136 SEXUAL selection: birds. PartIL 



domestic fowl and pigeon. The barbs coalesce towards 

 the extremity of the shaft to form the oval disc or 

 ocellus, which is certainly one of the most beautiful 

 objects in the world. This consists of an iridescent, 

 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 disc per- 

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

 of the concentric zones are destitute, to a greater or 

 less degree, of their barbules, so that a part of the disc 

 is surrounded by an almost transparent zone, which 

 gives to it a highly-finished aspect. But I have else- 

 where described*^ 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 por- 

 " tions 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 surrounding zones like- 

 wise shew traces, as may be seen in the drawing 

 (fig. 53), of indentations, or rather breaks. These in- 

 dentations are common to the Indian and Javan pea- 

 cocks (Pavo cristatus and P. muiicus) ; and they seemed 

 to me to deserve particular attention, as probably con- 

 nected 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 

 must formerly have existed many species which pre- 

 sented every successive step between the wonderfully 

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



•^' 'Variation of Animals and Plants under Domestication,' vol. i. 

 p. 254. 



