302 THE STRUCTURE AND LIFE OF BIRDS CHAP. 



sack on each side of its neck. The Umbrella Bird 

 has a kind of fleshy dewlap thickly covered with 

 scale-like blue feathers. The Cassowary carries a blue 

 horny elevation at the top of his head. 



Patterns. 



On first thoughts the patterns in which we find the 

 colours distributed on the surfaces of birds and butter- 

 flies seem to show an infinite variety. But on in- 

 vestigation it proves not nearly so great. There is 

 one law which always operates in wild animals, the 

 law of bilateral symmetry — i.e., the right and left sides 

 are always coloured very nearly alike. In birds, the 

 head is often of one colour, the breast of another, and 

 so forth ; but this can hardly be called a pattern. As 

 a rule the patterns are varieties of lines or spots, and 

 this is true, not only of birds, but of other classes of 

 animals ; among mammals, for instance, of the deer 

 and the great carnivora. The ocellus, or peacock eye, 

 is a spot in its most perfect development ; the centre 

 is surrounded by one or more rings of different colours 

 or of different shades. The delicate shading is more 

 beautiful in the ocelli of the Argus Pheasant than in 

 those of the Peacock, though the colours are not so 

 brilliant. And in this bird one and the same feather 

 shows how the ocellus grows out of a line through 

 the transition stage of a vague ellipse. As we look at 

 it, the work of Nature's " 'prentice hand " and mature 

 skill seem presented to us at once, suggesting the 

 gradual stages by which the species has developed 

 its magnificence. There was a time when Argus 



