560 



SCIENCE. 



[N. S. Vol. XIV. No. 354. 



some sort of barrier, past or present. The 

 chief barriers which limit marine fishes are 

 the presence of land, the presence of great 

 oceans, the differences of temperature aris- 

 ing from differences in latitude, the nature 

 of the sea bottom and the direction of 

 oceanic currents. That which is a barrier 

 to one species may be an agent in distribu- 

 tion to another. The common shore fishes 

 would perish in deep waters almost as 

 surely as on land, while the open Pacific is 

 a broad highway to the albacore or the 

 sword-fish. 



Again, that which is a barrier to rapid 

 distribution may become an agent in the 

 slow extension of the range of a species. 

 The great continent of Asia is undoubtedly 

 one of the greatest of barriers to the wide 

 movement of species of fish, yet its long 

 shore-line enables species to creep, as it 

 were, from bay to bay, or from rock to rock ; 

 till, in many cases, the same species is 

 found in the Eed Sea and in the tide-pools 

 or sand-reaches of Japan. In the North 

 Pacific, the presence of a range of half- 

 submerged volcanoes, known as the Aleu- 

 tian and the Kurile Islands, has greatly 

 aided the slow movement of the fishes of 

 the tide-pools and the kelp. To a school 

 of mackerel or of flying fishes these rough 

 islands would form an insuperable barrier. 



TEMPERATURE THE CENTRAL FACT IN DISTRI- 

 BUTION. 



It has long been recognized that the mat- 

 ter of temperature is the central fact in all 

 problems of geographical distribution. 

 Few species in any group freely cross the 

 frost-line, and except as borne by oceanic 

 currents, few species extend their range far 

 into waters colder than those in which the 

 species is distinctively at home. Knowing 

 the average temperature of the water in a 

 given region, we know in general the types 

 of fishes which must inhabit it. It is the 

 similarity in temperature and physical 



conditions, not the former absence of bar- 

 riers, which chiefly explains the resem- 

 blance of the Japanese fauna to that of the 

 Mediterranean or the Antilles. This fact 

 alone must explain the resemblance of the 

 Arctic and Antarctic faunae. 



AGENCY OP OCEAN CURRENTS. 



We may consider again for a moment the 

 movements of the great currents in the 

 Pacific as agencies in the distribution of 

 species. 



A great current sets to the eastward, 

 crossing the ocean just south of the Tropic 

 of Cancer. It extends between the Gilbert 

 and the Marshall Islands and passes on 

 nearly to the coast of Mexico, touching the 

 Galapagos Islands, Clipperton Island and 

 especially the Eevillagigedos. This at once 

 accounts for the number of Polynesian 

 species found on these Islands, about which 

 they are freely mixed with immigrants from 

 the mainland of Mexico. 



From the Eevillagigedos* the current 

 moves northward, passing the Hawaiian 

 Islands and thence onward to the La- 

 drones. The absence in Hawaii of many 

 of the characteristic fishes of the Society 

 Islands and the Gilbert Islands is doubt- 

 less due to the long detour made by these 

 currents, as the conditions of life in these 

 groups of islands are not very different. 

 Between the Gilbert Islands and Samoa 

 there is also a return current to the west, 

 and northeast of Hawaii is a great spiral 

 current, moving with the hands of the 

 watch, forming what is called Fleurieu's 

 Whirlpool. This does not reach the coast 

 of California. This fact may account for 

 the almost complete distinction in the shore 

 fishes of Hawaii and California.f 



* Clarion Island and Socorro Island. 



t A few Mexican shore fishes, Chfetodon humeralis, 

 Galeichthys dasycephalus, Hypsoblennius parvipinnis, 

 have been wrongly accredited to Hawaii by some 

 misplacement of labels. 



