EMIGRA TION AND E VOL UTION. 1 1 7 



wards into Europe is doomed inevitably to destruc- 

 tion. The Rose-coloured Pastor [Pastor ro.seus) is 

 another species that has evidently permanently 

 increased its western range by very similar means ; 

 and even now vast flocks occasionally wander into 

 new districts, like irruptions from congested areas 

 seeking vent for their superabundant life. The ob- 

 servant ornithologist may often remark the occa- 

 sional and often very extensive irruptions of much 

 commoner birds, species less likely to arrest universal 

 attention, that take place, and this unusual abun- 

 dance is probably the result of a general exodus of 

 surplus population from some overcrowded district. 

 The irruptic emigrations of such remarkable birds 

 as Sand Grouse or Pastors are noticed at once, whilst 

 those of commoner species are apt to be over- 

 looked, or their importance under-estimated, or even 

 entirely ignored. The vast flights of Common Jays 

 (Garnilus glandarius), for instance, that were noticed 

 passing Heligoland in the autumn of 1882, for 

 three days in succession, was probably an irruptic 

 emigration of surplus population from a congested 

 district. Similar irruptic waves of Goldcrests (Re- 

 giiliis cristatas) are occasionally remarked. Such 

 instances as the above are more or less exceptional 

 events at the present day, a period, as I have already 

 remarked, of long-continued stability, primarily due 

 to low eccentricity of the earth's orbit ; but they 

 enable us to form some slight idea of what Emi- 

 gration must have been during epochs of great 

 disturbance. 



