BIRDS. 489 



gradation could be given, it seems the best plan to take 

 one or two strongly marked cases, for instance that of the 

 peacock, in order to see if light can be thrown on the steps 

 by which this bird has become so splendidly decorated. 

 The peacock is chiefly remarkable from the extraordinary 

 length of his tail-coverts; the tail itself not being much 

 elongated. The barbs along nearly the whole length of 

 these feathers stand separate or are decomposed; but this 

 is the case with the feathers of many species and with some 

 varieties of the domestic fowl and pigeon. The barbs 

 coalesce toward the extremity of the shaft forming the 

 oval disk or ocellus, which is certainly one of the most 

 beautiful objects in the world. It consists of an iridescent, 

 intensely blue, indented center, surrounded by a rich green 

 zone, this by a broad coppery-brown zone, and this by five 

 other narrow zones of slightly different iridescent shades. 

 A trifling character in the disk deserves notice; the barbs 

 for a space along one of the concentric zones are more or 

 less destitute of their barbules, so that a part of the disk is 

 surrounded by an almost transparent zone, which gives it a 

 highly finished aspect. But I have elsewhere described* 

 an exactly analogous variation in the hackles of a sub- 

 variety of the gamecock in which the tips, having a metallic 

 luster, " are separated from the lower part of the feather 

 by a symmetrically shaped transparent zone composed of 

 the naked portions of the barbs." The lower margin or 

 base of the dark blue center of the ocellus is deeply 

 indented on the line of the shaft. The surrounding zones 

 likewise show traces, as may be seen in the drawing (fig. 

 54), of indentations, or rather breaks. These indentations 

 are common to the Indian and Javan peacocks (Pavo 

 cristatus and P. muticus) ; and they seem to deserve 

 particular attention as probably connected with the devel- 

 opment of the ocellus; but for a long time I could not 

 conjecture their meaning. 



If we admit the principle of gradual evolution there 

 must formerly have existed many species which presented 

 every successive step between the wonderfully elongated 

 tail-coverts of the peacock and the short tail-coverts of all 

 ordinary birds; and again between the magnificent ocelli 



* "Variation of Animals and Plants under Domestication," vol. i. 

 p. 254. 



