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Show .'EXUAL i;ELEC'l'JON : BIHD.'. l'.\lt'l' n. vVithrcspcr·t to the slight individual differences wb ich arc common, in a greater or lrss degree, to all th ' mcmhers of the same species, we have every reason to bclicYe that they arc by far the most important for the work of selection. Secondary sexual characters arc eminently liable to vary, Loth with animals in a ~:!!tate of nature and under domestication.40 There is also rca on to believe, as we have seen in our eighth chapter, that variations ar more apt to occur in the male than in the female sex. All these contingencies arc highly favourable for s xual selection. Whether characters thus acquired are transmitted to one sex or to Loth sexes, depends exclusively in most cases, a I hope to shew 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 sliO'ht differences between the sexes of birds arc simpl; the result of variability with sexuallylimited inheritance, without the aid of sexual selection, or whether they have been augmented through this latter process. I do not here refer to the innumerable instances in which the male displays splendid colours or other ornaments, of which the female partakes only to a slight degree; for these cases are almost certainly clue to characters primarily acquired by the male, having been transferred, in n, greater or less degree, to the female. But what arc we to conclude with respect to certain birds in which, for instance, the eyes differ sliO'htly in colour in the two sexes? n In some cases th~ ey s differ conspicuously; thus with the storks <~O On the c points sec al ·o 'V nriation of Animals and Plants under Domestication,' vol. i. p. 253; vol. ii. p. 73, 75. 41 Se , for iust:mce, on the irides of a Podiea anJ Gallicrex in 'Ibis,' vul. ii. 1 GO, p. 20G; nncl vol. v. 1803, p. 4:W. CHAP. XIV. VARIABILITY. 129 of the genus Xenorhynchus those of the male are blackish-haze], whilst those of the females are gamLoge- yellow; with many hornbills (Buceros), as I hear from Mr. Blyth,42 the males have intense crimson, and the females white eyes. 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 Chili 43 that the iris in the condor, when about a year old, is dark-brown, but changes at maturity into yellowish-brown in the male, and into bright red in the female. The male has also a small, longitudinal, leaden-coloured, fleshy crest or comb. ·with many . gallinaceous birds the comb is highly ornamental, and assumes vivid colours during the act of courtship; but what are we to think of the dull-coloured comb of the condor, which does not appear to us in the least ornamental? The same question may be asked in regard to various other characters, such as the knob on the base of the beak of the Chinese goose (Anser cygnoides), which is much larger in the male than in the female. No co1·tain answer can be given to these questions; but we ought to be cautious in assuming that knobs and various fleshy appendages cannot be attractive to the female, when we remember that with savage races of man 42 Sec A.lso J erdon, 'Birds of Indio,' vol. i. p. 243-245. ' 1 3 'Zoology of the Voyage of H.M.S. Beagle,' 1841, p. 6. w~~ K |
