138 LIFE IN THE SEA [CH. 



been found that the activity of the denitrifying 

 bacteria is greatest at about 25 C., that is ap- 

 proximately the temperature of the tropical seas, 

 and it is practically arrested at freezing point. 

 Therefore if these bacteria are universally distributed 

 over the seas of the world they must be most active 

 in equatorial regions and least active at the bottom 

 of the ocean, and in circumpolar seas. It is possible 

 to account- for the destruction of the excess of 

 nitrogen compounds entering the sea by assuming 

 their activity. If they are present in tropical waters 

 we may also account for the poverty of these seas in 

 nitrates and nitrites, and we should have a convincing 

 explanation of the poverty of tropical seas in vegetable 

 plankton. 



One is tempted to follow up these hints by a 

 speculation as to past modes of nutrition of marine 

 animals, and as to the kinds of food-stuff which they 

 had at their disposal. No fact of biological chemistry 

 is stranger than this that nitrogen should be the 

 element which is so intimately associated with the 

 processes of life. For of nearly all chemical elements 

 it is this one which is the most sluggish and inert. 

 Yet its compounds with carbon, hydrogen and oxygen 

 are those which exhibit the greatest degree of 

 complexity known to us, and the reactions of these sub- 

 stances with each other, and with simpler compounds 

 constitute what we recognise as the phenomena of life. 



