darwin and modern science-第14章
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ls which caused the greatest degree of excitement were preferred； it reached as high a pitch of perfection as was possible to it。 I shall confine myself here to the comparatively recently discovered fragrance of butterflies。 Since Fritz Muller found out that certain Brazilian butterflies gave off fragrance 〃like a flower；〃 we have become acquainted with many such cases； and we now know that in all lands； not only many diurnal Lepidoptera but nocturnal ones also give off a delicate odour； which is agreeable even to man。 The ethereal oil to which this fragrance is due is secreted by the skin…cells； usually of the wing； as I showed soon after the discovery of the SCENT…SCALES。 This is the case in the males； the females have no SPECIAL scent…scales recognisable as such by their form； but they must； nevertheless； give off an extremely delicate fragrance； although our imperfect organ of smell cannot perceive it； for the males become aware of the presence of a female； even at night； from a long distance off； and gather round her。 We may therefore conclude； that both sexes have long given forth a very delicate perfume； which announced their presence to others of the same species； and that in many species （NOT IN ALL） these small beginnings became； in the males； particularly strong scent…scales of characteristic form （lute； brush； or lyre…shaped）。 At first these scales were scattered over the surface of the wing； but gradually they concentrated themselves； and formed broad； velvety bands； or strong； prominent brushes； and they attained their highest pitch of evolution when they became enclosed within pits or folds of the skin； which could be opened to let the delicious fragrance stream forth suddenly towards the female。 Thus in this case also we see that characters； the original use of which was to bring the sexes together； and so to maintain the species； have been evolved in the males into means for exciting the female。 And we can hardly doubt； that the females are most readily enticed to yield to the butterfly that sends out the strongest fragrance；that is to say； that excites them to the highest degree。 It is a pity that our organs of smell are not fine enough to examine the fragrance of male Lepidoptera in general； and to compare it with other perfumes which attract these insects。 （See Poulton； 〃Essays on Evolution〃； 1908； pages 316； 317。） As far as we can perceive them they resemble the fragrance of flowers； but there are Lepidoptera whose scent suggests musk。 A smell of musk is also given off by several plants： it is a sexual excitant in the musk…deer； the musk…sheep； and the crocodile。
As far as we know； then； it is perfumes similar to those of flowers that the male Lepidoptera give off in order to entice their mates； and this is a further indication that animals； like plants； can to a large extent meet the claims made upon them by life； and produce the adaptations which are most purposive；a further proof； too； of my proposition that the useful variations； so to speak； are ALWAYS THERE。 The flowers developed the perfumes which entice their visitors； and the male Lepidoptera developed the perfumes which entice and excite their mates。
There are many pretty little problems to be solved in this connection； for there are insects； such as some flies； that are attracted by smells which are unpleasant to us； like those from decaying flesh and carrion。 But there are also certain flowers； some orchids for instance； which give forth no very agreeable odour； but one which is to us repulsive and disgusting； and we should therefore expect that the males of such insects would give off a smell unpleasant to us； but there is no case known to me in which this has been demonstrated。
In cases such as we have discussed； it is obvious that there is no possible explanation except through selection。 This brings us to the last kind of secondary sexual characters； and the one in regard to which doubt has been most frequently expressed；decorative colours and decorative forms； the brilliant plumage of the male pheasant； the humming…birds； and the bird of Paradise； as well as the bright colours of many species of butterfly； from the beautiful blue of our little Lycaenidae to the magnificent azure of the large Morphinae of Brazil。 In a great many cases； though not by any means in all； the male butterflies are 〃more beautiful〃 than the females； and in the Tropics in particular they shine and glow in the most superb colours。 I really see no reason why we should doubt the power of sexual selection； and I myself stand wholly on Darwin's side。 Even though we certainly cannot assume that the females exercise a conscious choice of the 〃handsomest〃 mate； and deliberate like the judges in a court of justice over the perfections of their wooers； we have no reason to doubt that distinctive forms （decorative feathers） and colours have a particularly exciting effect upon the female； just as certain odours have among animals of so many different groups； including the butterflies。 The doubts which existed for a considerable time； as a result of fallacious experiments； as to whether the colours of flowers really had any influence in attracting butterflies have now been set at rest through a series of more careful investigations； we now know that the colours of flowers are there on account of the butterflies； as Sprengel first showed； and that the blossoms of Phanerogams are selected in relation to them； as Darwin pointed out。
