MIGRATION — MEINERTZHAGEN. 343 
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 differentiation 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 ex- 
pansion radiates from the bird's original home. It is interesting 
to note that the Charadriidae are believed by Seebohm to have origi- 
nated 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 {Galerlda cristata and its subspecies), 
whose original home was probably central and western Asia. This 
species has now amplified its distribution from France to Korea; 
and south to Sierra Leone and Senegambia, on the west coast of 
Africa, and Abyssinia and Somaliland, on the east coast; and to 
Ceylon. It would appear from an examination of this distribution 
that expansion has followed coast lines, which, as pointed out by 
Hartert (Novit. Zool., xx, 1913, p. 76), is a tendency not only among 
migratory but among such sedentary species as the white owl, chough, 
cirl bunting, and others. But here, in the case of Galerida cristata^ 
we see expansion and differentiation progressing concurrently; and 
there can be little doubt that the crested lark, a hardy species capable 
of residence in the snows of central Europe and Asia or in the heat 
of the Red Sea littoral, will not check its expansion till the Cape 
Seas arrest its progress. Its advent on the west coast of Europe is 
probably of comparatively recent date, for it has never established 
itself in Great Britain, though there can be little doubt it would have 
done so during the last century if its efforts had not been checked by 
the greed for rare birds. 
The shore lark {Eremophila alpestris flava), which in compara- 
tively recent times has become a common breeding species in Arctic 
Norway, affords a good illustration of gradual expansion to the west. 
At the same time as expansion of breeding range, these birds opened 
out a new line of migration about 1847 (Gaetke) and became a com- 
mon bird of passage at Heligoland in spring and autumn. This fact 
is of particular interest, as other northern species {Phylloscopus 
horealis horealis, Anthus gustavi^ and Emheriza pusiUa) have, in 
spite of westward extension of their breeding range, rigidly adhered 
to their ancient migration route and winter quarters in southeast 
Asia. Cooke (Migration of Birds, p. 6) further illustrates the 
