The geographical distribution of animals, with a study of the relations ...

;chap, x.] THE PAL^IARCTIC REGION. 191No doubt all of them could be advantageously again subdivided,in a detailed study of the geographical distribution ofspecies.But in a general work, which aims at treating all partsof the world with equal fulness, and which therefore is confinedalmost wholly to the distribution of families and genera, suchfurther subdivision would be out of place. It is even difficult,in some of the classes of animals, to find peculiar or evencharacteristic genera for the present sub-regions ; but they allhave well marked climatic and physical differences, and thissuffi-leads to an assemblage of species and of groups which areciently distinctive./. Central and Northern Europe.This sub-region, which may perhaps be termed the " European,"is zoologically and botanically the best known on the globe. Itcan be pretty accurately defined, as bounded on the south bythe Pyrenees, the Alps, the Balkans, the Black Sea, and theCaucasus range ; and by the Ural Mountains, or perhaps morecorrectly the valley of the Irtish and Caspian Sea, on the eastwhile Ireland and Iceland are its furthest outliers in the west.To the north, it merges so gradually into the Arctic zone thatno demarcation is possible. The great extent to which thissub-region is interpenetrated by the sea, and the prevalenceof westerly winds bringing warmth and moisture from an oceaninfluenced by the gulf-stream, give it a climate for the mostpart genial, and free from extremes of heat and cold. Itis thus broadly distinguished from Siberia and NorthernAsia generally, where a more extreme and rigorous climateprevails.The whole of this sub-region is well watered, being penetratedby rivers in every direction ; and it consists mainly ofplains and undulating country of moderate elevation, the chiefmountain ranges being those of Scandinavia in the north-west,and the extensive alpine system of Central Europe.But theseare both of moderate height, and a very small portion of thensurfaceis occupied either by permanent snow-fields, or bybarren uplands inimical to vegetable and animal life. It is, in