The relationship between the palaearctic and nearctic faunas 303 



FIG. 57. Bembidion trans- 

 parens Gebl. Scandinavian 

 specimens. 



a = macropterous form (fly- 

 ing) 



b = intermediate form (flight- 

 less) (very rare) 



c = brachypterous form 

 (flightless). 



Short wing is a dominant 

 and long-winged specimens 

 are homozygotes. 



subspecies radiate toward Europe and eastern North America, argues in favour 

 of the "overlapping area" as an old centre. 



When pronounced and geographically concentrated, this area is situated west 

 of the Bering Strait, which supports the generally accepted idea that the main 

 Holarctic centre of speciation was in northeastern Asia and that the traffic over 

 the Bering land-bridge, irrespective of period, was most intense from the west 

 to the east (Simpson, 1947, p. 628 a.f.). 



Yet it is important to realise that a circumpolar area is not the "climax" stage 

 of a perpetual and even dispersal from an ancient evolutionary centre of the species. 

 It is but one stage in a complex procedure involving intermittent expansion and 

 regression. Species may once have been circumpolar which now no longer are so, 

 for instance the Muskox {Ovibos moschatus L.). The repeated glaciations prevented 

 any species from being continuously circumpolar during the entire Pleistocene 

 period, but at the same time they caused pulsating advances toward the south which 

 in many cases, left permanent outposts in southern mountains. This gave rise to 

 boreoalpine types of distribution (Holdhaus & Lindroth, 1939; Holdhaus, 1954), for 

 instance in the Three-toed Woodpecker {Picoides tridactylus L.; fig. 50). It is 

 difficult to establish the period in which the area of a boreoalpine species was 

 formed, but it is reasonable to assume that the time of the largest ice-sheet, which, 



