LIMNOLOGICAL PROBLEMS 
399 
no one has tried to bring together what is known regarding the 
changes these communities undergo systematically and biologically in 
the direction from north to south, nor can we attempt to do so here 
in this brief sketch. We shall restrict ourselves to remarking merely 
that these connnunities follow the same laws which control the life of 
land and sea. They reach their highest development in the tropics ; 
for every latitude from north to south we meet with new types both 
in the vegetal and in the animal kingdom. Still, it must be main- 
tained that the littoral fauna and flora of fresh water, especially amongst 
the lower organisms, contain many cosmopolitan species, and that even 
amongst the higher a great many enjoy an extremely wide distribution. 
With regard to the plankton communities our knowledge until 
about 1890 was very insignificant. In the last twenty years the 
investigation of the European fresh-water plankton has been carried 
out with great energy ; some knowledge of the arctic and the tropical 
fresh-water plankton (especially that of the great African lakes) has 
been gained. 
During the last ten years I have been much occupied with plankton 
studies, and in my work on PlanMon Investigations in the Danish Lakes, 
1904-8 (Copenhagen), I have endeavoured to bring together all that 
could throw light on the life-histories of the plankton forms. In the 
following, in accordance with Sir John Murray's wishes, I shall try to 
give the main points of these investigations, taking as my starting- 
point one of the greatest peculiarities of the fresh-water plankton, 
viz. its cosmopolitanism. From this as basis and as a means to 
understanding the nature of this cosmopolitanism the opportunity 
is afforded of mentioning the lines of investigation which have in recent 
years specially occupied the investigators in this great field of work. 
PART IL— THE PLANKTON COMMUNITIES, THEIR 
GEOGRAPHY AND LIFE-HISTORY 
The fresh-water plankton is characterised by its well-marked cosmo- 
politanism. I consider this cosmopolitanism as an established fact for 
a great many plankton organisms, and I believe that it can be seen 
with distinctness from most of the plankton papers of recent times. 
This subject has been more closely discussed in chap. xii. of my 
Plankton Investigations. In their interesting work. The British Fresh- 
water Phytoplankton, 1909, chap, xii., Messrs West have maintained 
that it does not hold good for the Desmidiacege. It may be remarked, 
however, that a very large number of these can only with difficulty be 
regarded as true plankton organisms, especially the true, typical lake 
plankton which was exclusively in my mind. Further, it must be 
