The Making of Species 
those whose legs are not yellow. Fowls which 
have yellow legs have also yellow skins. In this 
connection the almost universal assumption of 
orange feet by domestic guinea-fowls is sig- 
nificant. Normally the feet of these birds are 
black, and their natural African habitat is a dry 
one. 
A grey or white colour appears to be corre- 
lated with resistance to cold. In birds this may 
perhaps be explained by the fact that the feathers 
in some light-coloured varieties are longer than 
in those of normally-coloured ones. Thus mealy- 
coloured canaries have longer feathers than 
brightly-coloured ones. 
The Arctic Skua, having no enemies to fear, 
stands in no need of protective colouration. It 
would therefore seem that the white-breasted 
form of this bird becomes more numerous as it 
nears the north pole, not because of the closer 
assimilation of its plumage to the colour of the 
snowy surroundings, but because the bird has to 
resist the greater degree of cold the farther north 
it finds itself. Similarly, in the region of the 
south pole the albino form of the Giant Petrel 
(Osstfraga gigantea) becomes common. Both 
these birds are themselves predatory and not 
liable to be preyed upon. 
The curious china-white legs of some desert 
birds—as, for example, coursers and larks—would 
seem to indicate a power of resisting the hot rays 
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