lo THE MIGRATION OF BRITISH BIRDS 



more to have made its appearance, and a portion of its 

 southern coast-hne extended far within the present Hmits 

 of Northern France and Belgium. It is during this era 

 that the range of the Greenland whale and the walrus 

 is reputed to have extended as far south as the coast of 

 Lincolnshire. This state of things probably endured 

 long enough for the w^aters of the two seas to meet 

 across the narrow isthmus which joined England to 

 P^rance, but was followed by terrestrial upheaval or 

 marine depression, and as the glacial period passed 

 away the Channel and the North Sea once more became 

 dry land. 



It will perhaps be most convenient to deal first with 

 the geographical mutations which our area has under- 

 gone during the last phase of Pliocene time, and from 

 the close of the Glacial Epoch down to the period when 

 the British Islands were finally severed from continental 

 land, so that we may better understand the various 

 points at issue, and be able to refer with clearness to 

 such changes whenever the exigencies of our investiga- 

 tions render such a course advisable or necessary. We 

 have already seen (see Map, p. 5) that a vast submarine 

 plateau or bank covered by a shallow sea extends far 

 into the Atlantic, some 50 miles beyond the British area 

 to the north and west, and some 150 or 200 miles to 

 the south off Finisterre, whence it is contracted down 

 the shores of the Bay of Biscay to Biarritz. Upon this 

 submarine plain many important physical changes have 

 occurred since the close of the Pliocene Period. We 

 possess sufficient evidence to suggest that in those 

 remote ages the North Sea was much more contracted, 

 and the land mass of our area was very much more 



