1 6 THE MIGRATION OF BRITISH BIRDS 



traversed by the Rhine with all our eastern rivers from 

 the Thames northward as its tributaries ; we picture 

 the Irish Sea as another plain, more rugged, perhaps, 

 in character, perhaps with a long central lake and an 

 ex-current river, draining parts of Ireland, England, and 

 Scotland. We picture the Bristol Channel as a verdant 

 valley with precipitous sides, watered by a Severn flow- 

 ing on to join the main river of the Irish Sea valley. 

 Further, yet again, we must picture the English Channel 

 as another broad plain watered by the Somme and the 

 Seine, then one river, with tributary streams from the 

 south of England, flowing south-west to fall into the 

 Atlantic, whose coast-line then extended many miles to 

 the west of Ouessant Island. 



We appear to possess no evidence of the time that 

 these geographical conditions endured. Sufficient time 

 must have elapsed, however, for the climate to become 

 so modified as to allow of the growth of a luxuriant 

 flora and of the emigration of vast numbers of birds and 

 animals from niore southern regions to which their range 

 had been contracted by extermination by the preceding 

 glacial period. Submergence seems, however, to have 

 been in progress, due either to terrestrial action or 

 marine disturbance, and Ireland became separated from 

 England before more than twenty-two of the forty 

 species of British Mammalia, and four of the thirteen 

 species of Reptilia and Amphibia had succeeded in 

 establishing themselves in that area. So far as birds 

 are any evidence the land must soon comparatively 

 have been invaded by the sea, until the contour of 40 

 fathoms was reached. This would bring Ireland almost 

 to its j)rcsent contour and isohitc it from England and 



