Real causes are always simple when they are once known. All anomalies in distribution cease to be such when the facts necessary to understand them are at our disposal. FOOTNOTES: [22] _Hæmulon._ [23] _Calamus._ [24] Clarion Island and Socorro Island. [25] A few Mexican shore fishes, _Chætodon humeralis_, _Galeichthys dasycephalus_, _Hypsoblennius parvipinnis_, have been wrongly accredited to Hawaii by some misplacement of labels. [26] The dragonets (_Callionymus_) are shore fishes of the shallowest waters in Europe and Asia, but inhabit considerable depths in tropical America. The sea-robins (_Prionotus_) are shore fishes in Massachusetts, semi-bathybial fishes at Panama. Often Arctic shore fishes become semi-bathybial in the Temperate Zone, living in water of a given temperature. A long period of cold weather will sometimes bring such to the surface. [27] "Introduction to the Study of Fishes." [28] Cyprinidæ. [29] "A Comparison of Antipodal Faunæ," 1887. [30] _Galaxias_, _Neochanna_, _Prototroctes_, and _Retropinna_. [31] The shovel-nosed sturgeon (_Scaphirynchus_ and _Kessleria_) and the paddle-fish (_Polyodon_ and _Psephurus_). [32] _Esox lucius._ [33] _Umrba_, the mud-minnow. [34] _Leuciscus hakuensis._ [35] _Eleotris fusca._ [36] _Awaous genivittatus._ CHAPTER XV. ISTHMUS BARRIERS SEPARATING FISH FAUNAS =The Isthmus of Suez.=--In the study of the effect of the Isthmus of Suez on the distribution of fishes we may first consider the alleged resemblance between the fauna of the Mediterranean and that of Japan. Dr. Günther claims that the actual identity of genera and species in these two regions is such as to necessitate the hypothesis that they have been in recent times joined by a continuous shore-line. This shore-line, according to Prof. A. Ortmann and others, was not across the Isthmus of Suez, but farther to the northward, probably across Siberia. =The Fish Fauna of Japan.=--For a better understanding of the problem we may give a brief analysis of the fish fauna of Japan. The group of islands which constitute the empire of Japan is remarkable for the richness of its animal life. Its variety in climatic and other conditions, its nearness to the great continent of Asia and to the chief center of marine life, the East Indian Islands, its relation to the warm Black Current or Kuro Shiwo from the south and to the cold currents from the north, all tend to give variety and richness to the fauna of its seas. Especially is this true in the group of fishes. In spite of the political isolation of the Japanese Empire, this fact has been long recognized and the characteristic types of Japanese fishes have been well known to naturalists.