12 SEAWEEDS 



water from large rivers also mark dividing lines of 

 coast areas of distribution, and profoundly modify the 

 character of the flora in their vicinity. The ocean 

 forms a far less effectual barrier, however, to the 

 dispersal of land plants than continental areas do to 

 seaweeds, which have no means of bridging them, 

 though many species are of sufficient range to enable 

 them to double capes from one region to another 

 geographically remote. The continental barrier of 

 Africa interposed between the tropical Atlantic and 

 the Indian Ocean offers a passage round the Cape 

 of Good Hope, warm enough to sustain a marine flora 

 with many subtropical types, but subject to the dis- 

 tracting influence of opposite hot and cold currents. 

 The result of a comparison of the floras of these two 

 tropical regions discloses the fact, that while the 

 genera are largely in common, the species are in a 

 high proportion different, and this is naturally most 

 strikingly true of orders like the Sifihonece, which 

 are only sparingly represented outside the tropics. 

 Areas of different temperature in the ocean have 

 thus to be added to continental areas as natural 

 barriers, since the ocean is never wholly fenced about 

 with land. Such areas of different temperature, 

 however, are effective barriers of themselves, as a 

 comparison of the north temperate marine flora with 

 the south temperate one shows. The heat barrier of 

 the tropical seas would be less effective if the cold 

 depths of the ocean were available for passage, but 

 such depths are dark, and moreover the colder waters 

 rise to the surface of warm seas, and thus disable 

 the transporting action ot cold currents. 



