Conclusions 



The animal species common to Europe and North America are numerous. If 

 conditions among the groups here treated in Chapter I, comprising altogether 

 908 species in common, are regarded as representative, a calculation on the basis of 

 the Swedish fauna (Gislen, 1940), in which these groups make 17,3 per cent of the 

 entire terrestrial and limnic fauna, would lead to the conjecture that the actual 

 number of Eur- American non-marine animal species amounts to more than 5,000. 

 The true figure is probably higher because the microfauna, here disregarded, in- 

 cludes forms with a distribution above average. 



A large part of the species in common are regarded as introduced with man into 

 either continent, or both, this element amounting to between 41 and 46 per cent 

 (375-417 species). As the birds, with no unintentionally introduced species, are 

 among the groups treated, these figures are probably not too high, if applied to 

 the calculated total of Eur-American species. 



Species introduced from Europe into North America are about ten times as 

 numerous as those transported in the opposite direction. This is explained by 

 the peculiar character of ballast traffic in olden times, sailing vessels going almost 

 exclusively in ballast on their way west, to Newfoundland and the Maritime Pro- 

 vinces of Canada on the one hand, to the Pacific Northwest on the other. The 

 main area of departure for these animals, mostly belonging to the soil fauna, was 

 southwestern England. 



A contributory reason why the contingent of introduced European animals is 

 particularly large in the coastal regions of northeastern and northwestern North 

 America is, of course, that the climate of these parts of the continent is most 

 similar to that of western Europe. 



Among groups treated, the number of animal species undoubtedly indigenous 

 in both continents exceeds 500. Most of them possess a more or less continuous 

 circumpolar distribution. A special interest, however, is connected with the 

 so-called Amphiatlantic (incl. "Westarctic") species, which are lacking or have 

 a broadly interrupted area on the Pacific side of the globe. They have often been 

 used as an argument in favour of earlier transatlantic land-connections. 



Attempts are made to demonstrate that the cases of Amphiatlantic, and similar, 

 distribution can be understood, partly by over-sea dispersal under present condi- 



