136 
SEXUAL selection: bikes. 
Part II. 
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 bine, indented centre, surrounded by a ricli 
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. 
