1 92 SCIENCE PROGRESS 



streams and their offshoots, as well as others which were 

 characteristic of water of mixed nature, or of water which 

 was normally present in the shallow seas and was characterised 

 by the presence of "Neritic" plankton forms. Other biologists 

 in Norway and Sweden followed up this line of research, and 

 there is now a very copious literature on the subject. Victor 

 Hensen, 1 of the University of Kiel, was the first to attempt an 

 estimate of the abundance of living organisms in a definite sea 

 volume, and he was followed by Brandt, Apstein, and others. 

 Hensen recognised the impossibility of attempting to estimate 

 the numbers of animals living on the sea-bottom or swimming 

 freely through the water, and therefore he devised apparatus 

 by means of which he estimated the abundance of the 

 planktonic organisms. These latter are microscopic in size, 

 and as they float passively about in the sea it was possible to 

 construct and use nets which could be lowered to the sea- 

 bottom and then hauled up again so as to filter a known volume 

 of water. In this way it became possible to estimate the 

 absolute numbers of the organisms exceeding a certain limit 

 of size which were present in the area investigated. Now it 

 should be noted that the primary objects of both of these 

 methods of investigation were different from those latterly 

 contemplated. Cleve and Pettersson attempted to determine 

 the origin of the water which was present in the sea off the 

 shores of the countries surrounding the Baltic. It was known 

 that the climate of these countries depended very greatly on 

 the temperature of the sea, and it was thought that by tracing 

 the origin of the water currents reaching the shores of the 

 Baltic it might be possible to obtain information which would 

 be of value for the forecasting of the seasons. The place of 

 origin of the currents could be determined by a study of the 

 plankton forms — the diatoms chiefly — contained in the sea, and 

 this was the object of the study of plankton-types. Hensen's 

 object in attempting the numerical estimation of the individuals 

 of the plankton was to ascertain what was the absolute number 

 of the useful fishes inhabiting a large area, such as the North 

 Sea. It is theoretically possible to determine this by estimating 

 the number of fish eggs present in the area investigated during 

 the spawning season of the fishes in question, for we know 



1 " U. d. Bestimmung des Planktons," Ber. 5, Komm. Wiss. Untersuch. Deutschen 

 Meere, Berlin, 1887. 



