4 A TEXTBOOK OF OCEANOGRAPHY 



the coastal plankton varies according to whether the rivers 

 discharging into the sea are fed by spring or autumn floods. 

 This makes it worth while to study the variation of the plankton 

 from month to month, so as to draw inferences as to the move- 

 ments of fish to and from the coast. 



It is, hdwever, by no means a case of a simple chain of 

 cause and effect. Migrations of fish are not simply affected 

 by plankton and hydrographie changes. The necessities of 

 the life of our important food fish include a suitable place for 

 reproduction and for the growth of the délicate younger stages 

 of life, and this makes the story more complex. The avoidance 

 of enemies and the feeding of fish on one another are also 

 factors to be taken into considération. 



Oceanography is thus concerned with the elucidation of 

 many intricate and complex problems of physical and bio- 

 logical nature, and an increased knowledge of the subject 

 contributes in many ways not only to purely scientifîc questions 

 such as the progress of évolution, but also to such practical 

 problems as the wiser and less destructive utilisation of great 

 natural assets such as marine fish and cetacea. 



The Extent of the Océan. 



Broadly speaking, the surface of the world was thought to 

 be composed of three parts of water to one part of land. 



Récent Polar explorations, particularly in South Polar 

 régions, hâve revealed the existence of enormous land masses 

 — for instance, Victoria Land and Graham Land. If we take 

 thèse into account, they will appreciably reduce the différence 

 previously thought to exist between the land and water surface 

 of the globe. Even so the most récent calculations give the 

 ratio of land to water as i : 2*43, or, expressed as percentages, 

 29"2 : 7o'8. So that it is approximately correct to say that the 

 océans are two and a half times as extensive as the land masses 

 of the earth's surface. 



The division of the world into water and land plays an 



