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 manuscript note-books and collections of W.J. Burchell, 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, Burchell 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 Burchell'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
Burchell 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 Burchell 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
Burchell 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 Burchell,
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. Pocock,
to whom I have submitted the specimen, tells me that it is not one of
the group of species hitherto regarded as ant-like, and he adds, "It is
most interesting that Burchell 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 Burchell 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, Burchell 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 Burchell 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 Burchell's manuscript refers to one
of the chief mimics of the highly protected Lycid beetles. The whole
assemblage 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 advertisement,
a danger-signal, in common with each other, and in common with the
beetles in the centre of the group.
How far does the constitution 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 analysed.) 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 illustrates 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 analis, and the Lycidae. It was with the utmost astonishment
and pleasure that I found this very resemblance had almost certainly
been observed by Burchell. 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 associated with Lycid beetles in flowers ("consocians cum
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. Burchell recognised the wide difference in affinity,
shown by the distance between the respective numbers; for his catalogue
is arranged to represent relationships. He observed, what students of
mimicry are only just beginning to note and record, the coincidence
between model and mimic in time and space 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 classification was made
by Darwin himself, as described in the following passage 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 classical 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' analogies--a most curious
subject; I hope you publish on it. I have for a long time wished to
know whether what Dr Collingwood asserts 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 illustrious naturalist who had anticipated 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 anticipated 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 passed 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, sexual 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
sexual 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 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.