430 The Descent of Man. Tabt II, 



Grad-ttion of Secondary Sexual Characters. — Cases of gradation 

 are important, as shewing 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 magniiicent colours 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 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 towards the extremity of the shaft forming 

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

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

 tensely blue, indented centre, 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 of 

 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 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 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 surrounding zones likewise shew 

 traces, as may be seen in the drawing (fig. 54), of indentations, 

 or rather breaks. These indentations are common to the Indian 

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

 seemed to deserve particular attention, as probably connected 

 with the development of the ocellus ; but for a long time I could ' 

 pot conjecture their meaning. 



*• ' Variation of Aaimab and Plants under Domestication,' vol. i. p. 2^4 



