298 THE CIRCULATION OF NITROGEN [PART III 



and (2) because a certain amount of denitrification must take 

 place. On the other hand the land gains nitrogen, (1) by the 

 transfer of organic material to it from the sea ; and (2) by fixation 

 of the elementary nitrogen of the atmosphere by electric dis- 

 charges, and by the activity of the bacteria which are associated 

 with certain plants. The sea is being depleted of its nitrogen, 

 (1) by the transfer of economic organic products to the land (the 

 fisheries) ; and (2) by denitrification. It gains nitrogen, (1) from 

 land drainage ; and (2) by nitrogen fixation by electric discharges ; 

 and nitrogen-combining bacteria. 



It is not possible to set, in balance-sheet fashion, these receipts 

 and outputs against each other. We know only very imperfectly 

 what are the approximate masses of nitrogen compounds in 

 circulation. We do know, however, that the sea is not becoming 

 appreciably richer or poorer in nitrogen compounds. Probably it 

 gives up to the land and atmosphere just as much of this element, 

 or its compounds, as it receives from those parts of nature. We 

 know also that the composition of the atmosphere has remained 

 very approximately the same during long periods of time. It too 

 must receive back as much nitrogen (from the reduction of 

 compounds of this element by bacterial life) as it yields to the 

 land and sea (from fixation by electric discharges, and nitrification 

 by micro-organisms). But we must not assume that the com- 

 position of the sea and the atmosphere remains quite constant. 

 Perhaps changes are proceeding which by-and-by may profoundly 

 influence life both on sea and land. Such changes are, however, 

 taking place very slowly indeed. 



In the present state of our knowledge this is all that we can 

 say. Quantitative methods of study of biological phenomena in the 

 sea, such as I have attempted to illustrate in this book, have only 

 recently been adopted, and are still too imperfectly developed to 

 enable us to trace numerically the course of circulation of the 

 ultimate food-substances of terrestrial and marine organisms. 



