424 THE DESCENT OF MAN. 



ocelli on. many Lepldoptera, lead us to concluds tliat their forma- 

 tion is not a complex process, but depends on some slight and 

 graduated change in the nature of the adjoining tissues. 



Gradation of Secondary Sexual Charaeters. — Cases of gradation 

 are important, as showing us that highly complex ornaments 

 may be acquired by small successive steps. In order to discover 

 the actual steps by which the male of any existing bird has 

 acquired his magniflSent colors or other ornaments, we ought 

 to behold the long line of his extinct progenitors; but this is 

 obviously impossible. We may, however, generally gain a clue 

 by comparing all the species of the same group, if it be a large 

 one; for some of them will probably retain, at least partially, 

 traces of their former characters. Instead of entering on tedious 

 details respecting various groups, in which striking instances 

 of gradation c6uld 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 towards the extremity of the shaft forming 

 the oval disc or ocellus, which is certainly one of the most beau- 

 tiful 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 disc deserves notice; the barbs, for a space along one ot 

 the concentric zones are more or less destitute of their barbules, 

 so that a part of the disc is surrounded by an almost transparent 

 zone, which gives it a highly flnlshed 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 luster, "are separated from the lovv^er 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 j'avan peacocks 

 (Pavo cristatus and P. muticus) ; and they seemed to deserve par- 

 ticular attention, as probably connected with the development of 



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



