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 

 (Ossifraga 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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