GEOGRAPHIC ISOLATION 81 



not, however, a thoroughfare for marine organisms. The factors which 

 tend to prevent ready passage of the Suez Canal include: (a) 

 Plankton-bearing currents are deflected from the Mediterranean 

 entrance at Port Said, and for ten months of the year differences in 

 water level and the prevailing winds cause currents in the canal to 

 flow from the Bitter Lakes toward Suez. (6) There is oil contamina- 

 tion at the Red Sea Entrance, (c) The canal passes through the 

 Bitter Lakes with heightened salt content. In Great Bitter Lake, the 

 bottom mud is devoid of life; this lies directly over an ancient salt 

 bed which is gradually being redissolved and presumably will vanish 

 by the end of the present century. The high salinity of these lakes 

 probably acts as a barrier even though bryozoans and isopods are 

 carried through its waters on the bottoms of canal barges, (d) A 

 further obstacle to free passage is presented by Timsah Lake, the 

 salinity of whose waters varies enormously from hour to hour. The 

 extent to which animals pass through the Suez Canal, despite these 

 obstacles, has not as yet been carefully determined. 23, 24 



Restriction to coastal waters leads to specific transformation in 

 the fishes also. Littoral sharks and rays are scarce about New Zealand, 

 but form a number of endemic species. 25 Out of 65 of the marine fishes 

 of St. Helena, 17 are confined to the island littoral. 26 The coastal fishes 

 of Micronesia and Polynesia are closely allied, while those of the 

 Hawaiian islands are mostly specifically, though not generically, dif- 

 ferent, evidently on account of the isolation of the latter group. 27 The 

 littoral fishes of the Antarctic Ocean form groups of species in definite 

 areas, like Tierra del Fuego, the Falkland Islands, South Georgia, 

 South Shetland, and Grahamsland, in which the same types appear as 

 vicarious species, subspecies, or races. 28 



Even mere lack of vagility, for sessile or slightly movable animals, 

 without any special barriers, may afford sufficient isolation for the 

 divergence of species. In the flounder, Pleuronectes jiesus, which still 

 possesses a considerable degree of freedom, the specimens from the 

 English coast (at Plymouth) run 30% "left-sided," whereas on the 

 German coast opposite, the "left-sided" forms are only 5.36%. 29 In 

 the starfish, Solaster papposus, of the northern seas the number of 

 arms varies from 10 to 14; but specimens from a given locality have 

 an almost uniform number of arms. In some places the 10-armed 

 form predominates, at others the 11-armed, the 12-armed at Tenby on 

 the English coast, the 13-armed on a stretch of the Greenland coast, 

 while those of the Kattegat are 14-armed; the specimens from given 

 areas agree also in other characters, such as the development of the 

 dorsal skeleton and number of paxillae. 30 



