THE RETURN OF THE SWALLOWS. ii 



would grow into what wc ordinarily call an instinct — that 

 is to say, an untaught habit. This is the stage at which 

 the migratory custom has always remained in America, 

 where broad stretches of land extend from the Arctic 

 region to the tropical forests, unbroken by any inter- 

 mediate zone of severing sea. 



In Europe, however, special circumstances have 

 added another and more complicated element to the 

 problem — the element of discontinuity. The Mediter- 

 ranean, the English Channel, and the Baltic practically 

 cut off the various parts of the swallows' summer hunt- 

 ing-grounds from their African wintering-places. To 

 get from England to Algiers, many swallows fly over 

 wide expanses of sea, far too broad to sec across, and 

 therefore quite destitute of landmarks. It is simple 

 enough to find one's way by land from Canada to 

 Mexico ; but it is quite another thing to find one's 

 way across the sea, without a compass, from Algeria 

 to Marseilles : yet this is the route annually taken 

 by one large body of northward-bound swallows. 

 Dr. Weismann, however, has suggested an ingenious 

 and fairly satisfactory explanation of the difficulty. 

 He points out that the lines taken by the swal- 

 lows and other migratory birds correspond on the 

 whole with the shallowest parts of the Mediterranean, 

 where it is most intersected by peninsulas and islands. 

 When the Mediterranean valley began to sink below 

 the sea-level it must at first have produced two or three 

 large lakes in the deepest portions of its bed ; and 

 between these lakes there must have been connecting 

 belts of land, now marked respectively by Sicily and 

 Italy, by Sardinia and Corsica, and by Gibraltar and 



