384 Lieut.-Col. R. Meinertzhagen on [Ibis, 



Geographical distribution and migration have been in the 

 past, are now, and always will be fluctuating, sometimes 

 imperceptibly, sometimes by leaps and bounds. The same 

 applies to the geographical races of a species. As distribu- 

 tion and migration alter, so do subspecies become evolved, 

 usually very gradually, but sometimes within the lifetime 

 o£ man. But the problems remain constant, and the laws 

 which govern these problems change but little. 



The extent of the geographical distribution or range of a 

 species, on which largely hinges the differentiation in both 

 species and subspecies, is due to :- — 



1. Gradual expansion or contraction. 



2. Periodic and regular migration. 



3. Sporadic migration, invasion, or extensive wanderings. 



4. Human agency, direct or indirect. 



A few cases will be taken to illustrate these problems 

 which so closely link Distribution, Migration, and Ditfereuti- 

 ation among birds. 



1. Gradual expansion or contraction. 



Birds have been known to gradually extend their range 

 into every point of the compass, and it will probably be found 

 that normal expansion radiates from the bird's original home. 

 It is interesting to note that the Charadriidse are believed by 

 Seebohm to have originated in the north, and the Swallows 

 have been credited with an early home not far removed from 

 the tropics. 



But it is more recent and current movement which now 

 concerns us. 



An example of gradual expansion to the south is well 

 illustrated by the range of the Crested Lark [Galerida 

 cristata and its subspecies), whose original home was 

 probably central and western Asia. This species has now 

 amplified its distribution from France to Corea, and south to 

 Sierra Leone and Senegambia on the west coast of Africa 

 and Abyssinia and Somalilaud on the east coast, and to 

 Ceylon. It would appear from an examination of this 



