Chap. XIV.] GRADATION OF CHARACTERS. 129 



centripetally or centrifugally. The feathers of the com- 

 mon guinea-fowl offer a good instance of white spots sur- 

 rounded by darker zones ; and whenever the white spots 

 ai'e large and stand near each other, the surrounding dark 

 zones become confluent. In the same wing-feather of the 

 Argus pheasant dark spots may be seen surrounded by a 

 pale zone, and white spots by a dark zone. Thus the for- 

 mation of an ocellus in its simplest 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 bearing in mind the zoned feathers of the mongrel 

 offspring from differently-colored fowls, and the extraor- 

 dinary variability of the ocelli in many Lepidoptera, the 

 formation of these beautiful ornaments can hardly be a 

 highly-complex process, and probably depends on some 

 slight and graduated change in the nature of the tissues. 



Gradation of Secondary Sexual Characters. — Cases 

 of gradation are important for us, as they show that it is 

 at least possible that highly-complex ornaments may have 

 been acquired by small successive steps. In order to dis- 

 cover the actual steps by which the male of any existing 

 bird has acquired his magnificent colors or other orna- 

 ments, we ought to behold the long line of his ancient and 

 extinct progenitors ; but this is obviously impossible. 

 "We may, however, generally gain a clew by comparing all 

 the species of a group, if it be a large one ; for some of 

 them will probably retain, at least in a partial manner, 

 traces of their former characters. Instead of entering on 

 tedious details respecting various groups, in which strik- 

 ing instances of gradation could be given, it seems the 

 best plan to take some one or two strongly-characterized 

 cases, for instance that of the peacock, in order to dis- 

 cover if any light can thus be thrown on the steps by 



