290 



CHAPTER III 



FIG. 50. The Three-toed Woodpecker {Picoides tridactylus L.). 

 Breeding range of a subarctic-highboreal, circumpolar bird, 

 boreoalpine in Eurasia. 



(Mainly according to "Check List", 193 1, Stegmann, 1938, Peters, 

 1948, and HoLDHAUS, 1954.) 



(From Rosenberg, Faglar i Sverige.) 



Yet these favourable conditions do not in themselves provide an exhaustive 

 explanation, except for flying or otherwise easily dispersed forms. This was shown 

 above (p. 264) regarding the Davis Strait which, though only 350 km. at its nar- 

 rowest, has functioned as a highly efficient barrier against the dispersal of soil- 

 bound animals. 



