24 ORIGIN OF LIFE IN AMERICA 



and having subsequently been dispersed southward by different 

 routes. He believed that the tendency of birds to migrate 

 northwards was due to a natural instinct to return to the 

 home of their ancestors. 



What I chiefly endeavoured to prove in this chapter was the 

 existence in pre-Glacial and early Glacial times of a land 

 bridge joining Scotland, Iceland, Greenland and Labrador. 

 The evidence in favour of such a land connection must be 

 largely, if not entirely, biological ; but the testimony, as far 

 as it goes, leads me to believe that the theory is well founded. 

 I shall allude to a similar land connection in another chapter 

 which probably joined North America and Asia. If the 

 climatic changes ushered in by the Glacial Epoch were pro- 

 duced by the closing of these two highways to the Arctic 

 Ocean, it is evident that the preceding warm period must 

 have been due to a greater flow of warm currents to the Arctic 

 Regions. 



A few years ago I drew attention to the fact that the animals 

 and plants found on the Faroes and Iceland in particular 

 imply the existence of a former land connection between Scot- 

 land and the latter country. The occurrence in Iceland of the 

 European field-mouse (Mus sylvaticus), of the snail Arianta 

 arbustorum, which, also inhabits the Faroes, of the beetle 

 Nebria gyllenhali, which likewise inhabits Greenland, and 

 many other forms not likely to have been conveyed by acci- 

 dental means, all favour the view that the fauna of Iceland 

 owes its existence mainly to the land "bridge referred to. But, 

 as I pointed out, Iceland also possesses a distinctly American 

 element in the snail Succinea groenlandica and others, while 

 some of the American plants have even invaded the continent 

 of Europe by the Greenland-Iceland land bridge. One of the 

 objections raised against this view is that the low tempera- 

 ture in the north would have prevented any faunistic inter- 

 change across the land bridge. The temperature, on the con- 

 trary, in Iceland, southern Greenland, Labrador and Scot- 

 land would have been considerably higher under such geo- 

 graphical conditions than it is now. If so, why should not the 

 whole fauna of northern North America have streamed across 

 this bridge to Europe and that of northern Europe to North 

 America ? If we examine the fauna of Canada we find that it 



