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 ORIGIN OF MIGRATION. 61 vital a part in the migration of birds, I have no exj^lana- tion to oifer. 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.