a. _Central American district_, in which we include, for the present, Lower California, shows so near an affinity to the tropical Atlantic that, if it were not separated from it by the neck of land uniting the two American Continents, it would most assuredly be regarded as a portion of the Fauna of the tropical Atlantic. With scarcely any exceptions the genera are identical, and of the species found on the Pacific side nearly one-half have proved to be the same as those of the Atlantic. The explanation of this fact has been found in the existence of communications between the two oceans by channels and straits which must have been open till within a recent period. The isthmus of Central America was then partially submerged, and appeared as a chain of islands similar to that of the Antilles; but as the reef-building corals flourished chiefly north and east of those islands, and were absent south and west of them, reef-fishes were excluded from the Pacific shores when the communications were destroyed by the upheaval of the land. _b_. The _Galapagoes district_ received its coast fauna principally from the Central American district, a part of the species being absolutely the same as on the coast of the Isthmus of Panama, or as in the West Indies. Yet the isolation of this group has continued a sufficiently long period to allow of the development of a number of distinct species of either peculiarly Atlantic genera (such as _Centropristis_, _Rhypticus_, _Gobiesox_, _Prionotus_), or at least tropical genera (such as _Chrysophrys_, _Pristipoma_, _Holacanthus_, _Caranx_, _Balistes_). A few other types from the Peruvian coast (_Doydixodon_), or even from Japan (_Prionurus_), have established themselves in this group of islands. A species of _Cestracion_ has also reached the Galapagoes, but whether from the south, north, or west, cannot be determined. The presence of the Atlantic fauna on the Pacific side is felt still farther west than the Galapagoes, some Atlantic species having reached the Sandwich Islands, as _Chætodon humeralis_ and _Blennius brevipinnis_. c. The _Peruvian district_ possesses a very limited variety of shore fishes, which belong, with few exceptions, like _Discopyge_, _Hoplognathus_, _Doydixodon_, to genera distributed throughout the tropical zone, or even beyond it. But the species, so far as they are known at present, are distinct from those of the Indo-Pacific, as well as of the tropical Atlantic; and therefore this district cannot be joined either to the Central American or the Galapagoes. IV.--THE SOUTHERN TEMPERATE ZONE. This zone includes the coasts of the southern extremity of Africa, from about 30° lat. S., of the south of Australia with Tasmania, of New Zealand, and the Pacific and Atlantic coasts of South America between 30° and 50° lat. S.