Shiwo district itself, which is distinctly tropical and contrasts strongly with that of the inshore bays behind it. This warm "Black Current," analogous to our Gulf Stream, has its origin in part from a return current from the east which passes westward through Hawaii, in part from a current which passes between Celebes and New Guinea. It moves northward by way of Luzon and Formosa, touching the east shores of the Japanese islands Kiusiu and Shikoku, to the main island of Hondo, flooding the bays of Kagoshima and Kochi, of Waka, Suruga, and Sagami. The projecting headlands reach out into it and the fauna of their rock-pools is distinctly tropical as far to the northward as Tokio. [Illustration: FIG. 178.--Sacramento Perch, _Archoplites interruptus_ Girard. Family _Centrarchidæ_. Sacramento River.] These promontories of Hondo, Waka, Ise, Izu, Misaki, and Awa have essentially the same types of fishes as are found on the reefs of tropical Polynesia. The warmth of the off-shore currents gives the fauna of Misaki its astonishing richness, and the wealth of life is by no means confined to the fishes. Corals, crustaceans, worms, and mollusks show the same generous profusion of species. A fifth faunal area, closely related to that of the Black Current, is formed by the volcanic and coral reefs of the Riu Kiu Archipelago. This fauna, so far as known, is essentially East Indian, the genera and most of the species being entirely identical with those of the islands about Java and Celebes. =Resemblance of the Japanese and Mediterranean Fish Faunas.=--It has been noted by Dr. Günther that the fish fauna of Japan bears a marked resemblance to that of the Mediterranean. This likeness is shown in the actual identity of genera and species, and in their relation to each other. This resemblance he proposes to explain by the hypothesis that at some recent period the two regions, Japan and the Mediterranean, have been united by a continuous shore-line. The far-reaching character of this hypothesis demands a careful examination of the data on which it rests. The resemblance of the two faunal areas, so far as fishes are concerned, may be stated as follows: There are certain genera[45] of shore fishes, tropical or semi-tropical, common to the Mediterranean and Japan, and wanting to California, Panama, and the West Indies, and in most cases to Polynesia also. Besides these, certain others found in deeper water (100 to 200 fathoms) are common to the two areas,[46] and have been rarely taken elsewhere. =Significance of Resemblance.=--The significance of these facts can be shown only by a fuller analysis of the fauna in question, and those of other tropical and semi-tropical waters. If the resemblances are merely casual, or if the resemblances are shown by other regions, the hypothesis of shore continuity would be unnecessary or untenable. It is tenable if the resemblances are so great as to be accounted for in no other way.