242 CHAPTER III 



The chance of spontaneous transatlantic dispersal at the present time 



We have now a selection of species, mostly animals but also some plants, which 

 more than any others are expected to show evidence of a direct exchange between 

 Europe and North America, without the interference of man. 



In order to estimate their evidence it is necessary to scrutinize the animals 

 more concretely, so to speak more "individually", than is possible on the basis of 

 the taxonomic and purely geographical arrangement of the preceding paragraphs. 

 They should be grouped according to biological properties which may affect 

 their powers of dispersal. 



In this respect, the following types are represented: — 



I. Actively flying animals. All the birds (16 species or subspecies) and all the 

 Lepidoptera (16 species), referred to the Amphiatlantic or in widest sense "Westarc- 

 tic" groups. Yet the different taxonomical and biological groups of birds and 

 Lepidoptera are unequally represented. 



The Amphiatlantic birds are all excellent flyers and closely connected with the 

 sea, 14 are Swimmers and both of the remaining (Charadrius hiaticula and Erolia 

 mariiima) are Waders breeding on the seashore. For most of them the Atlantic 

 breeding area is almost continuous, with stations on the Faeroes, in Iceland, Green- 

 land, &c. (fig. 27). Exceptions are, in the first place the three species of Terns 

 {Sterna dougalli, S. hirundo, Thalasseus sandvicensis) with a more southerly breeding 

 area, but the Terns belong to the veritable gipsies of the Atlantic Ocean {vide, for 

 instance, KuUenberg, 1946); in the second place Morus bassanus (fig. 28) which 

 is lacking in Greenland, and Catharacta skua, only doubtfully recorded as breeding 

 there. 



The small breeding area of Charadrius hiaticula in arctic North America (inch 

 Greenland) is historically connected with the eastern side of the Atlantic, as dem- 

 onstrated by its route of migration along the European coast. The main popula- 

 tion of Erolia maritima even in Greenland stays over the winter. 



It cannot be denied that all Amphiatlantic birds are easily able to cross 

 the Atlantic or that individual birds actually do so. The Swimmers especially 

 are able to rest and take food on the open sea. Continents more than oceans 

 constitute obstacles to their distribution. No inland birds and no weak flyers 

 are represented among Amphiatlantic birds. The Passeriformes are completely 

 lacking. 



The 16 species of Lepidoptera include 13 Noctuid moths (fam. Noctuidae), that 

 is a good 80 per cent, 2 Geometers (fam. Geometridae; Cidaria sabini and Scopula 

 frigidaria), and one Butterfly {Colias nastes). These figures are not at all in accord- 



