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Show 27G '£HE PRINCIPLES OF PAnT II. are said to occur. Dr. Burt vVilder 18 has tabulated the cases of 152 individuals with supernumerary digits, of which 86 were males, and 39, or less than ha1f, females ; the remaining 27 being of unknown sex. It should not however be overlooked that women would more freq~ently e;deavour to conceal a deformit.y of this kind than men. Whether the large proportwnal uum ber of deaths of the male offspring of man and apparently of sheep, compared with the female offspring, before, during, and shortly after birth (see supplement), has any relation to a strono·er tendency in the organs of the male to vary and tl~us to become abnormal in structure or function, I will not pretend to conjecture. In various classes of animals a few exceptional cases occur in which the female instead of the male has acquired well pronounced secondary sexual characters, such as brighter colours, greater size, strength, or pugnacity. vVith birds, as we shall hereafter see, there has sometimes been a complete transposition of the ordinary characters proper to each sex ; the females having become the more eager in courtship, the males remaining comparatively passive, but apparently selecting, as we may infer from the results, the more attractive females. Certain female birds have thus been rendered more highly coloured or otherwise ornamented, as well as more powerful and pugnacious than the males, these characters being transmitted to the female offspring alone. It may be suggested that in some cases a double process of selection has been carried on ; the males having selected the more attractive females, and the latter the more attractive males. This process however, though it might lead to the · modification of both sexes, IS 'Massachusetts Medical Soc.' vol. ii. No. 3, 1868, p. (). CHAP. VIII. SEXUAL SELECTION. 277 would not make the one sex different from the other, unless indeed their taste for the beautiful differed· but thi.s is a supposition too improbable in the case of any ammal, excepting man, to be worth considering. There are, however, many animals, in which the sexes resemble each other, both being fur-nished with the same ornaments, which analogy would lead us to attribute to the agency of sexual selection. In such cases it may be suggested with more plausibility, that there has been a double or mutual process of sexual selection; the more vigorous and precocious females having selected the m?re attractive and vigorous males, the latter having reJected all except the more attractive females. But from what we know of the habits of animals, this view is hardly probable, the male being generally eager to pair with any female. It is more probable that the ornaments common to both sexes were acquired by one sex, generally the male, and then transmitted to the offspr~ ng of both sexes. If, in~eecl, during a lengthened perwd the males of any spemes were greatly to exceed the females in number, and then during another lengthened period under different conditions the reverse were to occur, a double, but not simultaneous, process of sexual selection might easily be carried on, by which the two sexes might be rendered widely different. We shall hereafter see that many animals exist, of which neither sex is· brilliantly coloured or provided with special ornaments, and yet the members of both sexes or of one alone have probably been modified through sexual selection. The absence of bright tints or other ornaments may be the result of yariations of the right kind never having occurred, or of the animals themselves preferring simple colours, such as plain black or white. Obscure colours have often been acquired through natural selection for the sake of protection, ancl |