294 ' THE CIRCULATION OF NITROGEN [PART III 



suggested to Brandt a possible explanation for the apparently 

 paradoxical distribution of life in the warm and cold seas. I have 

 already stated the evidence in favour of the belief that the colder 

 seas of the temperate and frigid zones are not less rich in life than 

 the Avarmer seas of the subtropical and tropical regions. Putting 

 this fact of distribution in such a way as to avoid any straining or 

 exaggeration of the case, we may say that on the land, the density 

 of life, particularly plant life, decreases very greatly as we pass 

 from the equator towards the poles, so that while there is a 

 luxuriant vegetation in the tropical and subtropical regions, there is 

 but little plant life on the polar land, even where the latter is not 

 covered with snow or ice. But this decrease in plant and animal 

 life does not take place in the sea as we proceed north or south 

 from the equator. Indeed the opposite appears to be the case. 



If it is the case that denitrifying bacteria are universally 

 present in the sea, and if the activity of these organisms is not 

 exactly compensated by the activity of equally ubiquitous nitrogen- 

 fixing bacteria, then it seems reasonable to suppose that a greater 

 amount of destruction of nitrogen salts will take place in the 

 warmer than in the colder seas. For it is clear that the activity 

 of denitrifying bacteria increases with the temperature. In the 

 tropical and subtropical sea-areas these micro-organisms are very 

 active and a considerable mass of nitrates, nitrites, and ammonia 

 must be reduced to elementary nitrogen. But in the cold polar 

 and temperate seas the activity of these bacteria must be greatly 

 restricted, or even arrested entirely, by the low temperature of 

 the water, and not nearly so much of the nitrogen salts will be 

 destroyed. Therefore the warmer sea-areas must be much less 

 rich in the nitrogenous food-salts, which are necessary for the 

 nutrition of the plants, than are the colder waters. To that extent 

 then there must be a lesser production of organic substance in 

 the equatorial, than in the polar sea-areas. 



While discussing such problems as these one is impressed with 

 the enormous gaps in our knowledge of the conditions of life in 

 the sea. It is quite evident that Brandt's hypothesis cannot be 

 the only one capable of explaining, provisionally at least, the facts 

 of distribution indicated above. Whatever explanations may be 

 given must take account of the distribution of the food-stuffs of 



