18 THE STORY OF LIFE IN THE SEAS. 



that of the other regions of the world, sinks to 

 the bottom, and gradually returns in a deep- 

 seated mass towards the Equator, where, welling 

 up from the bottom, it replaces the heated layers 

 of the surface. It is almost impossible to deter- 

 mine with accuracy the rapidity and exact 

 direction of these deep-sea currents. It is ex- 

 tremely probable that they are immensely 

 modified by the irregularities of the bottom and 

 the outline of the coast banks, but their exact 

 topography must remain for the present one of 

 the secrets of the abyss that are not revealed to 

 us. All that can be said is, that the warm 

 surface water which passes from the Tropics 

 towards the North and South is replaced by 

 deep-seated Polar currents, which account for 

 the extremely cold water that is found at great 

 depths in the ocean-basins, and also for some of 

 the peculiarities of the marine Fauna, which will 

 be referred to later on. 



The character of the sea-bottom in various 

 parts of the world must be referred to before 

 passing on, for there can be little doubt of the 

 important effect it has upon the Fauna. In the 

 neighbourhood of continents the bottom of the 

 sea varies very considerably. The great rivers 

 bring with them in suspension the products of 

 the wear and tear of mountains and valleys ; the 

 coast line, washed by the continuous ebb and flow 

 of the tides, contributes some of its substance to 

 the formation of the sea-bottom ; and the count- 

 less millions of animals and plants of the shallow 

 waters leave their skeletons and shells as they 

 die to form an integral part of the floor of the 



