GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 1477 



surface of the Torrid Region, and not one-fourth as much coast line, 

 facts which should be regarded in comparing the number of species of 

 the two. 



Before leaving this subject of the Map, we add a few brief remarks, 

 in a popular way, on the origin of the peculiar forms and positions 

 presented by the isothermal lines of the ocean. The great currents of 

 the globe are admitted to be the causes that produce the flexures and 

 modify the courses of these lines. These currents are usually of great 

 depth, and consequently the deflecting land will be the deeply seated 

 slopes off a coast, beyond ordinary soundings. 



The eastern coasts of the continents either side of the equator, feel 

 the influence of a warm equatorial current, which flows westward over 

 each ocean, and is diverted north and south by the coasts against 

 which it impinges, and more or less according to the direction of the 

 coast. 



The western coasts of the continents, on the contrary, receive a 

 strong polar current. In the southern oceans, it flows from the west- 

 ward, or southward and westward, in latitudes 45° to 65° south, and is 

 brought to the surface by the submarine lands or the submarine slopes 

 of islands or continents ; reaching the continents of Africa and South 

 America, it follows along the western coast towards the equator. The 

 same current, being divided by the southern cape of America, flows 

 also, with less volume up the eastern coast, either inside of the warmer 

 tropical current, or else on both sides of it. In the Northern Seas, 

 the system of polar currents is mainly the same, though less regular; 

 their influence is felt on both eastern and western coasts, but more 

 strongly on the eastern. In the Atlantic, the latter reduces the tem- 

 perature of the waters three or four degrees along the north coast of 

 South America, as far nearly as Cape St. Roque. 



The cold currents are most apparent along the coasts of continents 

 and about islands, because they are here brought to the surface, the 

 submarine slopes lifting them upward, as they flow on. The limits 

 of their influence towards the equator depends often on the bend of 

 the coast; for a prominent cape or a bend in the outline will change 

 the exposure of a coast from that favourable to the polar current to 

 that favourable to the tropical, or the reverse. Thus it is at Cape 

 Hatteras, on the coast of the United States ; Cape Yerde, on Western 

 Africa ; Cape Blanco, on western South America, etc. 



370 



