ECTLY secures reproduction。
In many cases THE ORGAN OF SMELL shows a similar improvement。 Many lower Crustaceans （Daphnidae） have better developed organs of smell in the male sex。 The difference is often slight and amounts only to one or two olfactory filaments； but certain species show a difference of nearly a hundred of these filaments （Leptodora）。 The same thing occurs among insects。
We must briefly consider the clasping or grasping organs which have developed in the males among many lower Crustaceans； but here natural selection plays its part along with sexual selection； for the union of the sexes is an indispensable condition for the maintenance of the species； and as Darwin himself pointed out； in many cases the two forms of selection merge into each other。 This fact has always seemed to me to be a proof of natural selection； for； in regard to sexual selection； it is quite obvious that the victory of the best…equipped could have brought about the improvement only of the organs concerned； the factors in the struggle； such as the eye and the olfactory organ。
We come now to the EXCITANTS； that is； to the group of sexual characters whose origin through processes of selection has been most frequently called in question。 We may cite the LOVE…CALLS produced by many male insects； such as crickets and cicadas。 These could only have arisen in animal groups in which the female did not rapidly flee from the male； but was inclined to accept his wooing from the first。 Thus； notes like the chirping of the male cricket serve to entice the females。 At first they were merely the signal which showed the presence of a male in the neighbourhood； and the female was gradually enticed nearer and nearer by the continued chirping。 The male that could make himself heard to the greatest distance would obtain the largest following； and would transmit the beginnings； and； later； the improvement of his voice to the greatest number of descendants。 But sexual excitement in the female became associated with the hearing of the love…call； and then the sound…producing organ of the male began to improve； until it attained to the emission of the long…drawn…out soft notes of the mole…cricket or the maenad…like cry of the cicadas。 I cannot here follow the process of development in detail； but will call attention to the fact that the original purpose of the voice； the announcing of the male's presence； became subsidiary； and the exciting of the female became the chief goal to be aimed at。 The loudest singers awakened the strongest excitement； and the improvement resulted as a matter of course。 I conceive of the origin of bird…song in a somewhat similar manner； first as a means of enticing； then of exciting the female。
One more kind of secondary sexual character must here be mentioned： the odour which emanates from so many animals at the breeding season。 It is possible that this odour also served at first merely to give notice of the presence of individuals of the other sex； but it soon became an excitant； and as the individuals 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 bot