26 GEOGEAPHICAL DISTRIBUTIOK. 



have as yet been discovered. Bounding this area on the north, and 

 extending from beyond the Ural Mountains (Kazan) to the northern 

 island of the Japanese group, there exists an almost continuous and 

 comparatively broad belt which is tenanted throughout its entire 

 extent, except where it overlaps the habitat of the common Euro- 

 pean G. glandarius, by a solitary species, known as Brandt's jay (G. 

 Brandti). Finally, in the southern island of Japan there are found 

 two species, G. Japonicus and G. Lidthi, the former of which, sin- 

 gularly enough, is the species which is most nearly allied to the 

 common European jay, although separated by the greatest distance 

 from it.' 



Generic Distribution. — The laws governing specific distribu- 

 tion are in considerable measure likewise applicable to the dis- 

 tribution of genera. Thus, we have genera that are restricted to 

 very limited areas, and, as a necessary consequence resulting from 

 specific distribution, those whose areas are coextensive with con- 

 tinental boundaries, or embrace portions of two or more continents ; 

 and, again, we have genera of a given family which occupy con- 

 tiguous, overlapping, or discontinuous provinces. The localisation 

 of a genus to an exceptionally narrowly circumscribed area, such 

 as we have seen in the case of the species of humming-birds of 

 the volcanic peaks of South America, can almost necessarily ob- 

 tain only there where the number of species belonging to the 

 genus is also exceptionally limited, or, more nearly, when the 

 genus is coextensive with a single species. Potamogale, which 

 comprises the single species P. velox, a singular otter-like insecti- 

 vore of the west coast of Africa, appears to be confined to the 

 region included between Angola and the Gaboon ; Choeropsis, 

 with the single species C. Liberiensis, an animal closely allied to 

 the true hippopotamus, inhabits, as far as is yet known, only the 

 wilds of Liberia; and, likewise, the singular carnivore constituting 

 the genus Ailurus (A. fulgens) has been met with only in the 

 Southeastern Himalayas. Instances of restriction are much more nu- 

 merously presented in the case of insular than of continental faunas, 

 whether the examples be taken from the class of birds or mam- 

 mals. 



Genera of very broad, or almost world-wide distribution, are of 

 frequent occurrence, both among the lower and higher animals. 

 Among the latter, in the class of birds, we have numerous examples 



