darwin and modern science-第102章
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dinarily able as Leibnitz。 The truth will not penetrate a preoccupied mind。〃 （To Sir J。 Hooker； July 28； 1868； 〃More Letters〃； I。 page 305。 See also the letter to A。R。 Wallace； April 30； 1868； in 〃More Letters〃 II。 page 77； lines 6…8 from top。）
There are many naturalists； especially students of insects； who appear to entertain an inveterate hostility to any theory of mimicry。 Some of them are eager investigators in the fascinating field of geographical distribution； so essential for the study of Mimicry itself。 The changes of pattern undergone by a species of Erebia as we follow it over different parts of the mountain ranges of Europe is indeed a most interesting inquiry； but not more so than the differences between e。g。 the Acraea johnstoni of S。E。 Rhodesia and of Kilimanjaro。 A naturalist who is interested by the Erebia should be equally interested by the Acraea； and so he would be if the student of mimicry did not also record that the characteristics which distinguish the northern from the southern individuals of the African species correspond with the presence； in the north but not in the south； of certain entirely different butterflies。 That this additional information should so greatly weaken； in certain minds； the appeal of a favourite study； is a psychological problem of no little interest。 This curious antagonism is I believe confined to a few students of insects。 Those naturalists who； standing rather farther off； are able to see the bearings of the subject more clearly； will usually admit the general support yielded by an ever…growing mass of observations to the theories of Mimicry propounded by H。W。 Bates and Fritz Muller。 In like manner natural selection itself was in the early days often best understood and most readily accepted by those who were not naturalists。 Thus Darwin wrote to D。T。 Ansted； Oct。 27； 1860： 〃I am often in despair in making the generality of NATURALISTS even comprehend me。 Intelligent men who are not naturalists and have not a bigoted idea of the term species； show more clearness of mind。〃 （〃More Letters〃； I。 page 175。）
Even before the 〃Origin〃 appeared Darwin anticipated the first results upon the mind of naturalists。 He wrote to Asa Gray； Dec。 21； 1859： 〃I have made up my mind to be well abused； but I think it of importance that my notions should be read by intelligent men； accustomed to scientific argument； though NOT naturalists。 It may seem absurd； but I think such men will drag after them those naturalists who have too firmly fixed in their heads that a species is an entity。〃 （〃Life and Letters〃 II。 page 245。）
Mimicry was not only one of the first great departments of zoological knowledge to be studied under the inspiration of natural Selection； it is still and will always remain one of the most interesting and important of subjects in relation to this theory as well as to evolution。 In mimicry we investigate the effect of environment in its simplest form： we trace the effects of the pattern of a single species upon that of another far removed from it in the scale of classification。 When there is reason to believe that the model is an invader from another region and has only recently become an element in the environment of the species native to its second home； the problem gains a special interest and fascination。 Although we are chiefly dealing with the fleeting and changeable element of colour we expect to find and we do find evidence of a comparatively rapid evolution。 The invasion of a fresh model is for certain species an unusually sudden change in the forces of the environment and in some instances we have grounds for the belief that the mimetic response has not been long delayed。
MIMICRY AND SEX。
Ever since Wallace's classical memoir on mimicry in the Malayan Swallowtail butterflies； those naturalists who have written on the subject have followed his interpretation of the marked prevalence of mimetic resemblance in the female sex as compared with the male。 They have believed with Wallace that the greater dangers of the female； with slower flight and often alighting for oviposition； have been in part met by the high development of this special mode of protection。 The fact cannot be doubted。 It is extremely common for a non…mimetic male to be accompanied by a beautifully mimetic female and often by two or three different forms of female； each mimicking a different model。 The male of a polymorphic mimetic female is； in fact； usually non…mimetic （e。g。 Papilio dardanus = merope）； or if a mimic （e。g。 the Nymphaline genus Euripus）； resembles a very different model。 On the other hand a non…mimetic female accompanied by a mimetic male is excessively rare。 An example is afforded by the Oriental Nymphaline； Cethosia； in which the males of some species are rough mimics of the brown Danaines。 In some of the orb…weaving spiders the males mimic ants； while the much larger females are non…mimetic。 When both sexes mimic； it is very common in butterflies and is also known in moths； for the females to be better and often far better mimics than the males。
Although still believing that Wallace's hypothesis in large part accounts for the facts briefly summarised above； the present writer has recently been led to doubt whether it offers a complete explanation。 Mimicry in the male； even though less beneficial to the species than mimicry in the female； would still surely be advantageous。 Why then is it so often entirely restricted to the female？ While the attempt to find an answer to this question was haunting me； I re…read a letter written by Darwin to Wallace； April 15； 1868； containing the following sentences： 〃When female butterflies are more brilliant than their males you believe that they have in most cases； or in all cases； been rendered brilliant so as to mimic some other species； and thus escape danger。 But can you account for the males not having been rendered equally brilliant and equally protected？ Although it may be most for the welfare of the species that the female should be protected； yet it would be some advantage； certainly no disadvantage； 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
