SEXUAL SELECTION 549 



are highly favorable for sexual selection. Whether charac- 

 ters 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 con- 

 clude with respect to certain birds in which, for instance, 

 the eyes differ slightly in color in the two sexes ? ** In some 

 cases the eyes differ conspicuously; thus with the storks of 

 the genus Xenorhynchus, those of the male are blackish 

 hazel, while those of the females are gamboge-yellow; with 

 many hornbills (Buceros), as I hear from Mr. Blyth,*' the 

 males have intense crimson eyes, and those of the females 

 are white. In the Buceros bicornia, the hind margin of the 

 casque and a stripe on the crest of the beak are black in 

 the male, but not so in the female. Are we to suppose that 

 these black marks and the crimson color of the eyes have 

 been preserved or augmented through sexual selection in 

 the males? This is very doubtful; for Mr. Bartlett showed 

 me in the Zoological Gardens that the inside of the mouth 

 of this Buceros is black in the male and flesh colored in the 

 female; and their external appearance or beauty would not 

 be thus affected, I observed in Chile" that the iris in the 

 condor, when about a year old, is dark broMm, but changes 

 at maturity into yellowish brown in the male, and into bright 

 red in the female. The male has also a small, longitudinal, 



''* See, for instance, <» the irides of a Podica and Gallicrez in "IbiB," 

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



^ See also Jerdon, "Birds of India," vol. i. pp. 243-346. 

 « "Zoology of the Voyage of H.M.S. Beagle, " 1841, p. 6. 

 Descent— Toi.. 11.-6 



