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 iso, 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 
