SPECIES OF THE GENUS NUMENIUS. 141 
the parents of the present genera. Most species of shore birds 
were then, no doubt, circumpolar, crossing and recrossing the 
Polar basin at will, their wandering habits originated and 
perpetuated by the annual necessity of making a short trip every 
winter into more tropical climates a little to the south, not in 
search of food, nor in search of warmth, but in search of light, 
until at last some species acquired the habit of feeding at night, 
and were then able to live all the year round in a region subject 
to annuaily recurring periods of perpetual night. The com- 
paratively small area of the Polar basin offered every facility for 
interbreeding, the condition of life remained the same throughout 
the shores of the Polar Sea, and neither isolation nor its usual 
concomitant differentiation took place. But the coming on of a 
Glacial Period changed all this. 
The first important break in the continuity of the area of 
distribution of the Curlews must have been a wedge of ice 
coming down from the North Pole and extending along the 
mountains of Greenland, and isolating the American Curlews 
from the European ones on the Atlantic coasts. This was 
probably speedily followed by two enormous glaciers, one 
extending from the North Pole to the Rocky Mountains, and the 
other to the mountains of Eastern Siberia. This great central 
glacier thus separated the Curlews into two great colonies, an 
Atlantic Coast and a Pacific Coast colony. Later on, when the 
Arctic ice filled the Polar basin, the Curlews retreating before it 
must again have been compelled to split into two parties, one 
following the Pacific shores of Asia and the other the Pacific 
shores of America. In like manner the Atlantic colony naturally 
split into parties, but it had three instead of only two available 
routes of escape from the ice. One party, no doubt, found its 
way down Baffin’s Bay, a second party followed the coast-line to 
Europe, whilst the third party, which followed the east coast of 
Greenland, finding its west coast already occupied by the 
Baffin’s Bay Curlews, naturally took the shortest flight to land, 
and also reached Europe vid Iceland and fhe Faroes. The 
theoretical distribution of the Curlews would, if this hypothesis 
of their origin be correct, resemble the following plan :— 
