GENERAL BIOLOGY 775 



doubt whether the method of work adopted has resulted in a 

 correct idea of the quantities of organisms which these hoop- 

 nets can capture per square metre of surface, and whether this 

 method recommends itself for adoption in the present state of 

 our knowledge. 



It is evident that the quantity of organisms present at any 

 given moment does not afford any gauge as to the " primitive 

 food" contained in the ocean. The quantity of such food 

 depends on the intensity of reproduction, which is entirely 

 unknown, from coccolithophoridae to fishes. For this reason 

 the volumes of plankton shown in Fig. 566 convey no idea of the 

 actual production of the ocean, a fact of which Hensen was 

 fully aware. The abundance in boreal waters only lasts a short 

 time, and during that time production is probably not by any 

 means so rapid as in the warm ocean. While the Hensen nets 

 thus capture only an arbitrary selection of organisms, the depths 

 from which the nets were hauled were also chosen in an 

 arbitrary manner. Hensen^ himself says, when describing the 

 copepoda : " The figures show that the copepoda usually live 

 still deeper than 200 metres, their density being, however, 

 insignificant." The results seem to have given rise to some 

 doubt in his mind as to the latter opinion. 



In Chapter IX., and when speaking of nutrition, I have 

 mentioned some of the investigations made on board the 

 " Michael Sars " regarding the capture of minute crustaceans in 

 closing-net hauls from various depths. The catches have been 

 classified in regard to number of species as well as to 

 volume, and the characteristic feature was that the greatest 

 number of species and the greatest volumes of these 

 crustaceans did not occur in the upper water-layers, but at 

 certain intermediate depths. In the Sargasso Sea the greatest 

 volumes were captured between 1000 and 500 metres, off 

 Newfoundland between 500 and 200 metres, and in the 

 Norwegian Sea (Station 113) between 1000 and 500 

 metres. In the Sargasso Sea a greater number of species (51) 

 was found in the deep hauls between 1000 and 500 metres 

 than in the "surface" hauls between 200 metres and the 

 surface (22). Certain species occurred at all depths, others only 

 in the deepest hauls. Our horizontal hauls showed that besides 

 these minute forms taken by the closing-nets there is a prolific 

 community of large crustaceans, prawns, etc., in deep water, 

 where many litres could be taken in each haul, while higher up 



^ "Das Leben im Ozean," Erg. d. Plankton-Expedition , Bd. v., Kiel, 1911. 



