SlRBS. 501 



they are, on tlie contrary, often broken up into smaller 

 spots, so that two or three rows run down to the same 

 ocellus. 



There still remains another very curious point, first 

 observed by Mr. T. W. Wood,* which deserves attention. 

 In a photograph, given me by Mr. Ward, of a specimen 

 mounted as in the act of display, it may be seen that on the 

 feathers which are held perpendicularly, the white marks 

 on the ocelli, representing light reflected from a convex 

 surface, are at the upper or farther end, that is, are directed 

 upward; and the bird while displaying himself on the 

 ground would naturally be illuminated from above. But 

 ^here comes the curious point, the outer feathers are held 

 almost horizontall}^, and their ocelli ought likewise to 

 appear as if illuminated from above, and consequently the 

 white marks ought to be placed on the upper sides of the 

 ocelli; and, wonderful as is the fact, they are thus placed! 

 Hence the ocelli on the several feathers, though occupying 

 very different positions with respect to the light, all appear 

 as if illuminated from above, just as an artist would have 

 shaded them. Nevertheless they are not illuminated from 

 strictly the same point as they ought to be; for the white 

 marks on the ocelli of the feathers, which are held almost 

 horizontally, are placed rather too much toward the farther 

 end; that is, they are not sufficiently lateral. We have, 

 however, no right to expect absolute perfection in a part 

 rendered ornamental through sexual selection, any more 

 than we have in a part modified through natural selection 

 for real use; for instance, in that wondrous organ the 

 human eye. And we know what Helmholtz, the highest 

 authority in Europe on the subject, has said about the 

 human eye, that if an optician had sold him an instrument 

 so carelessly made, he would have thought himself fully 

 justified in returning it. f 



We have now seen that a perfect series can be followed, 

 from simple spots to the wonderful ball-and-socket orna- 

 ments. Mr. Gould, who kindly gave me some of these 

 feathers, fully agrees with me in the completeness of the 

 gradation. It is obvious that the stages in development 



*TLe " Field," May 28, 1870. 



f " Popular Lectures on Scientific Subjects," Eng. trans., 1873, pp. 

 219, 227, 269. 390. 



