2. A comparison of the specifically Neotropical with the specifically North American types shows that no two regions can be more dissimilar. It is only in the intervening borderland, and in the large West Indian Islands, that the two faunæ mix with each other. We need not enter into the details of the physical features of Central America and Mexico--the broken ground, the variety of climate (produced by different altitudes) within limited districts, the hot and moist alluvial plains surrounding the Mexican Gulf, offer a diversity of conditions most favourable to the intermixture of the types from the north and the south. But yet the exchange of peculiar forms appears to be only beginning; none have yet penetrated beyond the debatable ground, and it is evident that the land-connection between the two continents is of comparatively recent date: a view which is confirmed by the identity of the marine fishes on both sides of Central America. Cuba--and this is the only island in the West Indies which has a number of freshwater fishes sufficient for the determination of its zoo-geographical relations--is inhabited by several kinds of a perch (_Centropomus_), freshwater mullets, Cyprinodonts, one species of Chromid (an _Acara_), and _Symbranchus marmoratus_. All these fishes are found in Central America, and as they belong to forms known to enter brackish water more or less freely, it is evident that they have crossed from the mainland of South America or from Central America. But with them there came a remarkable North American type, _Lepidosteus_. _Lepidosteus viridis_, which is found in the United States, has penetrated on the mainland to the Pacific coast of Guatemala, where it is common at the mouth of the rivers and in brack-water lakes along the coast; it probably crossed into Cuba from Florida. A perfectly isolated type of fishes inhabits the subterranean waters of the caves of Cuba (two species of _Lucifuga_). The eyes are absent or quite rudimentary, as in most other cave animals. Singularly, it belongs to a family (_Ophidiidæ_), the members of which are strictly marine; and its nearest ally is a genus, _Brotula_, the species of which are distributed over the Indo-Pacific Ocean, one only occurring in the Caribbean Sea. This type must have witnessed all the geological changes which have taken place since Cuba rose above the surface of the sea. A similar mixture of forms of the Tropical and Temperate types of Freshwater fishes takes place in the south of South America; its details have not yet been so well studied as in the north; but this much is evident that, whilst in the East Tropical forms follow the Plate river far into the Temperate region, in the West the Temperate Fauna finds still a congenial climate in ranges of the Andes, situated close to, or even north of, the Tropic.