THE GLACIAL RANGE CONTRACTION, ETC. 85 



These species appear to reach the British Islands by 

 way of the Danube and Rhine valleys. They are also, 

 significantly enough, unknown in the Iberian Peninsula., 

 although two of them, the Red-backed Shrike and the 

 Marsh Warbler, are common enough in France during 

 summer. Mr, Dresser professes to have identified 

 Marsh Warblers from Malaga, but Mr. Howard Saun- 

 ders, in whose collection they were, very rightly con- 

 siders them to be nothing but Reed Warblers, Acro- 

 cepJialus ariindinacetis. This shows how important a 

 correct knowledge of Migration may be, even in the 

 identification of specimens. These birds are all very 

 late migrants (amongst the last species to appear in 

 spring), as we have already seen is the case with birds 

 from the south-east. All summer birds of passage from 

 the south-east to Western Europe are late migrants; 

 and this fact seems to suggest that the habit was ac- 

 quired during the passing away of the vast glaciers 

 from Central Europe, when the summers were probably 

 shorter than they are now — the migration then par- 

 taking of the character which is the present feature of 

 the phenomenon in the Arctic regions, where the season 

 is very late and very short. Species from the direct 

 south — from North-west Africa and Iberia — had in 

 those remote ages f'and as they still continue to have) a 

 much earlier route open to them, enabling them to push 

 northwards sooner in the spring (a habit which is still 

 continued), owing to the ameliorating influence of the 

 Gulf Stream and the milder glacial conditions. It may 

 also be remarked that none of these late migrants from 

 the south-east are dominant or widely dispersed in the 

 British area — the Red-backed Shrike being principally 



