; future investigations 
may indeed increase the number of exceptions, but the fundamental 
result of this review will hardly be changed. 
Compared with this phenomenon, the supposed maintenance of 
sharply delimited areas of distribution for certain fixed genera and 
species is of quite secondary importance. If we try by means of such 
areas, which appear at present apparently natural and well defined 
for some species within certain groups of animals, to divide the fresh- 
water plankton into similar well-marked zoo- and phyto-geographical 
territories like those of other communities, we find that the attempt 
quite fails. 
