GENERAL BIOLOGY 727 



continental slopes of the Norwegian Sea. From these localities 

 the young stages spread over the whole sea, including the coast 

 banks and the fjords of Norway. During summer only young 

 individuals are met with, immediately recognizable by the 

 presence of large oil-globules. These minute calani constitute 

 the main nourishment upon which more or less directly the 

 animal life of the Norwegian Sea depends. Even the enormous 

 whalebone whales feed on calani. During the last months of 

 the year the number of calani decreases enormously, and in 

 winter only a few adult individuals remain. 



In Chapter VI. Gran gives an account of Lohmann's 

 attempts at calculating the relation between the increment in 

 pelagic plants and the consumption of plants by animals in the 

 fjords at Kiel during the course of a year. According to 

 Lohmann's calculations the volume of plants increases daily by 

 30 per cent, which increase may be used up by animals without 

 endangering the existence of the plant-stock. Copepoda and 

 other multicellular animals are supposed to need a daily supply 

 of food equivalent to about one-tenth of their own weight. 

 Starting from these assumptions Lohmann attempts to calculate 

 the relation between production and consumption in the course 

 of the year, and arrives at the conclusion that there is generally 

 a surplus of plants except in the winter. For details I refer to 

 the table on p. 384, recording the daily increment of various 

 food producers during the year, which varies greatly from summer 

 to winter, the relation amounting sometimes to 35 : i. 



In tropical and subtropical waters no seasonal changes of Conditions 

 this kind appear to take place. At least all the tow-nettings ^l^'^""^^^ 

 taken in the tropics by various expeditions have always yielded 

 remarkably uniform catches in the upper layers, which are the 

 ones most thoroughly examined, these catches being very small 

 compared with similar catches during summer in boreal waters. 

 As instances of this I may mention that the closing-nets of the 

 " Michael Sars " when hauled from 200 metres to the surface in 

 the Sargasso Sea yielded on the average 3 c.c. of plankton, 

 while in the Norwegian Sea from 85 to 225 c.c. were obtained 

 in numerous similar hauls.^ Similar results were obtained 

 during the German Plankton Expedition. 



It is, however, at present impossible to form any idea 

 whether the volumes thus obtained really tell us anything what- 

 ever about the annual production. First of all in boreal waters 

 we have to deal with the enormous seasonal changes. Secondly, 



^ Damas and Koefoed, /oc. cit. 



