CHAP. X.] THE PALiEARCTIC REGION. 191 



No doubt all of them could be advantageously again sub- 

 divided, in a detailed study of the geographical distribution of 

 species. But in a general work, which aims at treating all parts 

 of the world with equal fulness, and which therefore is confined 

 almost wholly to the distribution of families and genera, such 

 further subdivision would be out of place. It is even difficult, 

 in some of the classes of animals, to find peculiar or even 

 characteristic genera for the present sub-regions ; but they all 

 have well marked climatic and physical differences, and this 

 leads to an assemblage of species and of groups which are suffi- 

 ciently 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. 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 pene- 

 trated 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 



