224 THE MIGRATION OF BRITISH BIRDS 



into our area, and that entrance at this former land 

 connection was sufficiently south to admit of normal 

 range extension throughout the greater part of the 

 British Islands say north of lat. 50i^ — a fact which is 

 proved by the strong migration east and south-east in 

 autumn towards the Strait of Dover. The same re- 

 marks apply to the migration (and past emigration) 

 across St. Georges Channel, only the latitude will then 

 have to be extended north to 52 10'. To say that birds 

 in autumn, for instance (the same remarks apply to 

 spring as well) are all making for the narrowest sea- 

 passage to the Continent is absolute nonsense. If birds 

 were guilty of breaking the Law of their dispersal so 

 flagrantly, why, I ask, do not the tens ol thousands that 

 cross the wide North Sea each season — between say 

 Heligoland and Hull — pass south to Calais before they 

 attempt to make the passage .^ \Vc know, of course, 

 that numbers do so cross at that narrow passage, but 

 the circumstance is entirely due to the line of emigra- 

 tion followed by the ancestors of those individuals, and 

 is merely a coincidence. 



Palmen's elaborate system of " Fly Lines," which he 

 postulated in his endeavovu' to trace the Migration 

 Routes of birds, arc m}'ths. No special route of migra- 

 tion is traversed ; species follow the course that their 

 range expansion has taken in past ages ; and the route 

 can only be regarded as a conivion one (as a " Fly Line " 

 in Palmen's meaning), in the sense that many species 

 have extended their areas of distribution along it. It 

 may be urged that in all countries traversed largely by 

 migrants, there are certain routes which arc much more 

 crowded than others ; that in some districts little or no 



