1 88 BRITISH FISHERIES 



suggestive lines of investigation opened up. The 

 amount of labour entailed by these methods is 

 very great Hensen tells us that he took a week, 

 working for eight hours per day, to enumerate the 

 organisms in a single catch, 1 but the results are 

 commensurate with the labour employed. The 

 most interesting, and those of most practical 

 value, were obtained by the enumeration of 

 the pelagic fish eggs in the Baltic and North 

 Seas. In the course of three voyages in the 

 North Sea in 1895^ Hensen and Apstein found, 

 as the result of 158 hauls with a quantitative 

 plankton net, an average number of 92 eggs and 

 larvae belonging to fishes used as human food 

 per square metre of surface. The surface area 

 of the North Sea is 547,623 million square metres, 

 and a simple calculation (after applying certain 

 corrections) showed that that sea contained in 

 1895 at least 157 billion eggs and larvae of food- 



1 Haeckel (in the Plankton- Studien) has referred to this statement 

 of Hensen's in a somewhat slighting manner, and has deprecated 

 generally the value of quantitative investigations in biological 

 science. Such a criticism seems very curious now, when quantitative 

 methods have become so largely developed in general biological 

 as well as in pathological investigations. The quantitative methods 

 in connection with problems of variation and heredity developed 

 by Heincke, Galton, and Karl Pearson are producing results of 

 notable value, and the methods of the enumeration of the red blood- 

 corpuscles of the human blood in health and disease, and of the 

 enumeration of microbes in sewage and other liquids, are essenti- 

 ally similar to Hensen's plankton estimations. 



2 Wisscnschaftliche Meeresuntersuchungen^ Kiel Kommission, Bd. 

 ii., Heft 2, 1897. 



