MIGRATION THEORIES 419 



spond so exactly with what geologists tell us must have 

 been the old coast-line, that it is difficult to believe it to 

 be only a coincidence. If we admit the theory that 

 migration became a fixed habit during the glacial period, 

 we must also admit that the difficulty of proving that the 

 old coast-line disappeared after the formation of the 

 instinct, is removed. The fact that the British red grouse 

 is entirely confined to our islands and is replaced by a 

 very nearly-allied, but perfectly distinct species on the 

 continent — the willow-grouse, seems to prove that in all 

 probability, after the extermination of bird-life from the 

 corner of Europe now occupied by Great Britain, by the 

 ice of the glacial epoch, it was again re-peopled with grouse 

 from the mainland. During the warm period which 

 followed the glacial epoch, we may fairly assume that the 

 absence of the present ice at the North Pole, and the 

 presence of an additional amount of ice at the South Pole, 

 might so alter the centre of gravity of the earth as to 

 leave the shallow portion of the German Ocean dry land, 

 and then the grouse might again find a home in England 

 without difficulty. It is obvious, however, that whether 

 the land-connection between England and the Continent 

 were formed by a difference in the level of the water, or 

 whether it were formed by a greater former elevation of 

 a part of the bed of the German Ocean, the severance of 

 Britain from the continent of Europe must have taken 

 place sufficiently long ago to allow for the differentiation 

 of the two species which has subsequently taken place. 

 The reader may perhaps be inclined to think that it is 

 quite unnecessary to assume any such land-connection in 

 •order to account for the existence of grouse on our island. 

 The grouse is a bird, and can fly, and pretty quickly too, as 

 any one who has shivered behind a butt in the inglorious 

 sport of grouse-driving knows. Why cannot the ancestors 



