1919-] Geographical Distribution and Migration. 381 



nuptial display,, the thickness or extent of the feathered 

 regions to climate, and variation in colour to climate or 

 local surroundings or food. A high temperature, a dry 

 atmosphere, and a bright light seem to produce that bleached 

 effect usual in desert forms. A temperate climate, moist air, 

 and a dull sky tend to dark plumage. Alpine and Arctic 

 forms display more white than is noticeable in the same bird 

 from farther south or from lower altitudes ; though we see 

 in the case of Corvus comix capellanus the brilliant glare of 

 the Persian Gulf having the same effect on plumage as the 

 glare from the Arctic snows has on many northern forms. 



It is curious that it is the influence of the breeding- 

 quarters which causes differentiation, the winter-quarters 

 and regions traversed on migration having little effect on 

 coloration or structure. Wide-ranging and common species 

 show the most variation, so long as their breeding-area is 

 not restricted, as in some of the Polar breeding-species. 

 It therefore seems likely that it is the nursery which tends 

 to differentiation. This is most remarkable in such birds 

 as Cuculus canorus and Micropus apus, whose nursery-life 

 scarcely extends to a quarter of the whole year, and among 

 which several well-marked geographical forms exist which 

 in some cases share a common winter-quarters. 



But it seems by no means proved that the breeding- 

 quarters of a species is necessarily its real home, though it 

 is undeniable that the present breeding-quarters of a species 

 produces the homing influence on spring migration. 

 Seebohm (Geogr. Dist. of the Charadriidse) has already 

 pointed out that it is possible that the present winter- 

 quarters of migratory birds breeding in northern latitudes 

 coincide with the old breeding-quarters of the same bird's 

 ancestors in the Post-Pliocene Glacial Period. It seems 

 probable that a species with a confined breeding-area and an 

 extensive range in winter had its original home in the con- 

 finied breeding-area to which it is most attached, for this area 

 is much more exact and local in influencing the bird's life, 

 and becomes the focus of its migrations. On the other hand, 

 it may be that a species with a wide breeding-range and a 



2 £ 3 



