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 24 
species to the genus. The Antarctic species are in 
98 genera, very nearly an average of 2? 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, 88 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 Fucacee and Laminariacec, 
the sea-wracks and tangles, we should not find even 
asingle genus in common; the common forms are 
