130 SEXUAL SELECTION: BIRDS. [Past II. 



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 to form 

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

 beautiful objects in the world. This consists of an irides- 

 cent, intensely blue, indented* centre, surrounded by a 

 rich green zone, and 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 per- 

 haps deserves notice ; the barbs, for a space along one of 

 the concentric zones are destitute, to a greater or less de- 

 gree, of their barbules, so that a part of the disk is sur 

 rounded by an almost transparent zone, which gives to it 

 a highly - finished aspect. But I have elsewhere de- 

 scribed 4T an exactly analogous variation in the hackles of 

 a sub-variety of the game-cock, in which the tips, having 

 a metallic lustre, " 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 centre of the ocellus is 

 deeply indented on the line of the shaft. The surround- 

 ing zones likewise show traces, as may be seen in the 

 drawing (Fig. 53), of indentations, or rather breaks. 

 These indentations are. common to the Indian and Javan 

 peacocks (Pavo cristatus and P. muticus) ; and they 

 seemed to me to deserve particular attention, as probably 

 connected with the development of the ocellus ; but for a 

 long time I could not conjecture their meaning. 



If we admit the principle of gradual evolution, there 



* 7 ' Variation of Animals and Plants under Domestication,' vol. i. p. 254. 



