4 A TEXTBOOK OF OCEANOGRAPHY 



the coastal plankton varies according to whether the rivers 

 [ischarging into the sea are fed by spring or autumn f 

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

 from month to month, so as to draw inferences as to tl 

 ments of fish to and from the coast. 



It is, however, by no means a case of a simple cha.n c 

 cause and effect. Migrations of fish are not simply affected 

 by plankton and hydrographic changes. The necess.Ues of 

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

 reproduction and for the growth of the delicate younger stage 

 of life and this makes the story more complex. Theavoidaiw 

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

 factors to be taken into consideration. 



Oceanography is thus concerned with the elucidation of 

 many intricate and complex problems of physical and 

 logical nature, and an increased knowledge of the subject 

 contributes in many ways not only to purely scientific questions 

 such as the progress of evolution, but also to such pract, 

 problems as the wiser and less destructive ut.l.sat.on of great 

 natural assets such as marine fish and cetac 



THE EXTENT OF THE OCEAN. 



Broadly speaking, the surface of the world was thought to 

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

 Recent Polar explorations, part.cularly in South 

 regions, have revealed the existence of enormous land 

 3or instance, Victoria Land and Graham Land. If we take 

 these into account, they will appreciably reduce the d.fferenc 

 previously thought to exist between the land and water surface 

 of the globe. Even so the most recent calculates g.ve the 

 ratio of land to water as i : r, or, expressed as percentages, 

 2 o- 2 70-8. So that it is approximately correct to say that - 

 oceans are two and a half times as extensive as the Ian, 

 of the earth's surface. 



The division of the world into water and lan< 



