14 THE MIGRATION OF BRITISH BIRDS 



probable. It is suggested, however, that during this 

 epoch the North Sea appeared once more, and that 

 the Atlantic invaded the English Channel. The low 

 grounds of Prussia are said also to have been sub- 

 merged about this time. 



There can be no doubt that periods of glaciation 

 have always been accompanied by great variation in 

 the relative level of sea and land, but whether so much 

 of this has been caused by elevation or subsidence of 

 vast areas of land, as many geologists so positively 

 assert, seems to the present writer somewhat doubtful, 

 I am disposed to agree with the late Dr. Croll, who has 

 conclusively shown, almost to the extent of absolute 

 demonstration, that oscillations of sea-level may be 

 produced by an alteration of the earth's centre of 

 gravity, caused by the greater amount of ice at one 

 pole than the other, the ocean conforming to such 

 alteration, and appearing to rise in one hemisphere 

 and to fall in the other. Thus the sea-level might rise 

 in a given area without any terrestrial movement what- 

 ever. Dr. Croll points out that the great ice masses 

 which were characteristic of the last Glacial Epoch would 

 cause a rise in the sea-level by altering the earth's 

 centre of gravity, and he suggests (according to Pro- 

 fessor Gcikie) " that some of our recent raised beaches 

 may indicate periods when the ice of north polar regions 

 attained a considerable augmentation, while, at the same 

 time, the ice of the Antipodes suffered a corresponding 

 diminution." The enormous ice-sheets which once 

 covered so much of the Northern Hemisphere may 

 therefore have caused sufficient change in the earth's 

 centre of gravity to produce an oscillation in the sea- 



