"The naked skin on the head of a vulture is generally considered as a direct adaptation for wallowing in putridity; and so it may be, _or it may possibly be due to the direct action of the putrid matter_; but we should be very cautious in drawing any such inference [i.e. as to utility] when we see the skin on the head of the clean-feeding male Turkey is likewise naked[158]." [157] _Ibid._ [158] _Origin of Species_, pp. 157-8. Similarly, in the _Descent of Man_ it is said:-- "Variations of the same _general_ nature have _often been taken advantage of_ and accumulated through sexual selection in relation to the propagation of the species, and through natural selection in relation to the general purposes of life. Hence, _secondary sexual characters, when equally transmitted to both sexes, can be distinguished from ordinary specific characters, only by the light of analogy_. The modifications acquired through sexual selection are often so strongly pronounced that the two sexes have frequently been ranked as distinct species, or even as distinct genera[159]." [159] _Descent of Man_, p. 615. As Mr. Wallace does not recognize sexual selection, he incurs the burden of proving utility (in the life-preserving sense) in all these "frequently" occurring cases where there are such "strongly pronounced modifications," and we have already seen in the text his manner of dealing with this burden. But the point here is, that whether or not we accept the theory of sexual selection, we must accept it as Darwin's opinion--first, that in their beginnings, as _specific_ characters, these sexual modifications were often of a merely "_general nature_" (or without reference to utility even in the life-embellishing sense), and only _afterwards_ "have often been taken advantage of and accumulated through _sexual_ selection": and, secondly, that "we know they have been acquired in some instances _at the cost not only of inconvenience, but of exposure to actual dangers_[160]." [160] _Ibid._ We may now pass on to some further, and even stronger, expressions of opinion with regard to the frequent inutility of _specific_ characters.