286 SUBDIVISIONS OF THE PALAEARCTIC REGION chap. 



trict north of the line formed by the Pyrenees,^ Alps, Carpathians, 

 and which, passing to the northward of the Aralo-Caspian district, 

 follows the great central mountain range of Asia until it reaches 

 the Sea of Japan, perhaps somewhere in the neighbourhood of 

 Vladivostok. 



(2) The Mediterranean Sub-region, i.e. the countries border- 

 ing on the Mediterranean, the Black and Caspian Seas, with the 

 Atlantic Islands. 



(3) The Central Asiatic Sub-region, i.e. Turkestan, Afghan- 

 istan, Thibet, and probably the districts of Mongolia and 

 Manchuria.2 



(1) The Septentrional Sub-region has been divided by some 

 writers into two provinces, the European and the Siberian. 

 There seems, on the whole, but little occasion to separate off 

 northern Asia, the characteristic of which is,- as will be seen 

 below, rather the gradual disappearance, as we proceed eastward, 

 of European species and genera, than the development of any 

 new and peculiar groups. The remarkable fauna of Lake Baikal 

 stands apart, not only from European, but also from the Siberian 

 types occurring in its immediate neighbourhood. 



On the whole, the Septentrional Sub-region is poor in species 

 except those which inhabit fresh water. This fact is probably 

 due to the extreme vicissitudes of temperature which prevail, and 

 it is interesting to notice that the number of land Mollusca 

 appears to touch its lowest point in districts where the annual 

 range of temperature is greatest. On the other hand, in the 

 western portions of the region, where the climate is moist and 

 temperature more equable, the Mollusca are considerably more 

 abundant and varied. 



The line which separates the Septentrional from the Mediter- 

 ranean Sub-region must of necessity be very roughly drawn, and 

 stragglers from the south will be found to make their way north- 

 ward, and vice versd, under favouring circumstances of tempera- 

 ture and geological formation. Jordan has noticed ^ that species 



1 South and south-western France, however, belong to the Mediterranean 

 Sub-region. 



2 The coast-line of north-east China, including Corea and Japan to north 

 Niphon, is much more definitely tropical than the adjacent inland districts. The 

 coast-line, therefore, must be placed in the Oriental Region, while the inland 

 districts belong to the Palaearctic Region. 



8 Biol. Centralbl. ii. p. 208. 



