F. Borgesen: General remarks. 495 



Dictyosphæria, Valonia and Codiiim, that have most species in 

 common in both oceans, while the Rhodophyceæ being surely of 

 more recent origin, show less correspondance, seems to strengthen 

 the supposition that the great number of species common to both 

 oceans is due to this earlier communication between the two 

 oceans. 



In this connection it is also of interest to mention Osten- 

 feld's^) conclusion regarding the marine phanerogamic plants. Of 

 these, 6 species which are surely the representatives of very old types, 

 occur in the West Indies and 4 of these are closely related to four 

 corresponding forms occurring in the Indo-Pacific ocean, and 

 this great resemblance is, according to Ostenfeld, only to be 

 understood by the species having migrated through the Tertiary Cen- 

 tral American strait into the Caribean Sea and, after having been 

 shut in here, they have developed into the species which we 

 now find in the West Indies. 



In just the same way, in fact, resemblances between zoolo- 

 gical groups have been accounted for. 



The result of the above-mentioned comparison can therefore 

 briefly be summed up thus: The algal flora of the West 

 Indian islands in question shows a strikingly great 

 resemblance to that of the Indo-Pacific ocean. This 

 applies especially to certain, undoubtedly very old, 

 groups of Chlorophyceæ. The Rhodophyceæ, on the other- 

 hand, show less resemblance to those from the Indo- 

 Pacific Ocean, being more closely related to the algal 

 flora occurring in the Mediterranean-Atlantic terri- 



torv. 



The great similaritybetweenthose two algal floras: 



the West Indian and the Indo-Pacific, which in our 

 days are so distinctly separated, has its natural ex- 

 planation in a prehistoric old connection between 

 the two oceans. 



1) Ostenfeld, C. H., On the geographical distribution of the Seagrasses 

 (Proc. Roy. Soc. Victoria, 27, (X. S.), Part 11, 1914, p. 179). 

 Ostenfeld, C. H., Havgræssernes Udbredelse i Verdenshavene, „Na- 

 turen", 1917. 



