darwin and modern science-第75章
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tage； for the unfortunate male to enjoy an equal immunity from danger。 For my part； I should say that the female alone had happened to vary in the right manner； and that the beneficial variations had been transmitted to the same sex alone。 Believing in this； I can see no improbability （but from analogy of domestic animals a strong probability） that variations leading to beauty must often have occurred in the males alone； and been transmitted to that sex alone。 Thus I should account in many cases for the greater beauty of the male over the female； without the need of the protective principle。〃 （〃More Letters〃； II。 pages 73； 74。 On the same subject〃the gay…coloured females of Pieris〃 （Perrhybris （Mylothris） pyrrha of Brazil）； Darwin wrote to Wallace； May 5； 1868； as follows： 〃I believe I quite follow you in believing that the colours are wholly due to mimicry； and I further believe that the male is not brilliant from not having received through inheritance colour from the female； and from not himself having varied； in short； that he has not been influenced by selection。〃 It should be noted that the male of this species does exhibit a mimetic pattern on the under surface。 〃More Letters〃 II。 page 78。）
The consideration of the facts of mimicry thus led Darwin to the conclusion that the female happens to vary in the right manner more commonly than the male； while the secondary sexual characters of males supported the conviction 〃that from some unknown cause such characters （viz。 new characters arising in one sex and transmitted to it alone） apparently appear oftener in the male than in the female。〃 （Letter from Darwin to Wallace； May 5； 1867； 〃More Letters〃； II。 Page 61。）
Comparing these conflicting arguments we are led to believe that the first is the stronger。 Mimicry in the male would be no disadvantage but an advantage； and when it appears would be and is taken advantage of by selection。 The secondary sexual characters of males would be no advantage but a disadvantage to females； and； as Wallace thinks； are withheld from this sex by selection。 It is indeed possible that mimicry has been hindered and often prevented from passing to the males by sexual selection。 We know that Darwin was much impressed （〃Descent of Man〃； page 325。） by Thomas Belt's daring and brilliant suggestion that the white patches which exist； although ordinarily concealed； on the wings of mimetic males of certain Pierinae （Dismorphia）； have been preserved by preferential mating。 He supposed this result to have been brought about by the females exhibiting a deep…seated preference for males that displayed the chief ancestral colour； inherited from periods before any mimetic pattern had been evolved in the species。 But it has always appeared to me that Belt's deeply interesting suggestion requires much solid evidence and repeated confirmation before it can be accepted as a valid interpretation of the facts。 In the present state of our knowledge； at any rate of insects and especially of Lepidoptera； it is probable that the female is more apt to vary than the male and that an important element in the interpretation of prevalent female mimicry is provided by this fact。
In order adequately to discuss the question of mimicry and sex it would be necessary to analyse the whole of the facts； so far as they are known in butterflies。 On the present occasion it is only possible to state the inferences which have been drawn from general impressions；inferences which it is believed will be sustained by future inquiry。
（1） Mimicry may occasionally arise in one sex because the differences which distinguish it from the other sex happen to be such as to afford a starting…point for the resemblance。 Here the male is at no disadvantage as compared with the female； and the rarity of mimicry in the male alone （e。g。 Cethosia） is evidence that the great predominance of female mimicry is not to be thus explained。
（2） The tendency of the female to dimorphism and polymorphism has been of great importance in determining this predominance。 Thus if the female appear in two different forms and the male in only one it will be twice as probable that she will happen to possess a sufficient foundation for the evolution of mimicry。
（3） The appearance of melanic or partially melanic forms in the female has been of very great service； providing as it does a change of ground…colour。 Thus the mimicry of the black generally red…marked American 〃Aristolochia swallowtails〃 （Pharmacophagus） by the females of Papilio swallowtails was probably begun in this way。
（4） It is probably incorrect to assume with Haase that mimicry always arose in the female and was later acquired by the male。 Both sexes of the third section of swallowtails （Cosmodesmus） mimic Pharmacophagus in America； far more perfectly than do the females of Papilio。 But this is not due to Cosmodesmus presenting us with a later stage of history begun in Papilio； for in Africa Cosmodesmus is still mimetic （of Danainae） in both sexes although the resemblances attained are imperfect； while many African species of Papilio have non…mimetic males with beautifully mimetic females。 The explanation is probably to be sought in the fact that the females of Papilio are more variable and more often tend to become dimorphic than those of Cosmodesmus； while the latter group has more often happened to possess a sufficient foundation for the origin of the resemblance in patterns which； from the start； were common to male and female。
