CHAP. XVII.] 
MAMMALIA. 
205 
American, have hut two genera and six species in that vast area. 
We might therefore, from these considerations alone, conclude 
that parnivora are a development of the northern hemisphere, 
and have been introduced into the Neotropical region at a com- 
paratively recent epoch. The claim of the Nearctic region to 
he kept distinct from the Palsearctic (with which some writers 
have wished to unite it) is well maintained hy 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 Ursidse 
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 
Yiverridae (except a few species in the fourth Palsearctic sub- 
region) and a large number of Felidse and Mustelidse. 
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 
Otariidee to the Ursidse ; but this must be very Temote, 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 
