286 THE CIRCULATION OF NITROGEN [PART III 



food. So we can easily construct series of animals each of which is 

 the food of the one higher in the series. Thus : — 



Diatoms — >► cockles — >► flounders — >- man ; 



Diatoms — >- oysters — >-man ; 



Peridinians — >■ copepods — ^ sprats — >- whiting — ^ cod — ^ man ; 

 and so on. 

 Thus the nitrogen compounds which are produced on the land 

 from the putrefactive decomposition of the dead bodies of animals 

 and plants or the excretions of the animals, are washed down into 

 the sea ; and a fraction of the total mass of nitrogen so transported 

 may again return to the land in the form of the bodies of useful 

 marine animals which have fed, directly or indirectly, upon this 

 nitrogenous drainage substance. 



Production by marine plants. The plants are the inter- 

 mediaries between the inorganic food salts of the sea and the 

 organic proteid, fat, and carbohydrate which form the food-stuffs of 

 the higher animals. Let us imagine that in an enclosed sea-area 

 into which land drainage percolates, plant life were suddenly to 

 cease to exist. It is almost certain that animal life would also 

 become extinct in such a case. Two conditions would produce 

 this result : (1) the constant addition of salts of nitrous and nitric 

 acids and ammonia would by-and-by render the water poisonous 

 to animals ; and (2) there would be no production of organic from 

 inorganic materials. Possibly a certain small proportion of the 

 latter substances would be utilised by some of the lower inverte- 

 brata ; but it would only be the nitrogenous compounds of the type 

 of urea or extractive substances that could so be utilised by 

 saprozoic animals ; and there is no evidence that nitrates or 

 ammonia salts could be used as food. The animals would feed 

 upon each other, of course, but among them there would be a 

 number such as the moUusca, or smaller Crustacea, which are 

 accustomed to eat the protophyta. With the extinction of the 

 latter the lower invertebrata would miss their accustomed food, 

 would cease to multiply, and would finally become extinct, their 

 disappearance being hastened by the ravages of the larger predatory 

 animals, such as the fishes. The latter would be the last survivors 

 in our microcosmos, and in time, their food also disappearing, they 



