Darwin and Modern Science Part 24

MIMICRY,--BATESIAN OR PSEUDAPOSEMATIC, MULLERIAN OR SYNAPOSEMATIC.

The existence of superficial resemblances between animals of various degrees of affinity must have been observed for hundreds of years.

Among the early examples, the best known to me have been found in the ma.n.u.script note-books and collections of W.J. Burch.e.l.l, the great traveller in Africa (1810-15) and Brazil (1825-30). The most interesting of his records on this subject are brought together in the following paragraphs.

Conspicuous among well-defended insects are the dark steely or iridescent greenish blue fossorial wasps or sand-wasps, Sphex and the allied genera. Many Longicorn beetles mimic these in colour, slender shape of body and limbs, rapid movements, and the readiness with which they take to flight. On Dec. 21, 1812, Burch.e.l.l captured one such beetle (Promeces viridis) at Kosi Fountain on the journey from the source of the Kuruman River to Klaarwater. It is correctly placed among the Longicorns in his catalogue, but opposite to its number is the comment "Sphex! totus purpureus."

In our own country the black-and-yellow colouring of many stinging insects, especially the ordinary wasps, affords perhaps the commonest model for mimicry. It is reproduced with more or less accuracy on moths, flies and beetles. Among the latter it is again a Longicorn which offers one of the best-known, although by no means one of the most perfect, examples. The appearance of the well-known "wasp-beetle" (Clytus arietis) in the living state is sufficiently suggestive to prevent the great majority of people from touching it. In Burch.e.l.l's Brazilian collection there is a nearly allied species (Neoclytus curvatus) which appears to be somewhat less wasp-like than the British beetle. The specimen bears the number "1188," and the date March 27, 1827, when Burch.e.l.l was collecting in the neighbourhood of San Paulo. Turning to the corresponding number in the Brazilian note-book we find this record: "It runs rapidly like an ichneumon or wasp, of which it has the appearance."

The formidable, well-defended ants are as freely mimicked by other insects as the sand-wasps, ordinary wasps and bees. Thus on February 17, 1901, Guy A.K. Marshall captured, near Salisbury, Mashonaland, three similar species of ants (Hymenoptera) with a bug (Hemiptera) and a Locustid (Orthoptera), the two latter mimicking the former. All the insects, seven in number, were caught on a single plant, a small bushy vetch. ("Trans. Ent. Soc. Lond." 1902, page 535, plate XIX. figs.

53-59.)

This is an interesting recent example from South Africa, and large numbers of others might be added--the observations of many naturalists in many lands; but nearly all of them known since that general awakening of interest in the subject which was inspired by the great hypotheses of H.W. Bates and Fritz Muller. We find, however, that Burch.e.l.l had more than once recorded the mimetic resemblance to ants. An extremely ant-like bug (the larva of a species of Alydus) in his Brazilian collection is labelled "1141," with the date December 8, 1826, when Burch.e.l.l was at the Rio das Pedras, Cubatao, near Santos. In the note-book the record is as follows: "1141 Cimex. I collected this for a Formica."

Some of the chief mimics of ants are the active little hunting spiders belonging to the family Attidae. Examples have been brought forward during many recent years, especially by my friends Dr and Mrs Peckham, of Milwaukee, the great authorities on this group of Araneae. Here too we find an observation of the mimetic resemblance recorded by Burch.e.l.l, and one which adds in the most interesting manner to our knowledge of the subject. A fragment, all that is now left, of an Attid spider, captured on June 30, 1828, at Goyaz, Brazil, bears the following note, in this case on the specimen and not in the note-book: "Black... runs and seems like an ant with large extended jaws." My friend Mr R.I. Poc.o.c.k, to whom I have submitted the specimen, tells me that it is not one of the group of species. .h.i.therto regarded as ant-like, and he adds, "It is most interesting that Burch.e.l.l should have noticed the resemblance to an ant in its movements. This suggests that the perfect imitation in shape, as well as in movement, seen in many species was started in forms of an appropriate size and colour by the mimicry of movement alone." Up to the present time Burch.e.l.l is the only naturalist who has observed an example which still exhibits this ancestral stage in the evolution of mimetic likeness.

