IN my report, in the preceding number of this Journal, on my observations in the North Sea, I referred briefly to the problem of the relation between the physical and biological conditions. This problem will afford scope for investigation for some time to come, and the purpose of the present article is to discuss and compare some of the most recent additions to our knowledge of the matter. The paper by Mr. H. N. Dickson, to which I referred in my previous report, was published in the Geographical Journal last March, under the title of “The Movements of the Surface Waters of the North Sea,” and in the Scottish Geographical Journal, in 1894, was published a series of papers by Professor Pettersson on “Swedish Hydrographic Research in the Baltic and the North Seas.” Professor Heincke has discussed the fish fauna of Heligoland, its composition and sources, in an interesting paper in the series issued under the title of “Wissenschaftliche Meeresuntersuchungen,” by the staff of the Biological Station at Heligoland, in association with the Commission for the Investigation of the German Seas, at Kiel. Professor Heincke's paper is contained in Bnd. I., Hft. 1 of this series (1894), and in the same volume are a number of papers dealing on similar lines with other divisions of the marine fauna of the Heligoland Bight.It will be most convenient and logical to start the present discussion with a consideration of the results of Professor Pettersson's work. He found that the Skagerack and Cattegat were filled with layers of water distinguished from one another by differences of salinity, and that the lower layers entered the channel as under–currents, and could be recognised at the surface somewhere in the North Sea.
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