ROUTES OF MIGRATION 223 



tlie North Sea across the low watershed, of which the 

 cliffs at Calais and Dover then formed the continuous 

 highest ridge, and completely isolated the British Islands 

 from continental land. 



With such facts as these before us, surely the cross- 

 ing of the Channel by migrant birds in spring and 

 autumn can be nothing very wonderful after all. The 

 submergence took place very slowly ; the doomed land 

 perhaps being swampy at first, then gradually studded 

 with lagoons, and finally becoming open narrow sea, 

 wider and wider. Throughout all this slow and 

 gradual change the migration (and emigration) of birds 

 went on, no single generation noticing a change, until 

 the complete transference of land to water had been 

 accomplished. The birds with dogged perseverance and 

 admirable persistency stuck to their old routes — the 

 only ones, be it remarked, of which they could possibly 

 have had any knowledge — and continue to stick to 

 them down to the present day. The Wheatears that 

 cross the wide expanse of sea to the south-west of 

 Ireland know of no easier and safer passage ; the 

 Redstarts that land in England on the South Hams of 

 Devon know of no narrower route further north and 

 east ; whilst those fortunate individuals that have 

 descended from the later settlers which entered our 

 area where the Channel now is narrowest, never experi- 

 ence normally the greater terrors of the prolonged flight 

 across that Channel in its wider aspects. I may also 

 remark that there can be little doubt that the reason the 

 stream of migration is so dominant at the Strait of 

 Dover is purely because the land connection there was 

 coincident with the dominant line of northern extension 



