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. 
