CURRENTS; TOPOGRAPHY 105 1 



rents. Thus even Middle and South Atlantic fish are carried by 

 the Gulf Stream to the Norwegian coast, though these do not gen- 

 erally propagate themselves in these northern waters. 



One mode of dispersal that must not be overlooked is that ef- 

 fected by the epiplankton. The transportation of the Crustacea, 

 Hydrozoa and other organisms, with the Sargassum, from the 

 Bahamas to the middle North Atlantic, is an example. Trunks of 

 trees from the forests of the Mississippi and the Orinoko are car- 

 ried northward by the Gulf Stream, past the coast of Norway, as 

 far as Spitzbergen, whence they are again carried southward and 

 cast ashore on the northeast coast of Iceland. Sea-weeds, molluscs 

 and other organisms arc frccjuently found attached to these trees, 

 having- made the long journey on their floating substratum. Seeds 

 of land plants commonly accompany these woods. 



The warm ocean currents also have an important influence on 

 the relative abundance of the benthonic life in the regions traversed 

 by them. Being rich in plankton, and thus supplying an abundance 

 of food to the animal life inhabiting the sea-l)ottom below it, it is 

 not surprising to find that here the bottom life is developed in the 

 greatest luxuriance, and that vast accumulations of organic lime- 

 stones occur in the littoral regions thus afifected. This is true, how- 

 ever, only of those portions of the littoral district which lie at a 

 sufficient depth below the surface to escape the motion of the 

 streaming water, which might otherwise prevent the attachment 

 of the benthos as it settles down. Thus, ofif Charleston, South 

 Carolina, in depths from 100 to 350 fathoms, the sea-floor is 

 but sparsely settled beneath the Gulf Stream. 



Topography. Next to the climate of the sea the topography 

 of the sea-bottom, and that'of the adjacent land, are the most pow- 

 erful factors in determining the distribution of marine organisms. 

 The facies of the ocean floor, or the material of which it is com- 

 posed, is perhaps the most significant part of sea-bottom topog- 

 raphy, though submarine ridges and barriers are of great im- 

 portance, especially when such barriers cut ofif marginal bodies of 

 water, the inhabitants of which may be prevented from interming- 

 ling. The separation thus produced may lead to the development 

 of local faunas and floras. The importance of the greater inequali- 

 ties of the sea-bottom and the submarine continental shelves and 

 deep oceanic basins that result from them, as well as the conforma- 

 tion of the coast-line, with its varying facies, has already been con- 

 sidered. (See Chapter IIP) 



Of all topographical features whicii influence the distribution of 

 marine organisms, northward and southward stretching bodies of 



