244 SIBERIA IN EUROPE. chap. XX. 



of migration. The periodical disappearance of the swallow 

 remained, however, almost as great a mystery as before. 

 The impulse of migration was called an instinct ; but did 

 not the less remain an unfathomable secret of Nature, and 

 the only cause that could be assigned for it was that it must 

 have been originally implanted in certain species at their 

 creation and denied to others. 



The discoveries of Darwin and Wallace have placed the 

 facts of migration in an entirely new light, and added a 

 new interest to a subject which has always been one of the 

 most fascinating departments of ornithology. The origin 

 of the habit of migration is still involved in much mystery. 

 It is probably a fact in the history of birds of comparatively 

 modern date. It is not confined to any one geographical 

 region, nor to any one family of birds, nor can we assume that 

 it will be present or absent in every species of the same genus. 

 The birds of the Nearctic region are as migratory as those 

 of the Palaearctic. Many birds visit South America and 

 Australia only during the breeding-season. If we include 

 as birds of the tropical regions those species which visit 

 them after having bred in the cooler regions, they will also 

 contain a considerable proportion of migrants, even though 

 no bird migrates there to breed. We may lay it down as 

 a law, to which there is probably no exception, that every 

 bird breeds in the coldest regions of its migrations. No bird 

 migrates to the tropics to breed because there is no hotter 

 region for it to migrate from. The well-authenticated 

 stories of birds breeding a second time in the place of their 

 winter migration probably have the fame scientific value as 



