264 CHAPTER III 



dispersed by sea-drift. The remaining 13 species, 7 of them lacking the power of 

 flight {vide table 7), have been unable to get across to Greenland. 



The. character of the interjacent sea as a faunistic obstacle is best illustrated by 

 maps of two functionally brachypterous species, the Ground-beetle Amara alpina 

 brunnipennis Dej. (fig. 40) and the Weevil Otiorrhynchus arcticus O. Fbr. (fig. 41), 

 both widespread on either side. It can be stated, without any exaggeration, that, 

 zcithin the Arctic of the entire northern circumpolar area, the comparatively narrow 

 strait between Baffin Island and Greenland has constituted the most effective barrier 

 to the dispersal of soil-bound animals. The Bering Strait has been far more sur- 

 mountable (below, p. 293). Though it should of course be admitted that Green- 

 land is a zoogeographical transition between the Palaearctic and Nearctic regions, 

 this island— if a choice is necessary— must be regarded as Palaearctic (a good reason 

 for continued Danish government!). 



How is this surprising extension of the European fauna over the islands of the 

 North Atlantic to be explained? 



In the first place it must be stated that the possibilities of passive dispersal (and 

 at least for the soil-bound, flightless fauna this is the effective agency) would seem 

 to favour an eastward transport across the Atlantic. Not only the Gulfstream 

 follows this course (fig. 32), the predominating winds are also western^ and the 

 barometric minima developed off the northeastern coast of North America, not- 

 withstanding all irregularity too well-known to meteorologists, move eastward. 

 Williamson (1954) has given good evidence for how stray birds on migration may 

 be sluiced across from North America by the aid of subsequent minimum pressures 

 (fig. 34). In exceptional cases, provided a minimum becomes immobilized to the 

 south of Iceland (Lindroth, 193 1, p. 524, fig. 42), favourable, mainly southeastern 

 winds may last long enough to make air-borne transport at least theoretically 

 possible, for instance from the British Isles to the Faeroes or Iceland, Salomonsen 

 (1951) has shown quite convincingly how a wind-driven flock of Field-fares (Tj/r^/w^ 



^ The air movements at high altitude may be different from the winds immediately 

 above the earths surface. Meteorological balloons, usually travelling above 20 kilometers, 

 have crossed the Atlantic in both directions. In Europe, the summer winds at this altitude are 

 usually eastern (Scherhag, 1948, p. 77 a. f.) but rather slow. The biogeographical consequences 

 of transported air-plankton is a question of survival, which should be tested experimentally. 



DiAGR. 7. Species of Colcoptera common to the Faeroes, Iceland, Greenland and 

 Baffin Island, and selected regions of the Holarctis. 



White = flying; black = flightless species. 



Dimorphic species counted as flightless where (entirely or partially) represented 

 by the short-winged form. — Different subspecies treated as species. 



Summary of table 7. 