Following the teachings of his day, Burch.e.l.l was driven to believe that it was part of the fixed and inexorable scheme of things that these strange superficial resemblances existed. Thus, when he found other examples of Hemipterous mimics, including one (Luteva macrophthalma) with "exactly the manners of a Mantis," he added the sentence, "In the genus Cimex (Linn.) are to be found the outward resemblances of insects of many other genera and orders" (February 15, 1829). Of another Brazilian bug, which is not to be found in his collection, and cannot therefore be precisely identified, he wrote: "Cimex... Nature seems to have intended it to imitate a Sphex, both in colour and the rapid palpitating and movement of the antennae" (November 15, 1826). At the same time it is impossible not to feel the conviction that Burch.e.l.l felt the advantage of a likeness to stinging insects and to aggressive ants, just as he recognised the benefits conferred on desert plants by spines and by concealment. Such an interpretation of mimicry was perfectly consistent with the theological doctrines of his day. (See Kirby and Spence, "An Introduction to Entomology" (1st edition), London, Vol. II.

1817, page 223.)

The last note I have selected from Burch.e.l.l's ma.n.u.script refers to one of the chief mimics of the highly protected Lycid beetles. The whole a.s.semblage of African insects with a Lycoid colouring forms a most important combination and one which has an interesting bearing upon the theories of Bates and Fritz Muller. This most wonderful set of mimetic forms, described in 1902 by Guy A.K. Marshall, is composed of flower-haunting beetles belonging to the family Lycidae, and the heterogeneous group of varied insects which mimic their conspicuous and simple scheme of colouring. The Lycid beetles, forming the centre or "models" of the whole company, are orange-brown in front for about two-thirds of the exposed surface, black behind for the remaining third.

They are undoubtedly protected by qualities which make them excessively unpalatable to the bulk of insect-eating animals. Some experimental proof of this has been obtained by Mr Guy Marshall. What are the forms which surround them? According to the hypothesis of Bates they would be, at any rate mainly, palatable hard-pressed insects which only hold their own in the struggle for life by a fraudulent imitation of the trade-mark of the successful and powerful Lycidae. According to Fritz Muller's hypothesis we should expect that the mimickers would be highly protected, successful and abundant species, which (metaphorically speaking) have found it to their advantage to possess an advertis.e.m.e.nt, a danger-signal, in common with each other, and in common with the beetles in the centre of the group.

How far does the const.i.tution of this wonderful combination--the largest and most complicated as yet known in all the world--convey to us the idea of mimicry working along the lines supposed by Bates or those suggested by Muller? Figures 1 to 52 of Mr Marshall's coloured plate ("Trans. Ent. Soc. Lond." 1902, plate XVIII. See also page 517, where the group is a.n.a.lysed.) represent a set of forty-two or forty-three species or forms of insects captured in Mashonaland, and all except two in the neighbourhood of Salisbury. The combination includes six species of Lycidae; nine beetles of five groups all specially protected by nauseous qualities, Telephoridae, Melyridae, Phytophaga, Lagriidae, Cantharidae; six Longicorn beetles; one Coprid beetle; eight stinging Hymenoptera; three or four parasitic Hymenoptera (Braconidae, a group much mimicked and shown by some experiments to be distasteful); five bugs (Hemiptera, a largely unpalatable group); three moths (Arctiidae and Zygaenidae, distasteful families); one fly. In fact the whole combination, except perhaps one Phytophagous, one Coprid and the Longicorn beetles, and the fly, fall under the hypothesis of Muller and not under that of Bates. And it is very doubtful whether these exceptions will be sustained: indeed the suspicion of unpalatability already besets the Longicorns and is always on the heels,--I should say the hind tarsi--of a Phytophagous beetle.

