CHAPTER V. 



EMIGRATION AND EVOLUTION. 



Confusion between Emigration and Migration— Definition of 

 Terms — Theories of Avian Distribution — Primary Causes 

 of Emigration — Local Causes of Emigration — Irruptic 

 Emigration — Irruptions of Sand Grouse — Irruptions of 

 Pastors, Jays, and Goldcrests — Chronic Emigration — Birds 

 extending their Range — Present Lines of Migration an Indi- 

 cation of Past Routes of Emigration — Ancient Routes of 

 Emigration — Emigration of Ouzels and Snipes — Recent 

 Avian Emigration in the British Islands — Emigrations of 

 House Sparrow — Emigration resulting in Migration — Emi- 

 gration and Evolution — North Polar Emigration caused by 

 Glacial Epoch — Island Species of Birds — Islands and 

 Routes of Migration — Influence on Insular Avifaunas — 

 Avifauna of the Galapagos Islands — Avifauna of the Ber- 

 mudas — Avifauna of British Islands — British Local Races — 

 Evidence of Emigration in British Islands — Importance 

 of Emigration. 



The subjects of the present chapter are so closely, 

 I may say inseparably, connected with Migration 

 that it becomes absolutely necessary to include 

 them if we desire to make the history and philosophy 

 of our subject even reasonably complete. We 

 have had, and shall continue to have, occasion to 

 allude casually to the Emigration of Birds ; we will 

 therefore devote the present chapter to its discus- 

 sion in greater detail. There is not a little popular 



