R O UTES OF MIGRA TION. 9 3 



over them is both extensive and regular ; and it is 

 also by the aid of islands that such vast areas 

 of sea are crossed as extend, for instance, between 

 Japan, the Malay Archipelago, and Australia. In 

 the same way deserts are crossed by stages from 

 oasis to oasis. It is also owing to this fact that 

 oceanic migration is practically unknown among 

 land birds, and that sea migration is so extensive. 

 For instance, the Wheatear {Saxicola cenanthe) can 

 and does travel from North Greenland or Spitz- 

 bergen to the Equator without having to cross the 

 sea for more than 300 miles at any point of its 

 route. From Greenland its fly-line crosses Ice- 

 land, the Faroes, the Shetlands, the mainland of 

 the British Islands, France, the Spanish Peninsula, 

 the Straits of Gibraltar, and ends in Africa. The 

 fly-line of birds travelling from Spitzbergen crosses 

 the Arctic Ocean over several small islands to 

 Scandinavia, thence either across by way of Fair 

 Isle and the Orkneys to the British Islands, or 

 down the continental coast-line to Heligoland, and 

 onwards to France and the South. There is pro- 

 bably more migration across the Mediterranean 

 than any other sea in the world. It is the great 

 divide between the summer and winter quarters of 

 nearly half the birds of the Pal^arctic region ; and 

 yet the passage over it is by no means a promis- 

 cuous one, but it is made with due regard to what 

 are evidently very ancient routes. All or nearly 

 all the migrants from the extreme west of Europe^ 

 including the British Islands, enter Africa by way 



