EMIGRATION AND EVOIUTION 115 



More local, but none the less certain, causes of 

 ■emigration may be found in the great numerical 

 increase of species, rendering the dispersal of the 

 surplus population into new regions indispensable 

 and imperative. This may happen in two ways. 

 Either a vast wave of surplus population may 

 suddenly spread out from the congested districts — 

 an irruption, or even series of irruptions, within a 

 comparatively short period of time, flowing across 

 wide areas until gradually spent ; or, under favour- 

 able conditions, a species may slowly extend its 

 range from a comparatively small centre until it 

 ultimately covers an enormous area. As an example 

 of Irruptic Emigration we have the intensely interest- 

 ing emigrations of Pallas's Sand Grouse [Sjjrrhaptes 

 paradoxus) from Central Asia, which have from 

 time to time occurred with startling suddenness. 

 This species for the past fifty years or so has 

 evidently been in a very restless and disturbed 

 state, and from time to time great waves of emi- 

 grants have been thrown out apparently to relieve 

 a congested area of distribution. Pallas's Sand 

 Grouse normally is an inhabitant of the vast plains 

 •or steppes that stretch continuously from north-east 

 Turkestan and South Siberia to Mongolia. In the 

 north it is a migratory bird, and the winter range 

 extends into North China in the east and the 

 Kirghiz Steppes north of the Aral Sea in the west. 

 Until 1859 ^^^s species was practically unknown to 

 western ornithologists, although Russian naturalists 

 had met with it from time to time in its far eastern 



