132 THE MIGRATION OF BIRDS. 



adjoining continent, and individuals are repeatedly 

 arriving to mix and interbreed with the island individ- 

 uals, thus preventing any possible differentiation 

 which might and undoubtedly would soon occur 

 through their isolation. In this case the effects of 

 any fortuitous emigration have been rapidly eradi- 

 cated, if ever they appeared, by the constant influx 

 of wandering individuals from the adjoining route 

 of migration. So long as this fly-line continues to 

 be recognized by migratory Nearctic species, the 

 Bermudas cannot possibly acquire any remarkable 

 or specialized endemic avifauna. 



Even the British Islands can furnish one or two 

 instances bearing on this interesting subject. These 

 islands are remarkably poor in endemic species, 

 partly owing to their separation from continental 

 Europe being so recent, and partly because they 

 are situated on a great route of migration, which 

 keeps the island individuals of almost every 

 species well mixed with continental individuals, and 

 thus by interbreeding checks any tendency to 

 variation being preserved by isolation. Endemic 

 British species of birds are therefore excessively 

 rare. The Red Grouse [Lago[ms scoticus) is cer- 

 tainly the most interesting, and may have been the 

 result of a fortuitous emigration of Willow Grouse 

 [Lagopus albus) from Scandinavia, or a colony of 

 the latter left isolated on British moors by the sub- 

 mergence of land between the Orkneys and that 

 country. Any way, the Red Grouse owes its 

 specific distinctness to the fact that its continental 



