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—ze., 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 
