LIMNOLOGICAL PKOBLEMS 
427 
lakes. No thorough investigations have been carried out with regard 
to their temperatures, and the other physical and chemical condi- 
tions are quite unknown. With regard to the life of the littoral 
flora and fauna we have only cursory and rather casual descriptions 
and observations ; the abyssal fauna we only know for some of the 
large African lakes, and as regards the plankton our knowledge is 
very unsatisfactory (Apstein, Lake Colorfibo, 1907, p. 201). I am of 
opinion that it is in the tropical lakes that we shall find the proofs of 
the correctness of many of the above-named theories. Shall we find 
seasonal variations in the plankton organisms of the tropical lakes ? 
Is the local variation very conspicuous ? Is the average size of the 
different plankton organisms smaller than in the temperate lakes ? Is 
the propagation chiefly digonic or monogenic ? What part is played 
by the resting-stages in the life of the species ? What is the period- 
icity of the plankton organisms ? Do they make vertical wanderings ? 
Is the relation of the tropical fresh-water plankton to that of the 
ocean closer than that between the lake plankton and oceanic plankton 
of the temperate zone ? Is v. Martens' supposition, " Die Aehnlich- 
keit der gesannnten Slisswasserfauna mit der gesannnten Meerfauna 
nimmt vom Pol gegen den Aequator zu,*" indisputable and of the 
same validity for all associations in the fresh-water lakes ? 
In my opinion, a thorough eocploration of one of the great tropical 
lakes is one of the inost desirable objects for the promotion of limnology. 
Attention is drawn involuntarily to the great African lakes, where 
Moore's investigations have given such valuable scientific results, and 
where the German investigations and those of the Messrs West have in 
so high a degree increased our knowledge of the fresh-water flora. 
That this investigation will be both very expensive and, owing to the 
climatic conditions, probably much more dangerous than many oceanic 
and polar explorations is unfortunately beyond doubt. 
When preparing my work on the plankton for the press I looked 
over the whole literature relating to fresh-water plankton, and was often 
astonished at the very great differences in the interpretation of the 
biology even of the most common plankton organisms. From the 
foregoing we have seen that the plankton, both with regard to their 
morphology and their biology, follow different lines in different lati- 
tudes. The great difl^erences in the interpretation of the biology and 
morphology by diff'erent investigators are therefore quite natural. 
What we in planktology as well as in every other part of limnology 
most of all need is simultaneous^ coherent investigatioiis in different 
latitudes from north to south. We need such investigations with 
regard to temperature. This has for a long time been clear to 
limnologists (John Murray, Forel, Pettersson, 1902) ; and a first paper 
on simultaneous temperature observations in Lakes Enare, Mjosen, 
