EMIGRATION AND EVOLUTION. 125 



which are in many places, if not absolutely identical, 

 still almost the same as those followed by the 

 Ouzels! So plainly are these past emigrations 

 indicated by present geographical distribution, that 

 one can almost venture to prophesy the discovery 

 of new species of Ouzel and Snipe in that tropical 

 chain of islands reaching across the Pacific, thus 

 making the area of distribution of Merula and 

 Scolopax continuous and complete. 



Instances of small but recent avian emigration 

 may even be met with in the British Islands. Such 

 species, for instance, as the Song Thrush {Turdus 

 musicus), Missel Thrush (Turdus viscivorus), and 

 the Rook (Corvus frugilegus) are in a more or 

 less acute state of emigration, gradually extending 

 their area of dispersal as circumstances may arise 

 favouring the increase. In Scotland these birds 

 are gradually following the planting of trees; and 

 I found it to be the invariable experience of 

 competent observers in Skye, that soon after a 

 plantation was formed, birds made their appearance 

 therein which had never been met with in the 

 neighbourhood before. The late Mr. Cameron 

 of Tallisker (Skye) gave me many interesting 

 details bearing upon the emigration of our common 

 resident birds. The Partridge {Perdix cinerea) 

 has followed the spread of corn cultivation in 

 Scotland. The House Sparrow {Passer domesticus) 

 has emigrated far and wide throughout the civilized 

 w^orld. This bird, according to Lyell, made its 

 first appearance on the Irtish when the Russians 



