The relationship between the palaearctic and nearctic faunas 



289 



(From Rosenberg, Faglar i Sverige.) 



FIG. 49. Breeding area of the Snowy Owl, Nyctea scandiaca L., an arctic, circumpolar 

 bird. The southern limit varies considerably according to supply of rodents. 



(Compiled from Ekman, 1922, Pleske, 1928, "Check List", 193 1, Stegmann, 1938, TiM- 



MERMANN, 1 949, SaLOMONSEN & GiTZ, 195O.) 



Thus, the circumpolar type dominates under high-arctic conditions and de- 

 creases relatively with decreasing latitude. 



Examples of circumpolar distribution are here shown by maps of one arctic 

 species, the Snowy Owl {Nyctea scandiaca L., fig. 49), one subarctic-highboreal 

 species, the Three-toed Woodpecker [Picoides tridactylus L., fig. 50), and one 

 generally boreal species, the Dragonfly Libellula quadrimaculata L. (fig. 51). This 

 latter is an excellent flyer with strong migratory tendencies; it is able to cross even 

 broad stretches of sea. 



Several reasons may be given which could explain why no faunal element has 

 been able to spread all around the globe to such a great extent as that of the High 

 North: — 



(a) The way along a given parallel is shorter the nearer it lies to the Pole. 



(b) The continents lie everywhere closer together about the latitude of the Polar 

 Circle than farther south. The most important distances are: 



Bering Strait 90 km. 

 Labrador-Greenland 750 km. 



Greenland-Iceland 330 km. 

 Iceland-Norway ca. 900 km. 



19 — 565597 Lindroth 



