128 THE MIGRATION OF BRITISH BIRDS 



land, goes up West Europe to lat. 64°. The three 

 Woodpeckers only known to breed in England in the 

 British Area penetrate in Scandinavia to lat. 6},° [Gccinus 

 viridis), to the Arctic Circle [Picus major), and to lat. 

 70° {Picus nihior) ! The Kentish Plover only breeds on 

 the south coast of England in our area, but on the 

 Continent it visits the south of Sweden for that purpose. 

 The Ruff formerly bred in England — there is no evidence 

 to suggest that it ever bred further north in our islands 

 — }-et it goes in summer as far north as continental land 

 extends in West Europe. Even such hardy aquatic 

 species as the Black-throated Diver and the Common 

 Tern breed no further north in our area than the 

 Scottish mainland, perhaps in the Orkneys, yet the 

 former is found in Scandinavia to the highest limits, 

 whilst the latter reaches the Arctic Circle. 



What is the explanation of these apparently anomal- 

 ous facts .'• We have already seen that birds are loth 

 to extend their emigrations across water areas, and we 

 might reasonably assume that our isolated position was 

 a check to any considerable increase of northern range 

 in that direction ; but I am convinced that these won- 

 derful variations in the northern limits of so many 

 species are not due to such a cause. After a prolonged 

 and careful study of the facts, I consider that this dis- 

 crepancy of distribution is entirely due to the dominant 

 line of Emigration followed by these species. The 

 range base of the vast bulk of the individuals of these 

 species breeding east say of E. long. 10^ was in the 

 south-east during the Glacial Epoch. Their normal 

 line of Post-Glacial Emigration north and north-west 

 from that Range Base or Refuge Area (III.) would 

 therefore be entirely beyond our limits. The i dividuals 



