102 A HISTORY OF BIRDS 



they have not become too highly specialised in the matter of 

 food, and so can avail themselves of supplies that in some form 

 or another are universal. 



That these migratory birds are the descendants of a line of 

 forebears of similar habits extending far back in time none can 

 doubt, extending back indeed to a time when the physical 

 geography of the world, in the matter of the distribution of 

 land and water, differed materially to what obtains at the 

 present day. This is a fact which is not only commonly 

 recognised by students of bird migration, but which is also 

 made to play what to some must seem rather too imaginary 

 a part. Thus it has been contended that many present-day 

 migration routes are taken irrespective of present areas of 

 land and water, and in conformity with the route followed by 

 their ancestors when these areas were otherwise than now. 

 That is to say, they cross such and such areas of water not 

 because they afford short cuts, but because these areas were 

 traversed ages ago when what is now a sea was then dry land. 



And just as submerged areas are now traversed purely for 

 traditional motives, so it is held certain land areas are now 

 skirted rather than crossed because they represent areas once 

 covered by sea. Thus Dr. Sharpe, in describing the route of 

 the Woodchat Shrike on its way to its winter quarters in Africa, 

 remarks that it " skirts the Sahara as if it were still a sea . . ." ; 

 yet he shows that the Willow Warbler, on the other hand, 

 " winters in the oases of the Northern Sahara," whence it 

 appears some individuals perform a further flight to Senegambia, 

 this species being found during the winter months both in 

 West and South Africa. The inference seems rather to be 

 that while the Willow Warbler can safely enter the Sahara, 

 being sure of food, the Woodchat cannot. Of course it is open 

 to argument that the Willow Warbler is the more recent species, 

 and therefore entertains no hereditary prejudices against this 

 region, but of this there is no evidence. 



Ancient and now submerged land surfaces, however, are 

 undoubtedly to be reckoned with in considering the subject of 

 migratory birds, for these submerged areas would seem to act 

 as barriers of a hitherto unsuspected kind. But before this 

 hypothesis can be profitably developed it will be necessary to 

 say a few words anent what has been called the "homing 



