CHAP. 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 introduced 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 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 

 Viverridffi (except a few species in the fourth. Palgearctic sub- 

 region) and a large number of Felidse and Mustelidee. 



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 Secondaiy 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 



