THE PROBLEM OF LATE LAND CONNECTIONS IN 

 THE NORTH ATLANTIC AREA 



Carl H, Lindroth 



Zoological Institute, University of Lund, Lund, Sweden 



The idea of an earlier land connection between Europe and North America 

 first occurred to biogeographers who wished to explain the striking similarity 

 of the flora and fauna of the two continents. It found its most ardent supporter 

 in R. F. Scharff (1907, 1909, 1911) and according to Th. Arldt (1917, p. 83) the 

 Tertiary North Atlantic land-bridge constituted "eine der am sichersten 

 feststehenden palaeogeographischen Tatsachen". The latest to express his 

 acceptance of this hypothesis was the Norwegian botanist E. Dahl (1958). 

 An Early Tertiary trans-Atlantic land connection has also been assumed on 

 paleontological evidence (cf. Simpson, 1940, p. 149; 1947, pp. 658, 666). 



An analysis of the animal species common to Europe and North America 

 (Lindroth, 1957) has shown, however, that the majority of these belong to 

 either of two about equally large groups: one, whose present distribution is 

 a result of unintentional transport by man, mainly in the direction from 

 Europe to North America; another, having a more or less complete circum- 

 polar area, where the migration between the two continents went through a 

 "back door", via Siberia and the Beringian land-bridge. This bridge is now 

 considered by geologists as well as biogeographers almost unanimously to 

 have functioned as a continuous land connection during each of the Pleisto- 

 cene glaciations. 



Exceptions from the rule that the faunal and floral exchange between 

 Europe and North America took place by the way of Asia may be expected 

 in the group of Amphi- Atlantic species (cf. Hulten, 1958) — provided the 

 plants and animals are indigenous in both continents. This is an important 

 restriction since several examples of Amphi- Atlantic distribution, such as that 

 of the common garden snail, Cepea hortensis L., used as arguments in favor 

 of a Eur-American connection, are now supposed to be the results of 

 introductions into North America. 



However, originally Amphi-Atlantic species do no doubt exist among 

 animals as well as plants though they are not numerous. It is our task to 

 explain their history and to decide whether an earlier continuous land connec- 

 tion between the two continents can be regarded as responsible for their 

 present distribution. If so, the islands of the North Atlantic, as remnants of a 



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