This most remarkable group which ill.u.s.trates so well the problem of mimicry and the alternative hypotheses proposed for its solution, was, as I have said, first described in 1902. Among the most perfect of the mimetic resemblances in it is that between the Longicorn beetle, Amphidesmus a.n.a.lis, and the Lycidae. It was with the utmost astonishment and pleasure that I found this very resemblance had almost certainly been observed by Burch.e.l.l. A specimen of the Amphidesmus exists in his collection and it bears "651." Turning to the same number in the African Catalogue we find that the beetle is correctly placed among the Longicorns, that it was captured at Uitenhage on Nov. 18, 1813, and that it was found a.s.sociated with Lycid beetles in flowers ("consocians c.u.m Lycis 78-87 in floribus"). Looking up Nos. 78-87 in the collection and catalogue, three species of Lycidae are found, all captured on Nov. 18, 1813, at Uitenhage. Burch.e.l.l recognised the wide difference in affinity, shown by the distance between the respective numbers; for his catalogue is arranged to represent relations.h.i.+ps. He observed, what students of mimicry are only just beginning to note and record, the coincidence between model and mimic in time and s.p.a.ce and in habits. We are justified in concluding that he observed the close superficial likeness although he does not in this case expressly allude to it.

One of the most interesting among the early observations of superficial resemblance between forms remote in the scale of cla.s.sification was made by Darwin himself, as described in the following pa.s.sage from his letter to Henslow, written from Monte Video, Aug. 15, 1832: "Amongst the lower animals nothing has so much interested me as finding two species of elegantly coloured true Planaria inhabiting the dewy forest! The false relation they bear to snails is the most extraordinary thing of the kind I have ever seen." ("More Letters", I. page 9.)

Many years later, in 1867, he wrote to Fritz Muller suggesting that the resemblance of a soberly coloured British Planarian to a slug might be due to mimicry. ("Life and Letters", III. page 71.)

The most interesting copy of Bates's cla.s.sical memoir on Mimicry ("Contributions to an Insect Fauna of the Amazon Valley". "Trans. Linn.

Soc." Vol. XXIII. 1862, page 495.), read before the Linnean Society in 1861, is that given by him to the man who has done most to support and extend the theory. My kind friend has given that copy to me; it bears the inscription:

"Mr A.R. Wallace from his old travelling companion the Author."

Only a year and a half after the publication of the "Origin", we find that Darwin wrote to Bates on the subject which was to provide such striking evidence of the truth of Natural Selection: "I am glad to hear that you have specially attended to 'mimetic' a.n.a.logies--a most curious subject; I hope you publish on it. I have for a long time wished to know whether what Dr Collingwood a.s.serts is true--that the most striking cases generally occur between insects inhabiting the same country." (The letter is dated April 4, 1861. "More Letters", I. page 183.)

The next letter, written about six months later, reveals the remarkable fact that the ill.u.s.trious naturalist who had antic.i.p.ated Edward Forbes in the explanation of arctic forms on alpine heights ("I was forestalled in only one important point, which my vanity has always made me regret, namely, the explanation by means of the Glacial period of the presence of the same species of plants and of some few animals on distant mountain summits and in the arctic regions. This view pleased me so much that I wrote it out in extenso, and I believe that it was read by Hooker some years before E. Forbes published his celebrated memoir on the subject. In the very few points in which we differed, I still think that I was in the right. I have never, of course, alluded in print to my having independently worked out this view." "Autobiography, Life and Letters", I. page 88.), had also antic.i.p.ated H.W. Bates in the theory of Mimicry: "What a capital paper yours will be on mimetic resemblances!

You will make quite a new subject of it. I had thought of such cases as a difficulty; and once, when corresponding with Dr Collingwood, I thought of your explanation; but I drove it from my mind, for I felt that I had not knowledge to judge one way or the other." (The letter is dated Sept. 25, 1861: "More Letters", I. page 197.)

Bates read his paper before the Linnean Society, Nov. 21, 1861, and Darwin's impressions on hearing it were conveyed in a letter to the author dated Dec. 3: "Under a general point of view, I am quite convinced (Hooker and Huxley took the same view some months ago) that a philosophic view of nature can solely be driven into naturalists by treating special subjects as you have done. Under a special point of view, I think you have solved one of the most perplexing problems which could be given to solve." ("Life and Letters", II. page 378.) The memoir appeared in the following year, and after reading it Darwin wrote as follows, Nov. 20, 1862: "... In my opinion it is one of the most remarkable and admirable papers I ever read in my life... I am rejoiced that I pa.s.sed over the whole subject in the "Origin", for I should have made a precious mess of it. You have most clearly stated and solved a wonderful problem... Your paper is too good to be largely appreciated by the mob of naturalists without souls; but, rely on it, that it will have LASTING value, and I cordially congratulate you on your first great work. You will find, I should think, that Wallace will fully appreciate it." ("Life and Letters", II. pages 391-393.) Four days later, Nov. 24, Darwin wrote to Hooker on the same subject: "I have now finished his paper...' it seems to me admirable. To my mind the act of segregation of varieties into species was never so plainly brought forward, and there are heaps of capital miscellaneous observations." ("More Letters", I.

