232 THE MIGRATION OF BRITISH BIRDS 



land birds, extends from Greenland, via Iceland, the 

 Faroes, St, Kilda, and the west coast of Scotland, to 

 Ireland. I have already dwelt at some length upon 

 the emigrations of species that followed many of these 

 Routes, so that it is unnecessary to enter into greater 

 details here. 



Before dismissing the subject of Routes it will now be 

 advisable to deal a little more fully with their inland 

 continuations. These, I need scarcely remark, are 

 extremely difficult to trace, owing to the sad lack of 

 an all-necessary series of long-continued observations. 

 There can be little doubt, especially in the case of locally 

 distributed species, that these Internal Routes are com- 

 plicated, although they will be found to conform exactly 

 to those laws which govern the migration of birds else- 

 where. We have already seen that birds follow most 

 closely in their present migrations the past line of 

 Emigration — however complicated and tortuous that 

 may be. Could we only define the limits of emigration, 

 we should find them to agree precisely with the present 

 Routes of Migration. We may safely put down as an 

 invariable rule that the Internal Routes of migrants 

 trend in the present direction of suitable breeding 

 conditions, or over such districts that were once suitable 

 areas for reproduction. River valleys, as long as they 

 trend northerly, arc certain routes for migrants ; for 

 there can be no doubt that from earliest times they were 

 favourable localities for range extension. Mountain 

 chains will just as surely indicate the routes followed by 

 other birds whose conditions of successful reproduction 

 arc only to be found in upland districts ; whilst lake 

 systems, swamps, heaths, woodlands, or cultivated dis- 



