INTRODUCTION 25 



of migration, but either establish themselves as residents in the 

 strange country, or perish. Nor do wanderers leave behind 

 them traditions of new routes. The only possible explanation 

 seems to be that the migrants are following old land lines, 

 which were quite clear at one time, but have now disappeared. 

 The shore-birds follow the old shore-lines, and the land-birds 

 the old land-line. Migration must have commenced when the 

 two lands, which are now separated, were contiguous, or nearly 

 so. In no part of the course would an island be so far otf at first 

 as to be invisible from an adjacent one. Then the land must 

 have gradually sunk. But the force of habit, handed down 

 from generation to generation, probably maintained the 

 migration, until it became an instinct. During the life of each 

 bird, the changes would be too small to be perceptible, and 

 only after many years had passed would the migrants find that 

 they were flying over a trackless ocean. 



The faculty of returning home, whatever the nature of it 

 may be, is not uncommon in the Animal Kingdom. Bees and 

 ants and many other insects possess it to a small degree. 

 Sea-snakes and turtles return to the same place to breed, 

 although, during their absence from land, they must have swum 

 many miles in many different directions. The faculty is also 

 possessed by penguins, petrels, and other birds. Several seals 

 undertake long oceanic migrations. As is well known, some 

 domesticated animals return to their homes after having been 

 taken long distances away ; and, finally, savages, after having 

 followed their quarry for several days, find their way home 

 again through dense forests. The faculty, however, is not 

 unerring, even with migratory birds. In the course of 

 migrations, large numbers of "stragglers" lose their way; 

 many perish at sea, and perhaps none regain the route after 

 having lost it. 



Before passing on to other questions, it may be asked, "Why 

 should some of the shore birds and the two cuckoos migrate to 

 New Zealand, while the swallows, which are certainly quite as 

 capable of undertaking the journey, do not come?" Possibly 

 the answer may be found in the geological history of birds. 



