PAST GEOGRAPHICAL MUTATIONS 5 



contingent to our own, in order to arrive at a reasonable 

 and probably correct interpretation of the facts now 

 presented. It is absolutely impossible to understand 

 the present migration and geographical distribution of 

 British birds, unless we take into consideration the past 

 emigrations of these species, or their common ancestral 

 forms ; for undoubtedly those ancient emigrations pre- 

 sent the only means now available b)' which we can 

 explain a very large number of apparently anomalous 

 facts. It is equally impossible to understand them 

 without taking into consideration the various geo- 

 graphical mutations and climatic changes which have 

 taken place in this area during late Tertiary and Post- 

 Tertiary time. It becomes necessary therefore to glance 

 briefly at the probable state of Europe, not so much 

 during the height of the Glacial Epoch, but more 

 especially during the closing periods of that awful era 

 and in the earlier stages of Post-Glacial time. 



The area which now forms the British Islands consists 

 of the elevated portions of a vast submarine plateau, 

 receding from the coasts of continental Europe, and for 

 the most part covered by a shallow sea varying from 

 fifteen to a hundred fathoms in depth. This plateau 

 extends from Denmark, some distance beyond the south 

 coast of Norway, the Shetlands, the west coast of 

 Ireland, and southwards along the French coasts of the 

 Bay of Biscay. Beyond this limit westwards the sea 

 rapidly deepens in a series of terraces from 500 and 

 1000 to 2000 fathoms. The greater part of this water 

 area, including most of the North Sea, ranges from 

 15 to 50 fathoms, but deeper trenches occur along 

 the centre of the bed of St. Georges Channel and the 



