I02 THE MIGRATION OF BRITISH BIRDS 



Plymouth Sound. As a general rule, even narrow seas 

 are an effectual bar to Emigration (in many cases wide 

 rivers are equally effective barriers), and their area 

 appears generally to have been crossed by these avian 

 colonists whilst such seas were dry land, and previous 

 to submergence, in such instances where we find an 

 abundant avifauna now separated from that of adjoining 

 districts by water areas. On the other hand, existing 

 water areas even of comparatively wide extent are in 

 the majority of instances no barrier to Migration or 

 Season Flight, inasmuch as the fly-lines of migratory 

 birds are followed with amazing persistence, and con- 

 tinue to be followed across recent seas, or seas slowly 

 becoming wider as submergence progresses. There can 

 be no doubt that at the time these routes were formed 

 the land surface was continuous, or nearly so ; indeed, 

 in a very great number of cases the geological evidence 

 confirms its continuity, and as birds only know, and 

 always and invariably follow, the one route by which 

 their area has been extended from winter quarters, 

 refuge areas, range bases, or centres of dispersal in 

 past ages, they must of necessity continue to follow 

 that route, notwithstanding the gradually widening 

 seas, or the formation of entirely new water areas, 

 which we know has taken place during a by no 

 means very remote past even in our own area, on 

 that route which lies between their breeding grounds 

 and their winter refuge. 



As showing the impassable nature of a sea-barrier, 

 and its influence on the avifauna of a sea-encircled area, 

 I may mention that no less than -^6 species of birds 

 breed in West continental Europe within the parallels 



