THE SEA BOTTOM 59 



and changes in temperature are primarily responsible for 

 the great differences in the faunas of tropical, temperate and 

 polar regions — differences in the " horizontal " distribution 

 of animals. Temperature and depth are the factors of 

 greatest importance controlling the " vertical " distribution 

 of animals from the shore into deep water. Owing to the 

 decrease in temperature, the horizontal differences become 

 less and less as the water gets deeper, thus we find similar 

 animals at great depths in the polar and equatorial regions 

 because the temperature is the same, while animals found in 

 shallow water in temperate seas, for example, have relatives 

 living in deeper water with a similar temperature in the 

 tropics. 



The shallow waters of the globe can be divided into 

 different regions corresponding to changes in the hori- 

 zontal distribution of animals, the result, as we have seen, of 

 differences in temperature. These, it must be noted, do not 

 follow parallels of latitude, but are greatly influenced by cold 

 and hot currents, an example of which is provided by the 

 western coasts of Europe bathed by the warm Gulf Stream 

 and the frozen coasts of Labrador — in similar latitudes — 

 which are washed by the cold Labrador current. We can 

 distinguish between an Arctic region, including the water 

 round the North Pole and " boreal " areas along the west 

 coast of Europe and in the North Pacific ; an Indo-Pacific 

 Region including the shores of India, East Africa, China, the 

 East Indies and Australia ; a West American Region along 

 the tropical shores of West America ; an East American 

 Region running southward from Newfoundland ; a West 

 African Region including both the Mediterranean and the 

 Guinea coast ; and finally an Antarctic Region which 

 includes the southern coasts of Australia, Africa, and South 

 America from the Argentine on the east to Ecuador on the 

 west. Within any one area the members of the marine 



