Comparison of the Arctic and Antarctic Marine Floras. 91 



change from our northern Fucacece to the Sargasso, and other allied 

 genera of the tropical belt, and then to other Fucacea again in the 

 south temperate and Antarctic seas, these last resembling the northern 

 forms in general facies, but yet generically distinct in most cases. 



When we compare the littoral Algae of the tropical Atlantic, viz., 859 

 species in 162 genera, with those of the tropical Indian Ocean, viz., 514 

 species in 139 genera, we get of these 173 species and 103 genera in common 

 (see Mem. XI.) a very considerable proportion. However, while these 

 oceans, or rather the tropical portions of them, may have been in 

 communication via the Cape by a change of climate sufficient to give 

 a tropical or sub-tropical character to the Cape, in comparatively recent 

 geological times, the heat barrier between the northern and southern 

 polar seas has always been there. The cold depths of the ocean do 

 not enter into our calculation in the case of Algae owing to the illu- 

 mination difficulty. These two polar marine floras have been separated 

 as long as there has been climate of any sort on the globe, and out of 

 their poor marine floras there are 54 species that occur north and 

 south of the tropical belt, and, so far as we know, not within it. 

 Whether this needs a new cosmical theory to account for it or not we 

 do not pretend to say, but it appears to support Dr. Murray's other 

 statistics, and is more significant in some respects than the agreement 

 of the plankton Algae, which have a more stable environment than the 



littoral forms. 



GEORGE MURRAY. 



ETHEL S. BARTON. 



