140 LIFE IN THE SEA [CH. 



assimilating the rich supply of food substance through 

 their surface. The result of their metabolism would 

 be the degradation of both carbon and nitrogen food 

 substance to the form of ammonia and carbon dioxide, 

 both substances on which they could no longer subsist. 

 It is also probable that degradation of the nitrogen 

 compounds would be carried on further than this 

 stage, and that free nitrogen would be formed, as 

 it is at present, by denitrifying organisms. For we 

 cannot but be struck by the fact that while there is 

 so little of nitrogen compounds in nature there is 

 nevertheless an enormous quantity of the element in 

 the atmosphere. We are tempted to regard this as 

 a residue formed by the degradation of compounds 

 of nitrogen by the agency of living organisms. 



There would thus arise a scarcity of both forms 

 of food-stuff, carbonaceous and nitrogenous. By 

 developing the power of photo-synthesis plant organ- 

 isms solved the question of getting sufficient carbon 

 food from carbon dioxide and water, under the 

 influence of light; and they became able to utilise 

 ammonia and nitrate that is they became proto- 

 trophic in habit. Organisms developing along the 

 animal line continued to be saprozoic, and many 

 still remain so ; but this became a difficult and 

 precarious mode of nutrition, as it still is ; and so the 

 two savage methods of procuring food by parasitism, 

 and by hunting and devouring other organisms were 



