184 BRITISH FISHERIES 



preserved for future study. This consists in the 

 measurement of its volume or weight, in the 

 chemical analysis for nutritive material, and in the 

 enumeration of the separate individuals belonging 

 to the different species of plants and animals caught. 

 All these determinations have been made by the 

 Kiel biologists, and the solution of the various 

 difficulties encountered in these investigations is a 

 striking tribute to German patience and technical 

 skill. Obviously, the results obtained can claim 

 to be only approximations to the truth. Many 

 sources of error exist, the principal of which are : 

 (i) the difficulty of measuring the exact quantity 

 of water filtered by the net ; (2) the fact that 

 many of the smaller organisms (diatoms) in the sea 

 slip through the meshes of the silk cloth ; and (3) 

 the fact that the distribution of the plankton is 

 rather irregular, so that an exact estimation of 

 the organisms present in one part of the sea may 

 not be true of adjacent regions. All these, how- 

 ever, are errors which are minimised as much as 

 possible, and will, no doubt, ultimately be got 

 rid of more or less completely. They certainly 

 do not justify the somewhat severe criticism 

 which Hensen's method has received, 1 both in 

 Germany and Britain : Hensen's results may, not 



1 As, for instance, in the partial and injudicial criticisms by Haeckel 

 in Plankton- Studicn> Fischer, Jena, 1895. An English translation of 

 this paper is published in the U.S. Fish Commissioner's Report for 

 1889-91, Washington, 1893. 



