Chap. XI\ r .] VARIABILITY. 123 



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 selection. 

 "Whether characters thus acquired are transmitted to one 

 sex or to both sexes, depends exclusively in most cases, as 

 I hope to show in the following chapter, on the form ' of 

 inheritance which prevails in the groups in question. 



It is sometimes difficult to form any opinion whether 

 certain slight differences between the sexes of birds are 

 simply the result of variability with sexually-limited in- 

 heritance, without the aid of sexual selection, or whether 

 thoy have been augmented through this latter process. I 

 do not here refer to the innumerable instances in which 

 the male displays splendid colors or other ornaments, of 

 which the female partakes only to a slight degree ; for 

 these cases are almost certainly due to characters primarily 

 acquired by the male, having been transferred, in a greater 

 or less degree, 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 ? 41 In 

 some cases the eyes differ conspicuously ; thus with the 

 storks of the genus Xenorhynehus 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, 42 the males have intense crimson, and the fe- 

 males white eyes. In the Huceros bicornis, the hind mar- 

 gin 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 

 Bupjfose that these black marks and the crimson color of 

 the eyes have been preserved or augmented through sex- 

 ual selection in the males? This is very doubtful; for 

 Mr. Bartlett showed me in the Zoological Gardens that 



41 See, for instance, on the irides of a Podica and Gallicrex. in c Ibis/ 

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



42 See also Jerdon, ' Birds of India,' vol. i. pp. 243-245. 



