darwin and modern science-第74章
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ortunity of testing it by the observation of living Nature。 It is by no means improbable that； had he been able to apply this test； Bates would have recognised that his division of butterfly resemblances into two classes；one due to the theory of mimicry； the other to the influence of local conditions；could not be sustained。
Fritz Muller's contributions to the problem of Mimicry were all made in S。E。 Brazil； and numbers of them were communicated； with other observations on natural history； to Darwin； and by him sent to Professor R。 Meldola who published many of the facts。 Darwin's letters to Meldola （Poulton； 〃Charles Darwin and the theory of Natural Selection〃； London； 1896； pages 199…218。） contain abundant proofs of his interest in Muller's work upon Mimicry。 One deeply interesting letter （Loc。 cit。 pages 201； 202。） dated Jan。 23； 1872； proves that Fritz Muller before he originated the theory of Common Warning Colours （Synaposematic Resemblance or Mullerian Mimicry）； which will ever be associated with his name； had conceived the idea of the production of mimetic likeness by sexual selection。
Darwin's letter to Meldola shows that he was by no means inclined to dismiss the suggestion as worthless； although he considered it daring。 〃You will also see in this letter a strange speculation； which I should not dare to publish； about the appreciation of certain colours being developed in those species which frequently behold other forms similarly ornamented。 I do not feel at all sure that this view is as incredible as it may at first appear。 Similar ideas have passed through my mind when considering the dull colours of all the organisms which inhabit dull…coloured regions； such as Patagonia and the Galapagos Is。〃 A little later； on April 5； he wrote to Professor August Weismann on the same subject： 〃It may be suspected that even the habit of viewing differently coloured surrounding objects would influence their taste； and Fritz Muller even goes so far as to believe that the sight of gaudy butterflies might influence the taste of distinct species。〃 （〃Life and Letters〃； III。 page 157。）
This remarkable suggestion affords interesting evidence that F。 Muller was not satisfied with the sufficiency of Bates's theory。 Nor is this surprising when we think of the numbers of abundant conspicuous butterflies which he saw exhibiting mimetic likenesses。 The common instances in his locality； and indeed everywhere in tropical America； were anything but the hard…pressed struggling forms assumed by the theory of Bates。 They belonged to the groups which were themselves mimicked by other butterflies。 Fritz Muller's suggestion also shows that he did not accept Bates's alternative explanation of a superficial likeness between models themselves； based on some unknown influence of local physico…chemical forces。 At the same time Muller's own suggestion was subject to this apparently fatal objection； that the sexual selection he invoked would tend to produce resemblances in the males rather than the females； while it is well known that when the sexes differ the females are almost invariably more perfectly mimetic than the males and in a high proportion of cases are mimetic while the males are non…mimetic。
The difficulty was met several years later by Fritz Muller's well…known theory； published in 1879 （〃Kosmos〃； May 1879； page 100。）； and immediately translated by Meldola and brought before the Entomological Society。 （〃Proc。 Ent。 Soc。 Lond。〃 1879； page xx。） Darwin's letter to Meldola dated June 6； 1879； shows 〃that the first introduction of this new and most suggestive hypothesis into this country was due to the direct influence of Darwin himself； who brought it before the notice of the one man who was likely to appreciate it at its true value and to find the means for its presentation to English naturalists。〃 （〃Charles Darwin and the Theory of Natural Selection〃； page 214。） Of the hypothesis itself Darwin wrote 〃F。 Muller's view of the mutual protection was quite new to me。〃 （Ibid。 page 213。） The hypothesis of Mullerian mimicry was at first strongly opposed。 Bates himself could never make up his mind to accept it。 As the Fellows were walking out of the meeting at which Professor Meldola explained the hypothesis； an eminent entomologist； now deceased； was heard to say to Bates： 〃It's a case of save me from my friends！〃 The new ideas encountered and still encounter to a great extent the difficulty that the theory of Bates had so completely penetrated the literature of natural history。 The present writer has observed that naturalists who have not thoroughly absorbed the older hypothesis are usually far more impressed by the newer one than are those whose allegiance has already been rendered。 The acceptance of Natural Selection itself was at first hindered by similar causes； as Darwin clearly recognised： 〃If you argue about the non… acceptance of Natural Selection； it seems to me a very striking fact that the Newtonian theory of gravitation； which seems to every one now so certain and plain； was rejected by a man so extraordinarily 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 th
