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Show 298 THE PRINCIPLES OF PART II. were to vary at birth or soon afterwardR, in some manner which at this age was of no service to them, the chance of the preservation of such variations would be small. vVe have good evidence under domestication how soon variations of all kinds are lost, if not selected. But variations which occurred at or near maturity, and which were of immediate service to either sex, would probably be preserved; as would similar variations occurring at an earlier period in any individuals which happened to survive. As this principle bas an important bearing on sexual selection, it may be advisable to give an imaginary illustration. vVe will take a pair of animals, neither very fertile nor the reverse, and assume that after arriving at maturity they live on an average for five years, p~·oducing each year five young. They would thus produce 25 offspring; and it would not, I think, be an unfair estimate to assume that 18 or 20 out of the 25 would perish before maturity, whilst still young and inexperienced; the remaining seven or five sufficing to keep up the stock of mature individuals. If so, we can see that variations which occurred during youth, for instance in brightness, and which were not of the least service to the young, would run a good chance of being utterly lost. Whilst similar variations, which occurring at or near maturity in the comparatively few individuals surviving to this age, and which immediately gave an advantage to certain males, by rendering them more attractive to the females, would be likely to be preserved. No doubt some of the variations in brightness which occurred at an earlier age would by chance be preserved, and eventually give to the male the same advantage as those which appeared later ; and this will account for the young males commonly partaking to a certain extent (as may be observed with many birds) of the bright colours of their CHAP. VIII. SEXUAL SE:r.ECTION. 299' adult male parents. If only a few of the successive· variations in brightness were to occur at a late age,. the adult male would be only a little brighter than the young male; and such cases are common. In this illustration I have assumed that the young varied in a manner which was of no service to them;. but many characters proper to the adult male would be actually injurious to the young,-as bright colours fi·om makinO' them conspicuous, or horns of large size from expending much vital force. Such variations in the younO' would promptly be eliminated through natural selection. vVith the adult and experienced males, on the other hand, the advantage thus derived in their rivalry with other males would often more than counterbalance exposure to some degree of danger. Thus we can understand how it is that variations which must originally have appeared rather late in life have alone_ or in chief part been preserved for the development. of secondary sexual characters; and the remarlmble com-· cidence between the periods of variability and of sexual selection is intelligible. As variations which give to the male an advantage in fighting with other males, or in finding, securing,. or charming the female, would be of no use to the female, they will not have been preserved in this sex either during youth or maturity. Consequently such variations would be extremely liable to be lost; and the · female as far as these characters are concerned, would be left un~odified, excepting in so far as she may have received them by transference from the male. No doubt if the female varied and transferred serviceable characters. to her male offspring, these would be favoured through sexual selection; and then both sexes would thus far be modified in the same manner. But I shall hereafter have to recur to these more intricate contingencies. |