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 Stphonew:, 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 barricrs 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 of cold currents. 
