14 SEAWEEDS 



marine floras brings out the interesting result that 

 there is a much higher proportion of forms in com- 

 mon between the two areas than might have been 

 expected. This is especially the case with the pelagic 

 or free-floating plant organisms of the open sea, and 

 without citing figures, since exact data are not avail- 

 able, this may be taken to be generally true. Such 

 forms are much less variable than littoral seaweeds. 

 If we compare the two littoral floras they will be 

 found to be of similar extent, viz. 259 species in the 

 Arctic and 269 in the Antarctic. The Arctic species 

 are, as has been said, in 111 genera, an average of 2 J 

 species to the genus. The Antarctic species are in 

 98 genera, very nearly an average of 2f species to the 

 genus. The genera common to both are 56, and the 

 species 41. However, pushing the inquiry a little 

 farther, it will be found that while some Arctic forms 

 occur in the South Temperate zone and not in the 

 Antarctic, similarly some Antarctic forms are found 

 in the North Temperate zone and not in the Arctic. 

 Adding these, we get 92 species in common to Arctic 

 and Antarctic, including adjoining regions, and it 

 would be much greater if we included the two Tem- 

 perate zones fully; but the object is to compare the 

 two cold-water floras as strictly as possible. Of these 

 92 species, 38 occur in the intervening tropical belt, 

 and if they are subtracted, we get 54 species in com- 

 mon to the two polar and adjoining waters which 

 have not/- been found within the tropics. If we were 

 to take the great seaweeds Fucacece and Laminariacece, 

 the sea-wracks and tangles, we should not find even 

 a single genus in common ; the common forms are 



