es, in that case
difficulties connected with the geographical distribution of animals are
not without significance, and are worthy of mention even though, by
themselves, they constitute but feeble and more or less easily explicable
puzzles which could not alone suffice either to sustain or to defeat any
theory of specific origination.

Many facts as to the present distribution of animal life over the world are
very readily explicable by the hypothesis of slight elevations and
depressions of larger and smaller parts of its surface, but there are
others the existence of which it is much more difficult so to explain.

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 t