n8 THE DARWINIAN THEORY 



perate and cold countries, yet the majority of tropical 

 birds are dull-coloured, and in some groups the 

 most brightly-coloured members are not tropical : 

 for instance, the Arctic ducks and divers are more 

 handsome than the tropical ones. The humming- 

 birds found in the Andes form another instance, for 

 here they are confined to lofty mountains, some- 

 times to a particular mountain, "just beneath the 

 line of perpetual snow, at an elevation of some 

 16,000 feet, dwelling in a world of almost constant 

 hail, sleet, and rain." Again, most tropical and 

 brightly-coloured birds are denizens of the forests, 

 and shaded from the direct action of the sun ; they 

 abound also near the equator, where cloudy skies 

 are very prevalent. In the case of flowers, Wallace 

 remarks that " in proportion to the whole number of 

 species of plants, those having gaily-coloured flowers 

 are actually more abundant in the temperate zones 

 than between the tropics." 



Non-significant Colours. 



Many colours are the incidental results of chemical 

 and physical structure, for the same reason that sul- 

 phate of copper is blue. The red colour of blood, the 

 white colour of fat, the silvery colour of the bladder 

 of many fish, the pigmented condition of the frog's 

 peritoneum, and the green tint of the bones of many 

 fish, are examples. The brilliant and varied colours of 

 deep-sea animals are probably devoid of any significa- 

 tion, and the green colour of grass and the blue colour 

 of the sky, for all we know, are non-significant. 



