236 THE MIGRATION OF BRITISH BIRDS 



Channel where it is four times as wide as where indi- 

 viduals breeding in or passing over more eastern counties 

 cross ? There can be but one answer to such a ques- 

 tion, and that is, that these individual birds follow a 

 route along which their ancestors increased the range 

 extension northwards, that they have no knowledge of 

 any other route, and arc not endowed with any instinc- 

 tive faculty that will help them to migrate by any easier 

 way. If the route be exceptionally dangerous, or the 

 difficulties in following it increase, it will still continue 

 to be followed, until every bird that follows it is exter- 

 minated ; it can never be changed {couf. pp. 223-226). 



The Route has been slowly learnt by the species as 

 the summer area increased. When the extension of 

 breeding range commenced the extent could probably 

 have been measured by yards rather than miles ; by a 

 short flight to a more distant lake or swamp ; a visit for 

 nesting purposes to some outlying grove or forest ; or a 

 trip north along a coast for a little way to a suitable 

 stretch of sand or shingle, or range of cliffs — prompted 

 chiefly by overcrowded quarters to the south. As the 

 species multiplied the range increased. Each year the 

 journey became longer, more distant from the base or 

 centre of dispersal ; so slowly that perhaps not more 

 than a mile or so might have been added to the northern 

 area of a species in a century, as favourable conditions 

 to extension were gradually presented. The flight back 

 in autumn was therefore at the beginning a very small 

 one, and never perhaps included any sea-passage what- 

 ever. ]Jut our Summer jVIigrants now, we know, cross 

 the Channel every year ; they do so because the line of 

 their past emigration extended across that area when it 



