B. THE DISTRIBUTION OF MARINE ANIMALS 



Introduction 



The ocean presents the most extensive habitat for living organisms. 

 If the total surface of the earth is reckoned at about 510,000,000 

 sq. km., more than two-thirds, or 361,000,000, are occupied by the 

 oceans and only 149,000,000 by land. If the level of the sea bottom were 

 equalized so that the oceans would be everywhere of the same depth, 

 that depth would be 3795 m., from which the volume of water contained 

 in the ocean basins may be reckoned at 1,370,000,000 cu. km. In con- 

 trast the mean elevation of the land is only about 700 m. If all the land 

 were submerged in the sea, only a relatively small amount of water 

 would be displaced. At present about 3,000,000,000 metric tons of 

 material from the land are being washed into the sea annually. 



A second comparison is invited. In many ways it is convenient to 

 discuss hydrobiology as a unit rather than its subdivisions of oceanog- 

 raphy and limnology. From the zoogeographic point of view, these sub- 

 divisions are very unequal; the inland waters, whose physical and 

 biotic characteristics constitute the subject matter of limnology, occupy 

 only a small fraction of the land surface. North America is rich in lakes 

 and has at least an average river and stream surface, yet Welch esti- 

 mates 1 that only about 2% of the land surface of this continent is 

 covered by inland waters. 



The oceanic mass of water forms a single continuous domain. It 

 falls into two main divisions, the Atlantic with the Arctic on one hand, 

 and the Pacific and Indian oceans on the other. The separation of these 

 is incomplete, and their waters are united by wide stretches at the 

 south, and at the north by the narrow opening of Bering Strait. While 

 a number of smaller divisions are essentially independent seas, such 

 as the Mediterranean or the Baltic, they are connected with the prin- 

 cipal oceans by open straits. Completely separated salt-water basins, 

 like the Aral and Caspian, are few and vanishingly small as compared 

 with the oceanic area. A division like that of the land into continents 

 is thus entirely wanting for the oceans. 



This vast space is everywhere inhabited by living organisms. It 

 contrasts with the atmosphere, whose inhabitants are constantly, or 

 for the greater part of their life, confined to the ground. The atmosphere 



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