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Show 23! SEXUAL SELECTION : BillDS. PAnT Il. exhibit a marked preference or antipathy for certain individual males. If it be admitted that tho females prefer, or arc unconsciously excited by the more beautiful males, then the males would slowly but surely be r~ndorc~ m01:o and more attractive through sexual selectwn. Th~t 1t is this sex which has been chiefly modified we may mfer from the fact that in almost every genu!:! in which the sexes differ, the males differ much more from each other than do tho females; this is well shewn in certain closelyallied representative species in which tho femal.es c~n hardly be distinguished, wl1ilst the ma~es .a1:e qmte. distinct. Birds in a state of nature offer md1 v1dual differences which would amply suffice for the work of sexual selection; but we have seen that they occasionally present more strongly-marked variations which recur s?. frequently that they would immediately be fix~d'. If they served to allure the female. Tho l~w~ .of var1at10n will have determined tho nature of the Imt1al changes, and largely influenced the final result. The grada. tions, which may be observed between the males. of allied species, indicate the nature of the steps wh10h have been passed through, and explain in the most interesting manner certain characters, such as tho indented ocelli of the tail-feathers of tho peacock, and .the wonderfully-shaded ocelli of the wing-feath~r~ of the Argus pheasant. It is evident that the bnlllant colours top-knots fine plumes, &c., of many malo birds ~annot ha~e been acquired as a protection; indeed they sometimes load to danger. That they are not clue to the direct and definite action of tho conditions of life, we may feel assured, because the females have been exposed to the same conditions, and yet often differ from the males to an extl:e~e deg1:ee. Although it is probable that changed conditions actmg CHAP. XVl. SUMMAllY. 235 during a lengthened period have produced some definite effect on both sexes, the m01·e important result will have been an increased tendency to fluctuating variability or to augmented individual differences; and such differ ·ences will have afforded an excellent groundwork for the action of sexual selection. The laws of inheritance, irrespectively of selection, appear to have determined whether the characters acquired by the males for the sake of ornament, for producing various sounds, and for fighting together, have been transmitted to the males alone or to both sexes, either permanently or periodically during certain seasons of the year. Why various characters should sometimes have been transmitted in one way and sometimes in another is, in most cases, not known; but the period of variability seems often to have been the determining cause. When the two sexes have inherited all characters in common they necessarily resemble each other; but as the successive variations may be differently transmitted, every possible gradation may be found, even within the same genus, from the closest similarity to the widest dissimilarity between the sexes. With many closely-allied species, following nearly the same habitl:l of life, the males have come to differ from each other chiefly through the action of sexual selection; whil. t the females have come to differ chiefly from partaking in a greater or lesser degree of the characters thus acquired by the males. rn1e effects, moreover, of the definite action of the conditions of life, will not have been masked in the females, as in the case of the males, by the accumulation through sexual selection of stronglypronounced colours and other ornaments. The individuals of both sexes, however affected, will have been kept at each successive period nearly uniform by the ft·ee intercrossing of many individuals. |
