ORIGIN OF MIGRATION. 61 



vital a part in the migration of birds, I have no explana- 

 tion to offer. We know, however, that it exists not only 

 in birds but in many other animals. It is this instinct, 

 aided by the " heredity of habit," which guides a bird 

 to its nesting ground. The Carrier Pigeon is taught its 

 lines of flight by gradually extending its journeys ; a 

 species establishes its routes of migration by gradually 

 extending its range. 