（5） In very variable species with sexes alike； mimicry can be rapidly evolved in both sexes out of very small beginnings。 Thus the reddish marks which are common in many individuals of Limenitis arthemis were almost certainly the starting…point for the evolution of the beautifully mimetic L。 archippus。 Nevertheless in such cases； although there is no reason to suspect any greater variability； the female is commonly a somewhat better mimic than the male and often a very much better mimic。 Wallace's principle seems here to supply the obvious interpretation。
（6） When the difference between the patterns of the model and presumed ancestor of the mimic is very great； the female is often alone mimetic； when the difference is comparatively small； both sexes are commonly mimetic。 The Nymphaline genus Hypolimnas is a good example。 In Hypolimnas itself the females mimic Danainae with patterns very different from those preserved by the non…mimetic males： in the sub…genus Euralia； both sexes resemble the black and white Ethiopian Danaines with patterns not very dissimilar from that which we infer to have existed in the non…mimetic ancestor。
（7） Although a melanic form or other large variation may be of the utmost importance in facilitating the start of a mimetic likeness； it is impossible to explain the evolution of any detailed resemblance in this manner。 And even the large colour variation itself may well be the expression of a minute and 〃continuous〃 change in the chemical and physical constitution of pigments。
SEXUAL SELECTION （EPIGAMIC CHARACTERS）。
We do not know the date at which the idea of Sexual Selection arose in Darwin's mind； but it was probably not many years after the sudden flash of insight which； in October 1838； gave to him the theory of Natural Selection。 An excellent account of Sexual Selection occupies the concluding paragraph of Part I。 of Darwin's Section of the Joint Essay on Natural Selection； read July 1st； 1858； before the Linnean Society。 （〃Journ。 Proc。 Linn。 Soc。〃 Vol。 III。 1859； page 50。） The principles are so clearly and sufficiently stated in these brief sentences that it is appropriate to quote the whole： 〃Besides this natural means of selection； by which those individuals are preserved； whether in their egg； or larval； or mature state； which are best adapted to the place they fill in nature； there is a second agency at work in most unisexual animals； tending to produce the same effect； namely； the struggle of the males for the females。 These struggles are generally decided by the law of battle； but in the case of birds； apparently； by the charms of their song； by their beauty or their power of courtship； as in the dancing rock…thrush of Guiana。 The most vigorous and healthy males； implying perfect adaptation； must generally gain the victory in their contests。 This kind of selection； however； is less rigorous than the other； it does not require the death of the less successful； but gives to them fewer descendants。 The struggle falls； moreover； at a time of year when food is generally abundant； and perhaps the effect chiefly produced would be the modification of the secondary sexual characters； which are not related to the power of obtaining food； or to defence from enemies； but to fighting with or rivalling other males。 The result of this struggle amongst the males may be compared in some respects to that produced by those agriculturists who pay less attention to the careful selection of all their young animals； and more to the occasional use of a choice mate。〃
A full exposition of Sexual Selection appeared in the 〃The Descent of Man〃 in 1871； and in the greatly augmented second edition； in 1874。 It has been remarked that the two subjects； 〃The Descent of Man and Selection in Relation to Sex〃； seem to fuse somewhat imperfectly into the single work of which they form the title。 The reason for their association is clearly shown in a letter to Wallace； dated May 28； 1864： 〃。。。I suspect that a sort of sexual selection has been the most powerful means of changing the races of man。〃 （〃More Letters〃； II。 page 33。）
Darwin； as we know from his Autobiography （〃Life and Letters〃； I。 page 94。）； was always greatly interested in this hypothesis； and it has been shown in the preceding pages that he was inclined to look favourably upon it as an interpretation of many appearances usually explained by Natural Selection。 Hence Sexual Selection； incidentally discussed in other sections of the present essay； need not be considered at any length； in the section specially allotted to it。
Although so interested in the subject and notwithstanding his conviction that the hypothesis was sound； Darwin was quite aware that it was probably the most vulnerable part of the 〃Origin〃。 Thus he wrote to H。W。 Bates； April 4； 1861： 〃If I had to cut up myself in a review I would have （worried？） and quizzed sexual selection； therefore； though I am fully convinced that it is largely true； you may imagine how pleased I am at what you say on your belief。〃 （〃More Letters〃； I。 page 183。）
The existence of sound…producing organs in the males of insects was； Darwin considered； the strongest evidence in favour of the operation of sexual selection in this group。 （〃Life and Letters〃； III。 pages 94； 138。） Such a conclusion has received strong support in recent years by the numerous careful observations of Dr F。A。 Dixey （〃Proc。 Ent。 Soc。 Lond。〃 1904； page lvi； 1905； pages xxxvii； liv； 1906； page ii。） and Dr G。B。 Longstaff （〃Proc。 Ent。 Soc。 Lond。〃 1905； page xxxv； 〃Trans。 Ent。 Soc。 Lond。〃 1905； page 136； 1908； page 607。） on the scents of male butterflies。 The experience of these naturalists abundantly confirms and extends the account given by Fritz Muller （〃Jen。 Zeit。〃 Vol。 XI。 1877； page
