This sub-region, which may perhaps be termed the "European," is zoologically and botanically the best known on the globe. It can be pretty accurately defined, as bounded on the south by the Pyrenees, the Alps, the Balkans, the Black Sea, and the Caucasus range; and by the Ural Mountains, or perhaps more correctly the valley of the Irtish and Caspian Sea, on the east; while Ireland and Iceland are its furthest outliers in the west. To the north, it merges so gradually into the Arctic zone that no demarcation is possible. The great extent to which this sub-region is interpenetrated by the sea, and the prevalence of westerly winds bringing warmth and moisture from an ocean influenced by the gulf-stream, give it a climate for the most part genial, and free from extremes of heat and cold. It is thus broadly distinguished from Siberia and Northern Asia generally, where a more extreme and rigorous climate prevails. The whole of this sub-region is well watered, being penetrated by rivers in every direction; and it consists mainly of plains and undulating country of moderate elevation, the chief mountain ranges being those of Scandinavia in the north-west, and the extensive alpine system of Central Europe. But these are both of moderate height, and a very small portion of their surface is occupied either by permanent snow-fields, or by barren uplands inimical to vegetable and animal life. It is, in {192}fact, to these, and the numerous lesser mountains and hills which everywhere diversify the surface of Europe, that the variety and abundance of its animal life is greatly due. They afford the perennial supplies to rivers, and furnish in their valleys and ever varying slopes, stations suited to every form of existence. A considerable area of Central Europe is occupied by uplands of moderate elevation, a comparatively small portion being flat and marshy plains.