320 



organisms than the total amount of heat received; for 

 daily changes affect chiefly the upper strata of the water, 

 which are directly influenced by the heat of the sun. At a 

 moderate depth below the surface, the stratum of mean tem- 

 perature is reached, this, where not affected by oceanic cur- 

 rents, varying mainly with the change in latitude. 



A combination of the climatic and topographic factors is 

 the chief cause of ocean currents, which are of great impor- 

 tance in the geographic distribution of marine organisms, 

 not only affecting the temperature of the regions through 

 which they pass, and, consequently, their faunas and floras,* 

 but also to a large extent determining the directions of 

 migrations. 



Next to the climate of the sea the topography of the sea 

 bottom, and that of the adjacent land, is the most powerful 

 factor in determining the distribution of marine organisms. 

 The fades of the ocean floor, or the material of which it is 

 composed, is perhaps the most significant part of sea-bot- 

 tom topography, though submarine ridges and barriers are 

 of great importance, especially when such barriers cut off 

 marginal bodies of water, the inhabitants of which may 

 be prevented from intermingling. The separation thus pro- 

 duced may lead to the development of local faunas and 

 floras. The importance of the greater inequalities of the sea 

 bottom, and the submarine continental shelves and deep 

 oceanic basins, due to them, as well as the conformation of 

 the coast-line, with its varying facies, will be considered 

 later. 



Of all the topographical features which influence the 

 distribution of marine organisms, northward and south- 

 ward stretching bodies of land, like the continents of North 

 and South America, are perhaps the most important. For 

 since they form a continuous barrier across the warmer 

 portions of the ocean, extending into the cold regions, the 

 migration of the warmer-water species from one side to the 



* Differences of fauna* and floras due to oceanic currents are well illustrated by the 

 marine life north and south of Cape Cod. 



