0.1 m.? bottom-samplers would prove to be practical here. In Denmark the fresh- 
water and saltwater are so closely connected, that this matter is of importance for 
both; the large dams we have constructed, for example at Salbæk-Vig, have 
in part been based on the German principles of carp culture, as the result of 
investigations made by Germans called in for the purpose; it is time that such 
undertakings should be studied also from the Danish side; as the results hitherto 
have not been specially good. An evaluation will probably be able to show, 
whether they can be carried out better, and will possibly throw light on whether 
quite other methods should not perhaps be introduced. At the present time all 
the organic material of the surrounding land is usually conducted out to the sea 
by means of canals, without being of any use to the ponds, and perhaps also 
without being of use to the sea. 
Naturally the evaluation will give results most quickly in the case of the 
small waters; but even in the large seas we must sooner or later come to 
it. In the sea the determination of the mass and production of the bottom-plants 
is made much easier by the fact, that they only grow in shallower water, as also 
that they are probably not eaten up, whilst living, by the animals as a rule; for 
an extremely large number of algae we might therefore take almost the whole of 
their maximum quantity present as the annual production, as many are annuals; 
in the case of the Zostera and similar plants, it will probably be easy to ascertain, 
by closer study of their growth, how much they produce annually. At the places 
where the bottom is covered with stones, or formed of rocks, determinations with 
the bottom-sampler may perhaps be impossible; but here as a rule it is possible 
to carry on diving operations. 
No large open water has been better and oftener investigated probably 
than the North Sea, its coasts in the south and east perhaps most of all; but 
these are so little covered with vegetation, that we involuntarily, on a long 
acquaintance with them; obtain the impression, that the bottom-vegetation on the 
open coasts is on the whole of very little importance; this however is not the 
case; journey over to the rocky coasts of England and Scotland, to Norway with 
its cliffs and reefs and fjords, at places very broad and still unmeasured and 
numerous, not to say innumerable; there we find vegetation which is of some 
importance; and all the sheltered fjords have their superfluity of algae and Zostera; 
nor do they keep these to themselves, but scatter them far and wide in the sea, 
as Boysen Jensen points out. And consider the quantities carried down into 
the North Sea by the large rivers of Middle Europe and England! 
What is the origin of the rich mud-lands (marshes) of Holland, Friesland, 
Holstein and Sleswig? Until we have found our way in these questions, and have 
learnt to use measures and weight, as Hensen has taught us for the plankton, 
we shall never rightly understand, what forms the basis of the great fish-wealth 
of the North Sea and its coastal waters or what we can expect to get from them. 
After I had learnt, what the fine detritus of the bottom and the water 
means for the lower animals, I endeavoured to fix up a small aquarium with 
water and bottom-soil of detritus from the Great Belt, the same mass of 
water being used always, never filtered aud only air drawn through it. It appears, 
that Asterias, Ophioglypha albida and Bucernmum, Fusus, Littorina Uttorea, Abra 
