THE RETURN OF THE SWALLOWS. 17 



from their African wintering-places. To get from Eng- 

 land to Algiers, many swallows fly over wide expanses 

 of sea, far too broad to see 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 an- 

 other 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 suggest- 

 ed an ingenious and fairly satisfactory explanation of the 

 difficulty. He points out that the lines taken by the 

 swallows 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 Tangiers, 

 with their uniting submarine banks. Of these the Span- 

 ish belt is still almost entire, and it offers no special diffi- 

 culty ; the others are now broken up into peninsulas or 

 islands. Dr. Weismann supposes that various flocks of 

 birds grew accustomed to proceed north or south along 

 one such connecting belt, while the land was still in pro- 

 cess of subsiding ; and that their descendants still con- 

 tinue to follow the same lines till they reach the final 

 headlands, and then fly straight over sea in a definite 

 direction till they sight the opposite land. The younger 

 birds follow their elders ; while the elders themselves 

 have learned the proper landmarks and directions from, 

 similarly following their own predecessors, and gradually 

 take the lead in their turn as the seniors drop off one 



