330 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



that a destruction of the most important nitrogen-con- 

 taining inorganic food supply of plants takes place to a 

 greater extent in the warmer seas, since the activity of the 

 bacteria in the lower temperatures of the arctic seas would 

 not be so great as in the higher temperatures of the 

 warmer seas, and consequently the same amount of des- 

 truction would not go on in the former case as in the latter. 



As a practical bearing of this question, it may be asked 

 whether the sewage deposited in or washed out to sea is, 

 as is commonly supposed, wasted ? The answer must be 

 in the negative. The bacteria present in the sea water 

 convert the sewage material into nitrates, nitrites and 

 ammonia salts, which can be used up by the planktonic 

 plants. These in turn are devoured by Copepoda and 

 similar animals. These are then the prey of fish, which 

 are in turn the food of man, himself ultimately destined 

 for bacteria, and so the cycle perpetually runs its course. 



About 19 million kilograms of nitrogen are yearly with- 

 drawn from the North Sea in the shape of edible fish. This 

 represents more than one-half of the nitrogen present in 

 the North Sea at any given time. It is therefore obvious 

 that the balance must be made up in the form of material 

 derived from the land, and in this respect the sewage 

 which annually drains into the North Sea cannot but be 

 of importance, and hence must not be regarded as wasted. 



Criticisms of the Hensen Method. 



The Hensen method has been severely, and it seems to 

 me, unfairly criticised by Haeckel. 1 



Haeckel particularly objects to the estimation of the 

 adult fish from the number of floating eggs, but he has 

 entirely misapprehended Hensen's point of view. Hensen 



1 Plankton Studien. Jena. G. Fischer. 1895. 



