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. (43. On these points see also Variation of Animals and
Plants under Domestication, vol. i. p. 253; vol ii. pp. 73, 75.) 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 favourable for sexual selection.
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 certain 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 splendid colours 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
colour in the two sexes? (44. See, for instance, on the irides of a
Podica and Gallicrex in Ibis, vol. ii. 1860, p. 206; and vol. v.
1863, p. 426.) In some cases the eyes differ conspicuously; thus with
the storks of the genus Xenorhynchus, those of the male are
blackish-hazel, whilst those of the females are gamboge-yellow; with
many hornbills (Buceros), as I hear from Mr. Blyth (45. See also
Jerdon, Birds of India, vol. i. pp. 243-245.), the males have intense
crimson eyes, and those of the females are white. In the Buceros
bicornis, 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 colour of the eyes have
been preserved or augmented through sexual selection in the males? This
is very doubtful; for Mr. Bartlett shewed me in the Zoological Gardens
that the inside of the mouth of this Buceros is black in the male and
flesh-coloured in the female; and their external appearance or beauty
would not be thus affected. I observed in Chile (46. Zoology of the
Voyage of H.M.S. Beagle, 1841, p. 6.) that the iris in the condor,
when about a year old, is dark-brown, but changes at maturity into
448