PAET III. 



METABOLISM IN THE SEA. 



CHAPTER X. 



THE CONDITIONS OF LIFE IN THE SEA. 



We see then that it is possible to group together all the 

 organisms of the sea in three great categories — the predatory 

 nektic animals, the sedentary animals and plants living on the 

 sea bottom, and the drifting microscopic life of the plankton. The 

 older methods of marine biological research have shewn how the 

 nature and abundance of the organisms of the fauna and flora of 

 the sea vary with certain physical conditions — with temperature, 

 depth of water, and nature of the sea bottom ; and have also 

 shewn that regular seasonal variations occur in the course of the 

 year in any one place. Then oceanographical investigations have 

 made us acquainted with the distribution of oceanic drifts and 

 currents, and with their varying intensity from season to season, 

 and from year to year. The results of the older methods of 

 research have been to shew that there is continual change in the 

 composition of life in the sea — that the total mass of organisms 

 there is in a state of " dynamical equilibrium," for there are 

 numerous factors in operation which set up unceasing change. 

 The temperature changes from season to season with a certain 

 regularity ; the intensity of sunlight also varies ; the salinity of 

 the sea water undergoes continual small changes; and the dis- 



