Ocean We may consider again for a moment the movements of the great cur rents in the Pacific as agencies in the distribu tion of species.
A great current sets to the eastward, cross ing 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, Clip perton Island, and especially the Revillagigedos. This may account for the number of Polyne sian species found on these islands, about which they are freely mixed with immigrants from the mainland of Mexico.
From the Revillagigedos the current moves northward, passing the Hawaiian Islands and thence onward to the Ladrones. The absence in Hawaii of many of the characteristic fishes of the Samoan Islands and the Gilbert Islands is perhaps due to the long detour made by these currents, as the conditions of life in these groups of islands are not very different. North east 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.
It is, of course, not necessary that the move ments of a species in an oceanic current should coincide with the direction of the current. Young fishes, or fresh-water fishes, would be borne along with the water. Those that dwell within floating bodies of seaweed would go whither the waters carry the drifting mass. But free-swimming fishes, as the mackerel or fly ing-fishes, might as readily choose the reverse direction. To a free-swimming fish, the tem perature of the water would be the only con sideration. It is thus evident that a current which to certain forms would prove a harrier to distribution, to others would be a mere con venience in movement.
Centres of We may assume in regard to any species, that it has had its origin in or near that region in which it is most abun dant and characteristic. Such an assumption must involve a certain percentage of error or of doubt, but in considering the mass of species, it would represent essential truth. In the same
fashion, we may regard a genus as being autoch thonous or first developed in the region where it shows the greatest range or variety of species. Those regions where the greatest number of genera are thus autochthonous may be regarded . as centres of distribution. So far as the marine fishes are concerned, the most important of these centres are found in the Pacific Ocean. First of these in importance is the East Indian Archipelago, with the neighboring shores of India. Next would come the Arctic Pacific and its bounding islands, from Japan to British i Columbia. Third in importance in this regard is Australia. Important centres are also found in temperate Japan, in California, the Panama region and in New Zealand, Chile and Pata gonia. The fauna of Polynesia is almost en tirely derived from the East Indies; and the shore fauna of the Red Sea, the Bay of Bengal and Madagascar, so far as genera are concerned, seems to be not really separable from the Indian fauna generally.
In the Atlantic the chief centre of distribu tion is the West Indies; the second is the Mediterranean. On the shores to the north ward or southward of these regions occasional genera have found their origin. This is true especially of the New England region, the North Sea, the Gulf of Guinea and the coast of Argen tina. The fish-fauna of the North Atlantic is derived mainly from the North Pacific, the differences lying mainly in the relative poverty of the North Atlantic. But, in certain groups common to the two regions, the migration must have been in the opposite direction; exceptions that prove the rule.
Realms of If we consider the fresh-water fishes alone we may divide the land areas of the earth into districts and zones, fairly agreeing with those marked out for mammals and birds. The river-basin, bounded by its water-sheds and the sea at its mouth, shows many resemblances, from the point of view of a fish, to an island considered as the home of a mammal. It is evident that with fishes the differences in latitude outweigh those of continental areas, and a primary divi sion into Old World and New World would not be tenable.