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.