174 THE BIOLOGY OF BIRDS 



to magnetic waves. At least nine hypotheses have been 

 suggested, but a basis of fact has still to be found. 



Professor Watson makes an interesting observation on 

 the duration of the nesting impulse when the normal 

 procedure is interrupted by experiments such as he made. 

 He finds that it remains strong for two or three weeks, and 

 this gives an illumining significance to the homing. As 

 we have said in our " Secrets of Animal Life," " the sea- 

 swallows are returning to their nests, that is, to activities 

 in which their life reaches its climax, to the continuance of 

 which they are impelled by a deep organic urge, by an 

 irresistible impulse which is not baulked by any waste of 

 seas," 



§ 6. Possible Origin of the Migratory Custom 



{a) The Robber- Crab {Birgus latro) of the Pacific 

 Islands has become thoroughly terrestrial, even to the 

 extent of climbing trees, but it has to migrate to the shore 

 to spawn, and its larvae swim for a time in the open sea. 

 It is a general biological rule that a new generation is 

 started in the ancestral haunt of the race. The migration 

 of the land crab from the hills to the shore is bound up 

 primarily with its previous colonisation of the dry land, and 

 secondarily with the fact that without great changes in the 

 life-history the spawning must take place in the sea. 



Several turtles have become pelagic animals, but they 

 have to come to a sandy shore to lay their eggs. When these 

 hatch and the young are able to fend for themselves there is 

 a return to the open sea. Similarly, some at least of the 

 sea-snakes, which live far from land, migrate to the shore 

 to give birth to their young, and protect these for some 

 time among the rocks. Seals, again, which have consider- 

 able emancipation from the solid earth, come to the shores 

 at the breeding season. That this is not a necessary inter- 

 ruption of the aquatic life of a mammal that has ceased to 

 be terrestrial is shown by the whales and dolphins. 



It is clear that the migratory custom has in many cases 



