Ohap. XIV. Birds — Gradation of Characters. 441 



There still remains another very cnrious point, first observed 

 by Mr. T. W. Wood, 51 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 

 further end, that is, are directed upwards ; and the bird whilst 

 displaying himself on the ground would naturally be illuminated 

 from above. But here comes the curious point, the outer 

 feathers are held almost horizontally, and their ocelli ought like- 

 wise 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. Never- 

 theless they are not illuminated from strictly the same point 

 as they ought to be ; for the wdrite marks on the ocelli of the 

 feathers which are held almost horizontally, are placed rather 

 too much towards the further end ; that is they are not suffi- 

 ciently lateral. We have, however, no right to expect absolute 

 perfection in a part rendered ornamental through sexual selec- 

 tion, 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, ho 

 would have thought himself fully justified in returning it. 52 



W T e have now seen that a perfect series can be followed, from 

 simple spots to the wonderful ball-and-socket ornaments. 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 exhibited by the feathers on the 

 same bird, do not at all necessarily shew us the steps passed 

 through by the extinct progenitors of the species ; but they 

 probably give us the clue to the actual steps, and they at least 

 prove to demonstration that a gradation is possible. Bearing in 

 mind how carefully the male Argus pheasant displays his plumes 

 before the female, asw 7 ell as the many facts rendering it probable 

 that female birds prefer the more attractive males, no one who 

 admits the agency of sexual selection in any case, will deny that 

 a simple dark spot with some fulvous shading might be converted, 

 through the approximation and modification of two adjoining 



61 The 'Field/ May 28, 1870. Subjects,' Eng. trans. 1873, pp. 219, 



Popular Lectures on Scientific 227, 269, 390. 

 20 



