488 THE DESCENT OF MAN. 



female represent the beautiful white spots in the male;* 

 and something of the same kind may be observed in the 

 two sexes of the Argus pheasant. However this may be, 

 appearances strongly favor the belief that, on the one hand' 

 a dark spot is often formed by the coloring matter being 

 drawn toward a central point from a surrounding zone, 

 which latter is thus rendered lighter; and, on the other 

 hand, that a white spot is often formed by the color being 

 driven away from a central point, so that it accumulates in 

 a surrounding darker zone. In either case an ocellus is the 

 result. The coloring matter seems to be a nearly constant 

 quantity, but is redistributed, either centripetally or cen- 

 trifugally. The feathers of the common guinea-fowl oifer 

 a good instance of white spots surrounded by darker zones; 

 and wherever the white spots are large and stand near each 

 other the surrounding dark zones become confluent. In 

 the same wing-feather of the Argus pheasant dark spots 

 Tnay be seen surrounded by a pale zone and white spots by 

 a dark zone. Thus the formation of an ocellus in its most 

 elementary state appears to be a simple affair. By what 

 further steps the more complex ocelli, which are surrounded 

 by many successive zones of color, have been generated, I 

 will not pretend to say. But the zoned feathers of the 

 mongrels from differently colored fowls, and the extraor- 

 dinary variability of the ocelli on many Lepidoptera, lead 

 us to conclude that their formation is not a complex 

 process, but depends on some slight and graduated change 

 in the nature of the adjoining tissues. 



Gradation of Secondary Sexual Characters. 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 magnificent 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 clew 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 



* Jeydon, " Birds of India," vol. iii, p, 517. 