page 214.)

Darwin was here referring to the tendency of similar varieties of the same species to pair together, and on Nov. 25 he wrote to Bates asking for fuller information on this subject. ("More Letters", I. page 215.

See also parts of Darwin's letter to Bates in "Life and Letters", II.

page 392.) If Bates's opinion were well founded, s.e.xual selection would bear a most important part in the establishment of such species. (See Poulton, "Essays on Evolution", 1908, pages 65, 85-88.) It must be admitted, however, that the evidence is as yet quite insufficient to establish this conclusion. It is interesting to observe how Darwin at once fixed on the part of Bates's memoir which seemed to bear upon s.e.xual selection. A review of Bates's theory of Mimicry was contributed by Darwin to the "Natural History Review" (New Ser. Vol. III. 1863, page 219.) and an account of it is to be found in the "Origin" (Edition 1872, pages 375-378.) and in "The Descent of Man". (Edition 1874, pages 323-325.)

Darwin continually writes of the value of hypothesis as the inspiration of inquiry. We find an example in his letter to Bates, Nov. 22, 1860: "I have an old belief that a good observer really means a good theorist, and I fully expect to find your observations most valuable." ("More Letters", I. page 176.) Darwin's letter refers to many problems upon which Bates had theorised and observed, but as regards Mimicry itself the hypothesis was thought out after the return of the letter from the Amazons, when he no longer had the opportunity 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 b.u.t.terfly resemblances into two cla.s.ses,--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 a.s.sociated with his name, had conceived the idea of the production of mimetic likeness by s.e.xual 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 pa.s.sed 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 b.u.t.terflies 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 b.u.t.terflies 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 a.s.sumed by the theory of Bates. They belonged to the groups which were themselves mimicked by other b.u.t.terflies. 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 s.e.xual selection he invoked would tend to produce resemblances in the males rather than the females, while it is well known that when the s.e.xes 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 b.u.t.terflies. 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 ma.s.s 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 antic.i.p.ated 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 ent.i.ty." ("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 cla.s.sification.

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 s.e.x.

Ever since Wallace's cla.s.sical memoir on mimicry in the Malayan Swallowtail b.u.t.terflies, those naturalists who have written on the subject have followed his interpretation of the marked prevalence of mimetic resemblance in the female s.e.x 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 darda.n.u.s = 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 s.e.xes mimic, it is very common in b.u.t.terflies 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 b.u.t.terflies 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 s.e.x alone. Believing in this, I can see no improbability (but from a.n.a.logy of domestic animals a strong probability) that variations leading to beauty must often have occurred in the males alone, and been transmitted to that s.e.x 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 s.e.xual characters of males supported the conviction "that from some unknown cause such characters (viz. new characters arising in one s.e.x 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 s.e.xual characters of males would be no advantage but a disadvantage to females, and, as Wallace thinks, are withheld from this s.e.x by selection. It is indeed possible that mimicry has been hindered and often prevented from pa.s.sing to the males by s.e.xual 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 s.e.x it would be necessary to a.n.a.lyse the whole of the facts, so far as they are known in b.u.t.terflies. 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 s.e.x because the differences which distinguish it from the other s.e.x 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 a.s.sume with Haase that mimicry always arose in the female and was later acquired by the male. Both s.e.xes 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 s.e.xes 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 s.e.xes alike, mimicry can be rapidly evolved in both s.e.xes 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 s.e.xes 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 s.e.xes 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 const.i.tution of pigments.

s.e.xUAL SELECTION (EPIGAMIC CHARACTERS).