THE GLACIAL RANGE CONTRACTION, ETC. 157 



menced, and that Ireland had become separated from 

 England long before it had received its fair share of the 

 birds that were spreading across our land as summer 

 visitors. Thus we have birds that visit England, and 

 even extend their summer flight to the north of Scot- 

 land, such as the Redstart, the Lesser Whitethroat, and 

 the Tree Pipit, but which do not visit Ireland, which 

 they normally should do, all conditions being equal ; the 

 water areas forming an impassable barrier to extension 

 of range thereto. These species also furnish another 

 proof in support of the Law which forbids emigration 

 southwards, as in the north-east of Ireland the water 

 passage to Scotland is no wider than the Strait of Dover 

 between England and France, which these summer birds 

 must cross and re-cross each year, and therefore no 

 barrier to emigration to Ireland, if such a line of exten- 

 sion were a normal one. Ireland is often held up as an 

 example of zoological poverty, and rightly so, in com- 

 parison with adjoining areas, but so far as birds are 

 concerned this poverty only exists among one class of 

 species alone, the Summer Migrants to our area, which 

 were prevented from entering the country in past ages 

 owing to submergence of the connecting land areas, and 

 because such species were late emigrants in this direc- 

 tion, arriving in the British Area, we have every reason 

 to believe, long after the dominant resident avifauna 

 was established, or the present winter visitors thereto 

 and coasting migrants across had spread northwards 

 over it. 



In order to render the subject of Avian Emigration to 

 the British Area fairly complete, I append below a table 

 of the species and races that are peculiar to this area, 



