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 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.