Certainly it is not possible to bring forward any convincing proof of the origin of decorative colours through sexual selection； but there are many weighty arguments in favour of it； and these form a body of presumptive evidence so strong that it almost amounts to certainty。
In the first place； there is the analogy with other secondary sexual characters。 If the song of birds and the chirping of the cricket have been evolved through sexual selection； if the penetrating odours of male animals；the crocodile； the musk…deer； the beaver； the carnivores； and； finally； the flower…like fragrances of the butterflies have been evolved to their present pitch in this way； why should decorative colours have arisen in some other way？ Why should the eye be less sensitive to SPECIFICALLY MALE colours and other VISIBLE signs ENTICING TO THE FEMALE； than the olfactory sense to specifically male odours； or the sense of hearing to specifically male sounds？ Moreover； the decorative feathers of birds are almost always spread out and displayed before the female during courtship。 I have elsewhere （〃The Evolution Theory〃； London； 1904； I。 page 219。） pointed out that decorative colouring and sweet…scentedness may replace one another in Lepidoptera as well as in flowers； for just as some modestly coloured flowers （mignonette and violet） have often a strong perfume； while strikingly coloured ones are sometimes quite devoid of fragrance； so we find that the most beautiful and gaily…coloured of our native Lepidoptera； the species of Vanessa； have no scent…scales； while these are often markedly developed in grey nocturnal Lepidoptera。 Both attractions may； however； be combined in butterflies； just as in flowers。 Of course； we cannot explain why both means of attraction should exist in one genus； and only one of them in another； since we do not know the minutest details of the conditions of life of the genera concerned。 But from the sporadic distribution of scent…scales in Lepidoptera； and from their occurrence or absence in nearly related species； we may conclude that fragrance is a relatively MODERN acquirement； more recent than brilliant colouring。
One thing in particular that stamps decorative colouring as a product of selection is ITS GRADUAL INTENSIFICATION by the addition of new spots； which we can quite well observe； because in many cases the colours have been first acquired by the males； and later transmitted to the females by inheritance。 The scent…scales are never thus transmitted； probably for the same reason that the decorative colours of many birds are often not transmitted to the females： because with these they would be exposed to too great elimination by enemies。 Wallace was the first to point out that in species with concealed nests the beautiful feathers of the male occurred in the female also； as in the parrots； for instance； but this is not the case in species which brood on an exposed nest。 In the parrots one can often observe that the general brilliant colouring of the male is found in the female； but that certain spots of colour are absent； and these have probably been acquired comparatively recently by the male and have not yet been transmitted to the female。
Isolation of the group of individuals which is in process of varying is undoubtedly of great value in sexual selection； for even a solitary conspicuous variation will become dominant much sooner in a small isolated colony； than among a large number of members of a species。
Anyone who agrees with me in deriving variations from germinal selection will regard that process as an essential aid towards explaining the selection of distinctive courtship…characters； such as coloured spots； decorative feathers； horny outgrowths in birds and reptiles； combs； feather…tufts； and the like； since the beginnings of these would be presented with relative frequency in the struggle between the determinants within the germ…plasm。 The process of transmission of decorative feathers to the female results； as Darwin pointed out and illustrated by interesting examples； in the COLOUR…TRANSFORMATION OF A WHOLE SPECIES； and this process； as the phyletically older colouring of young birds shows； must； in the course of thousands of years； have repeated itself several times in a line of descent。
If we survey the wealth of phenomena presented to us by secondary sexual characters； we can hardly fail to be convinced of the truth of the principle of sexual selection。 And certainly no one who has accepted natural selection should reject sexual selection； for； not only do the two processes rest upon the same basis； but they merge into one another； so that it is often impossible to say how much of a particular character depends on one and how much on the other form of selection。
（b） NATURAL SELECTION。
An actual proof of the theory of sexual selection is out of the question； if only because we cannot tell when a variation attains to selection…value。 It is certain that a delicate sense of smell is of value to the male moth in his search for the female； but whether the possession of one additional olfactory hair； or of ten； or of twenty additional hairs leads to the success of its possessor we are unable to tell。 And we are groping even more in the dark when we discuss the excitement caused in the female by agreeable perfumes； or by striking and beautiful colours。 That these do make an impression is beyond doubt； but we can only assume that slight intensifications of them give any advantage； and we MUST assume this SINCE OTHERWISE SECONDARY SEXUAL CHARACTERS REMAIN INEXPLICABLE。
The same thing is true in regard to natural selection。 It is not possible to bring forward any actual proof of the selection…value of the initial stages； and the stages in the increase of variations； as has been alread
