The line which separates the Septentrional from the Mediterranean Sub-region must of necessity be very roughly drawn, and stragglers from the south will be found to make their way northward, and _vice versâ_, under favouring circumstances of temperature and geological formation. Jordan has noticed[364] that species which in southern countries are not confined to any particular quality of soil are in more northern latitudes found only on limestone, which absorbs more heat than other formations. Conversely, the higher elevations of the Alps, Pyrenees, and even Carpathians are like islands in a sea, and support a thoroughly northern fauna, quite strange to that of the plains below. Thus _Helix harpa_ Say, a completely boreal shell, which is at home in Canada, Sweden, Lapland, and the Amoor district, is found on the Riffel Alp, at a height of 6000 feet.[365] _Vertigo arctica_ Wall., a species abundant in Lapland, North Siberia, Iceland, and Greenland, occurs on the high Alps of the Tyrol. _Circumpolar Species._--A certain number of species are common to the extreme north both of the Palaearctic and Nearctic regions, and are, in fact, circumpolar. The number of these species, however, is so small, not exceeding about 40 species (= 16 genera), that it seems hardly worth while creating a special sub-region for their reception, particularly as no genus is peculiar. At the same time the fact is instructive as illustrating the close connexion of the northern districts of the two regions, a connexion which was no doubt more intimate in recent geological times than it is now. The circumpolar genera are as follows. The list decisively sets forth the superior hardiness of the fresh-water as compared with the land genera:-- Valvata 1 sp. Bithynia 1 „ Vitrina 1 „ Hyalinia 4 „ Helix 2 „ Patula 2 „ Pupa 3 „ Cionella 1 „ Succinea 1 „ Limnaea 7 „ Planorbis 5 „ Aplecta 1 „ Physa 1 „ Anodonta 1 „ Unio 1 „ Pisidium 1 „