The distribution either of animals possessing the power of flight, or of inhabitants of the ocean, is, of course, easily to be accounted for; the difficulty, if there is really any, must mainly be with strictly terrestrial animals of moderate or small powers of locomotion and with inhabitants of fresh water. Mr. Darwin himself observes,[142] "In regard to fish, I believe that the same species never occur in the fresh waters of distant continents." Now, the Author is enabled, by the labours and through the kindness of Dr. Guenther, to show that this belief cannot be maintained; he having been so obliging as to call attention to the following facts with regard to fish-distribution. These facts show that though only one species which is absolutely and exclusively an inhabitant of fresh water is as yet known to be found in distant continents, yet that in several other instances the same species _is_ found in the fresh water of distant continents, and that very often the same _genus_ is so distributed. The genus _Mastacembelus_ belongs to a family of fresh-water Indian {146} fishes. Eight species of this genus are described by Dr. Guenther in his catalogue.[143] These forms extend from Java and Borneo on the one hand, to Aleppo on the other. Nevertheless, a new species (_M. cryptacanthus_) has been described by the same author,[144] which is an inhabitant of the Camaroon country of _Western_ Africa. He observes, "The occurrence of Indian forms on the West Coast of Africa, such as _Periophthalmus_, _Psettus_, _Mastacembelus_, is of the highest interest, and an almost new fact in our knowledge of the geographical distribution of fishes." _Ophiocephalus_, again, is a truly Indian genus, there being no less than twenty-five species,[145] all from the fresh waters of the East Indies. Yet Dr. Guenther informs me that there is a species in the Upper Nile and in West Africa. The acanthopterygian family (_Labyrinthici_) contains nine freshwater genera, and these are distributed between the East Indies and South and Central Africa. The Carp fishes (Cyprinoids) are found in India, Africa, and Madagascar, but there are none in South America. Thus existing fresh-water fishes point to an immediate connexion between Africa and India, harmonizing with what we learn from Miocene mammalian remains. On the other hand, the Characinidae (a family of the physostomous fishes) are found in Africa and South America, and not in India, and even its component groups are so distributed,--namely, the _Tetragonopterina_[146] and the _Hydrocyonina_.[147] Again, we have similar phenomena in that almost exclusively fresh-water group the Siluroids.