GENERAL BIOLOGY ^^^ 



investigation to be applied, like all other means, according to 

 circumstances. 



Hensen invented his method for the purpose of investigating 

 the floating or suspended life in the sea, which he termed 

 " plankton." This plankton is, however, very difficult to define, 

 for among the profusion of organisms, ranging from the 

 minutest plants, the coccolithophoridae, to large crustaceans 

 and fishes, there is an enormous variety in size, in activity, 

 and consequently in the faculty of avoiding the appliances of 

 capture. In many investigations, therefore, the word plankton 

 may be taken to signify practically " the catch made in the 

 hoop-net constructed by Hensen, when new and in perfect 

 working order." But does this selection among the organisms 

 of the sea correspond to an arrangement peculiar to the 

 organisms in nature ? All our experience shows that the 

 catching power of the Hensen net is restricted, firstly, 

 because, as shown in Chapter VI,, an important group of plants 

 (the coccolithophoridae) may pass through the meshes of even 

 the finest silk nets, and secondly, because the selection of 

 animals actually taken is very limited, consisting of unicellular 

 animals, minute crustaceans, sagittidae, etc., while the large 

 crustaceans, schizopoda, decapoda, and even small fish-fry, 

 mostly avoid the net. This limited power of capture alone is 

 apt to affect our ideas of marine life in a perfectly arbitrary 

 manner ; but another objection to the universal application of 

 the Hensen method arises from the fact that in large areas 

 the conditions do not correspond to the theoretical condi- 

 tions on which the method is based, for in theory the dis- 

 tribution of the organisms is regarded as something like the 

 even distribution of the molecules of a gas encased in a box or 

 aquarium. 



In 1885 Hensen made an expedition in the " Holsatia" and 

 in 1889 another in the " National," during which vertical hauls 

 were made with his nets in shallow water from bottom to 

 surface, and in the ocean mostly from 200 metres to the 

 surface. The volumes of organisms taken during these cruises 

 have been represented graphically in Fig. 566, reproduced 

 from Steuer's text-book. In this figure the track of the cruise 

 has been used as horizontal axis, and lines have been drawn 

 vertically (as ordinates) to show the relative volumes taken per 

 square metre of surface. These volumes are very great 

 in the Irminger Sea and in the North Sea (amounting to 

 166.9 c.c), and very small in the Sargasso Sea as well as in 



