90 THE MIGRATION OF BRITISH BIRDS 



prevented the Iberian and North-West African Hypolais 

 polyglotta from extending its range north of the Seine 

 and reaching our islands, as normally we should have 

 expected it to have done. It is possible that similar 

 influences may have succeeded in preventing such forms 

 as Einbcrirja hortulana, Acroccphaliis turdoidcs, and A. 

 aqiiaticHs from extending their breeding range to us. 



Again, not one of the species tabulated above can be 

 fairly regarded as dominant or widely dispersed in the 

 British Area. The reason these species are not domi- 

 nant in South-west and West-central Europe, except 

 in one or two isolated cases which onh- tend to prove 

 the rule, is probably because thc}^ were prevented by 

 climatic and glacial conditions from reaching that area 

 until the vast northern Emigration of birds from Refuge 

 Area II. had taken place, which we have every reason 

 to believe occurred earlier and under more favourable 

 auspices — due to Gulf Stream influence — than was the 

 case with birds whose emigrations progressed from the 

 south-east. Before these south-eastern species had suc- 

 ceeded in colonizing West Europe, or before the)^ had 

 crossed the once glaciated portions of Central I'^urope, 

 these temperate western lands were occupied b\' wide- 

 spread and dominant species from the south which liad 

 become well established (or it may be the sea had 

 separated l?ritain from continental Europe) before thc}- 

 arrivcd so far to the north-west — two powerful checks to 

 the western progress of these eastern birds. We have 

 every reason to believe that had these birds come up 

 from the south, instead of from the remote south-east, 

 they would either have been regular \isitors to or 

 residents in our area ; unquestionably their most im- 



