EMIGRATION AND EVOLUTION. 127 



does. The Song Thrushes return regularly about 

 the first week in February, or about this very 

 date, the loth. I did not see a single bird all 

 the winter." That this dispersal into cooler areas 

 has necessitated the adoption of regular migratory 

 habits is absolutely certain; and in every known 

 case of this kind of emigration we find that the 

 migratory fly-lines follow the direction of that 

 emigration, either to the point w^here it commenced, 

 or to the point where the regular trunk line of 

 passage leading to the usual winter quarters of 

 the species may be joined. 



From the above series of facts I think it will 

 be reasonably evident that emigration is not only 

 very closely associated with migration, but is even 

 occasionally the means of its initiation. But emi- 

 gration has also played other important parts in 

 Avian Philosophy. The vast, wdde-reaching in- 

 fluence of emigration on the Evolution of Species 

 can never be sufliciently esdmated. Everywhere 

 we find evidence to indicate that emigration has 

 been one of the most powerful aids to segregation. 

 Without it the number of exisdng species would 

 undoubtedly be enormously reduced; whilst the 

 beaunful and wonderful dispersal of avian life 

 throughout the world would have presented a 

 very diflerent aspect. The diflferentiation of vast 

 numbers of species can be directly traced to 

 emigradon, often leading to the complete isoladon 

 of numbers of individuals, and bringing them 

 under the influence of new conditions of existence. 



