228 THE MIGRATION OF BRITISH BIRDS 



route in autumn arc all hardy species which must ha\c 

 been among the first to enter Britain after the Ice Age 

 had passed away, and when it was practically a compact 

 continental area. No Irish Sea then existed ; Ireland 

 was then probably better suited to the requirements of 

 all Passerine species than any other part of Britain, due 

 entirely to its proximity to the open sea and warm 

 ocean currents, and from there emigration eastwards 

 undoubtedly commenced, as is proved by the migration 

 to that area of so many of these east to west species. 

 The Starling, the Sky Lark, and the Short-eared Owl 

 are especially good instances of this line of past Emigra- 

 tion, as they migrate in such v^ast numbers, palpable to 

 every observer, right across England to Ireland, where 

 they are known to occur in enormous quantities in 

 autumn, starting on their return journey east in spring. 

 I may also remark, again, that the Short-cared Owl is 

 not known even to breed in Ireland now, although 

 common enough there in winter. It may also be stated 

 that none of this east to west migration is perceptible in 

 the south-west of England, although there is a con- 

 siderable amount of the migration from the north-east 

 observable even down to Land's End {conf. table, p. 

 132). Here, then, is eloquent testimony of the impas- 

 sable nature of a water area to Emigration, and of the 

 futility of such an obstacle to arrest Migration estab- 

 lished previous to the submergence ! We have seen 

 how Emigration has been effectually checked by the 

 comparatively narrow St. Georges Channel, and now 

 see how the wide Irish and North Seas — formed after 

 successful emigration — are impotent to arrest the Migra- 

 tion of species whose range was gradually extended 



