186 KILIF DAHL 



elements into Iceland and Greenland, it is concluded that the land connection 

 was broken first between Iceland and America, and the strong European 

 influence in the biota of Iceland corroborates this. Consequently, the western 

 Low Arctic elements in Greenland and Iceland must be yoanger than the 

 Amphi-Atlantic and eastern elements and this is borne out also by the endemics 

 in Iceland, most of which have many close relatives on the European side of 

 the Ocean but only a few on the American side. It is not denied that some 

 species originally might have dispersed across the Bering Sea or have attained 

 their distribution by long-distance dispersal, but for the majority of the biota 

 this explanation does not seem feasible. 



It has been emphasized that the Amphi-Atlantic plants are Arctic -Sub- 

 Arctic, whereas very few Temperate plants have this distribution pattern. 

 This implies that the climate on our hypothetical land connection must 

 have been favorable to Arctic-Sub-Arctic plants but not to Temperate 

 plants. The same fact is brought out by an analysis of altitudinal limits in the 

 Scandinavian flora; very few species unable to grow in the birch belt occur 

 west of the Atlantic Ocean. In this element there is a high over-representation 

 of types adapted to long-distance dispersal (Dahl, 1959). It is concluded that 

 the climate on the land-bridge was about the same as is found in the birch 

 belt of Scandinavia or of Iceland today. 



This permits us to propose an age for the land connection. In Mid-Tertiary 

 times the climate was warmer than at present and only in later times, during 

 Pliocene and Pleistocene, do we find climatic conditions comparable to the 

 present-day climate. It is therefore concluded that the connection existed at 

 least s late as the Pliocene. 



GEOLOGICAL EVIDENCE OF A FORMER LAND CONNECTION 

 BETWEEN EUROPE AND AMERICA 



The biogeographic data presented suggest the existence of a former land 

 connection between Europe and America as late as in the Pliocene. However, 

 one cannot conclude from the distribution that the land-bridge existed and 

 then explain the distribution pattern by the means of the land-bridge. This 

 becomes circular reasoning; thus independent support in the form of geological 

 observations is necessary in order to establish the existence of a land connec- 

 tion. 



It is tempting to invoke the Wegener hypothesis of continental drift to 

 explain the features, and the data presented above may be taken as support 

 for the hypothesis of continental drift. However, this is a very controversial 

 matter and at present it seems unwise to build too much on the Wegener 

 hypothesis. 



There are, however, other indications suggesting the existence of a land 

 connection between Europe and America during Tertiary times. In the Eocene 



