BIRDS. 483 



over the other it would soon have been multiplied to the 

 exclusion of the latter. If, for instance, the male pied 

 ravens, instead of being persecuted by their comrades, had 

 been highly attractive (like the above pied peacock) to the 

 black female ravens their numbers would have rapidly 

 increased. And this would have been a case of sexual 

 selection. 



With rpspect to the slight individual differences which 

 are common, in a greater or less degree, to all the mem- 

 bers of the same species, we have every reason to believe 

 that they are by far the most important for the work of 

 selection. Secondary sexual characters are eminently 

 liable to vary, both with animals in a state of nature and 

 under domestication.* There is also reason to believe, as 

 we have seen in our eighth chapter, that variations are 

 more apt to occur in the male than in the female sex. All 

 these contingencies are highly favorable for sexual selec- 

 tion. Whether characters thus acquired are transmitted to 

 one sex or to both sexes depends, as we shall see in the 

 following chapter, on the form of inheritance which 

 prevails. 



It is sometimes difficult to form an opinion whether cer- 

 tain slight differences between the sexes of birds are simply 

 the result of variability with sexually limited inheritance 

 without the aid of sexual selection or whether they have 

 been augmented through this latter process. I do not here 

 refer to the many instances where the male displays splen- 

 did colors or other ornaments of which the female partakes 

 to a slight degree; for these are almost certainly due to 

 characters primarily acquired by the male having been 

 more or less transferred to the female. But what are we to 

 conclude with respect to certain birds in which, for 

 instance, the eyes differ slightly in color in the two sexes ?f 

 In some cases the eyes differ conspicuously; thus with the 

 storks of the genus XenorhyncMis, those of the male are 

 blackish - hazel, while those of the female are gamboge- 

 yellow; with many hornbills (Buceros), as I hear from Mr. 

 Blyth,| the males have intense crimson eyes, and those of 



*0n these points see also " Variation of Animals and Plants under 

 Domestication," vol. i, p. 253; vol. ii, pp. 73, 75. 



j- See, for instance, on the irides of a Podica and Gallicrex in 

 " Ibis," vol. ii, 1860, p, 206; and vol. v. 1863, p. 426. 



:}:See also Jerdon, " Birds of India," vol. i, pp. 243-245, 



