THE WEST INDIAN FAUNA. 17 



secondary, or tertiary times. This extension of geographical 

 range in the case of so many of the species of the Caribbean 

 fauna is most instructive. As has been observed in several 

 groups of invertebrates, and in fishes, the presence of identi- 

 cal species on the two sides of the Isthmus of Panama points 

 to a comparatively recent communication between the Atlantic 

 and Pacific, while the presence of cosmopolitan species at such 

 distant points as the Caribbean, Australia, and the Red Sea in- 

 dicates a connection which could have been effected only by 

 migration on the floor of the ocean or in the track of currents. 



The sponges apparently have a wide geographical distribu- 

 tion, many of them being cosmopolitan. A number of mol- 

 lusks also have an extraordinary geographical range, from 

 Northern Europe to the Cape of Good Hope or to Patagonia. 

 Others are found in the seas of Great Britain, at the Cape of 

 Good Hope, and in the Southern Ocean. Others again are 

 denizens of the arctic and antarctic seas, or extend from the 

 northern parts of the Pacific to the Kerguelen Islands. 



A number of species of deep-sea corals and gorgonians ex- 

 tend northward in deep water from the Caribbean district along 

 the east coast of the United States. A few species of simple 

 corals like Flabellum and Fungia have a great geographical and 

 bathymetrical range. Half a dozen species of corals are com- 

 mon to the northern seas of Europe and the Straits of Florida. 

 From the geographical distribution of the corals, and their affin- 

 ity with the tertiary fossils of Italy, Pourtales came to the con- 

 clusion that the tertiary deep-sea fauna of Europe has as it were 

 migrated westward and maintained itself, while the greater part 

 of the contemporaneous forms of the West Indian deep sea 

 have become extinct. 



The collections obtained by the " Blake " in the Caribbean 

 district are superior, as regards the number of duplicates, to 

 those made by the " Challenger." Many species occur, not only 

 in large numbers, but also at several localities ; so that it has 

 been possible to study their range of variation in a more satis- 

 factory manner than hitherto. This opportunity has proved of 

 immense value in revealing the existence of many intermediate 

 forms between types which were considered quite distinct. 



