126 THE AflGRATION OF BIRDS. 



commenced to till the soil. About 150 years ago 

 it spread up the Obb, and four years later had 

 emigrated 500 miles still further to the east in 

 this river valley. Thence it pushed onwards 

 through the Yenesay to the Lake Baikal district, 

 and is now abundant throughout Siberia, within 

 these limits, wherever civilization has spread. 

 Many instances might be given where various 

 species have gradually become much more abund- 

 ant, even in our islands. All this evidence 

 tends to show that Chronic Emigration is far from 

 being absolutely quiescent at the present day ; 

 rather must we presume that it is ever ready to 

 break out much more acutely, even with irruptic 

 virulence, whenever a stimulating cause may 

 arise. 



In many cases chronic emigration, or even 

 irruptic emigration, may lead to the adoption of 

 migratory habits, if the winters of an invaded 

 district be too severe for constant residence therein. 

 I have just alluded to the Song Thrush as a species 

 in a state of chronic emigration in the British 

 Islands. Even a very marked migration takes 

 place during wdnter, which would lead us to infer 

 that this species has only recently extended its 

 range so far north. In Scotland this migration 

 is even more pronounced, as the Duke of Argyll 

 has most obligingly informed me. He writes from 

 Inveraray : " At this moment our Song Thrushes 

 have just returned. They almost all leave us for 

 the winter season, although the Blackbird never 



