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cfiAP. XVII.] MAMMALIA. 205"
American, have but two genera and six species in that vast area.
We might therefore, from these considerations alone, conclude
that Carnivora are a development of the northern hemisphere,
and have been introdiTced into the Neotropical region at a com-
paratively recent epoch. The claim of the Nearctic region to
be kept distinct from the Palsearctic (with which some writers
have wished to unite it) is well maintained by its possession of
at least six species of Mephitis, or skunk, a group having no
close allies in any other region,—and the genera Procyon and
Bassaris,—for the latter, ranging from the high lands of Guate-
mala and Mexico to Texas and California, may be considered
a Nearctic rather than a Neotropical form. In the other
families, the most marked feature is the total absence of UrsidaB
from the Ethiopian region. The great mass of the generic
forms of Carnivora, however, are found in the Oriental and
Ethiopian regions, which possess all the extensive group of
Viverridse (except a few species in the fourth Palsearctic sub-
region) and a large number of Eelidae and Mustelidae.
Aquatic Carnivora.—The aquatic Carnivora present no very
marked features of distribution, except their preference for cold
and temperate rather than tropical seas. Their nearest approxi-
mation to the terrestrial group, is supposed to be that of the
Otariidse to the Ursidse ; but this must be very remote, and the
occurrence of both seals and bears in the Miocene period, shows,
that until we find some late Secondary or early Tertiary formation
rich in Mammalian remains, we are not likely to get at the tran-
sition forms indicating the steps by which the aquatic Carnivora
were developed. The most interesting special fact of distribu-
tion to be noticed, is the occurrence of seals, closely allied to
those inhabiting the northern seas, in the Caspian, Lake Aral,
and Lake Baikal. In the case of the two first-named localities
there is little difficulty, as they are connected with the North Sea
by extensive plains of low elevation, so that a depression of less
than 500 feet would open a free communication with the ocean.
At a comparatively recent epoch, a great gulf of the Arctic ocean
must have occupied the valley of the Irtish, and extended to the
Caspian Sea ; till the elevation of the Kirghiz Steppes cut off the