CHAPTER IV. 



THE GLACIAL RANGE CONTRACTION AND POST- 

 GLACIAL EMIGRATION OF BRITISH BIRDS {continued). 



Anomalous Facts — Analysis of the Facts suggested by preceding 

 Table — Table demonstrating the two Dominant Lines of Post- 

 Glacial Emigration in the extreme West of Europe — Analysis 

 of Table — Variations in the Northern Limits of Species — 

 Ancient Line of Emigration from the British Area Eastwards 

 into Continental Europe — The North Sea Plains — Their 

 gradual Submergence and its Effect on Birds^-Table of East 

 and North-east Emigrants — Analysis of Table — Influence of 

 Temperature on Birds — Effects on Birds of Isolation of British 

 Area from Continental Land — Professor Geikicon Emigration 

 to the British Area — Emigration to Ireland — Impossibility of 

 Southern Emigration to this Area from Scotland — Migration 

 of Birds in the Valley of the Petchora — Emigration of Birds 

 within the British Archipelago — Resident Species — Table of 

 Resident Species — Analysis of Table — Absence of Birds from 

 Ireland — Summer Migrants — Table of Summer Migrants — 

 Analysis of Table — Table of Autumn MigiMnts to, and Coasting 

 Migrants over, the British Islands — Analysis of Table — Table 

 showing the Proportional Distribution of Species over the 

 British Area — Deductions from the Facts — Table of Endemic 

 British Species and Races — Resume of Present and Preceding 

 Chapters — Importance of New Law of Dispersal — Extermin- 

 ating Effects of Glacial Epoch — Effects of Cold Winters — The 

 Dartford Warbler — Importance of Southern Range Bases. 



One or two facts embodied in the table at the close of 

 the preceding chapter call for special mention. It may- 

 be remarked as somewhat anomalous, for instance, that 

 the Gannet {Sula dassana) breeds on the Faroes and 



