Descent of Man [ 1871]

Charles Darwin [ 1809 - 1882 ]

Chapter XI - Insects, Continued - Order Lepidoptera. Butterflies and Moths


  IN this great Order the most interesting points for us are the
differences in colour between the sexes of the same species, and
between the distinct species of the same genus. Nearly the whole of
the following chapter will be devoted to this subject; but I will
first make a few remarks on one or two other points. Several males may
often be seen pursuing and crowding round the same female. Their
courtship appears to be a prolonged affair, for I have frequently
watched one or more males pirouetting round a female until I was
tired, without seeing the end of the courtship. Mr. A. G. Butler
also informs me that he has several times watched a male courting a
female for a full quarter of an hour; but she pertinaciously refused
him, and at last settled on the ground and closed her wings, so as
to escape from his addresses.
  Although butterflies are weak and fragile creatures, they are
pugnacious, and an emperor butterfly* has been captured with the
tips of its wings broken from a conflict with another male. Mr.
Collingwood, in speaking of the frequent battles between the
butterflies of Borneo, says, "They whirl round each other with the
greatest rapidity, and appear to be incited by the greatest ferocity."

  * Apatura iris: The Entomologist's Weekly Intelligence, 1859, p.
139. For the Bornean butterflies, see C. Collingwood, Rambles of a
Naturalist, 1868, p. 183.

  The Ageronia feronia makes a noise like that produced by a toothed
wheel passing under a spring catch, and which can be heard at the
distance of several yards: I noticed this sound at Rio de Janeiro,
only when two of these butterflies were chasing each other in an
irregular course, so that it is probably made during the courtship
of the sexes.*

  * See my Journal of Researches, 1845, p. 33. Mr. Doubleday has
detected (Proc. Ent. Soc., March 3, 1845, p. 123) a peculiar
membranous sac at the base of the front wings, which is probably
connected with the production of the sound. For the case of
Thecophora, see Zoological Record, 1869, p. 401. For Mr. Buchanan
White's observations, the Scottish Naturalist, July, 1872, p. 214.

  Some moths also produce sounds; for instance, the males
Theocophora fovea. On two occasions Mr. F. Buchanan White* heard a
sharp quick noise made by the male of Hylophila prasinana, and which
he believes to be produced, as in Cicada, by an elastic membrane,
furnished with a muscle. He quotes, also, Guenee, that Setina produces
a sound like the ticking of a watch, apparently by the aid of "two
large tympaniform vesicles, situated in the pectoral region"; and
these "are much more developed in the male than in the female."
Hence the sound-producing organs in the Lepidoptera appear to stand in
some relation with the sexual functions. I have not alluded to the
well-known noise made by the death's head sphinx, for it is
generally heard soon after the moth has emerged from its cocoon.

  * The Scottish Naturalist, July, 1872, p. 213.

  Giard has always observed that the musky odour, which is emitted
by two species of sphinx moths, is peculiar to the males;* and in
the higher classes we shall meet with many instances of the males
alone being odoriferous.

  * Zoological Record, 1869, p. 347.

  Every one must have admired the extreme beauty of many butterflies
and of some moths; and it may be asked, are their colours and
diversified patterns the result of the direct action of the physical
conditions to which these insects have been exposed, without any
benefit being thus derived? Or have successive variations been
accumulated and determined as a protection, or for some unknown
purpose, or that one sex may be attractive to the other? And, again,
what is the meaning of the colours being widely different in the males
and females of certain species, and alike in the two sexes of other
species of the same genus" Before attempting to answer these questions
a body of facts must be given.
  With our beautiful English butterflies, the admiral, peacock, and
painted lady (Vanessae), as well as many others, the sexes are
alike. This is also the case with the magnificent Heliconidae, and
most of the Danaidae in the tropics. But in certain other tropical
groups, and in some of our English butterflies, as the purple emperor,
orange-tip, &c. (Apatura iris and Anthocharis cardamines), the sexes
differ either greatly or slightly in colour. No language suffices to
describe the splendour of the males of some tropical species. Even
within the same genus we often find species presenting extraordinary
differences between the sexes, whilst others have their sexes
closely alike. Thus in the South American genus Epicalia, Mr. Bates,
to whom I am indebted for most of the following facts, and for looking
over this whole discussion, informs me that he knows twelve species,
the two sexes of which haunt the same stations (and this is not always
the case with butterflies), and which, therefore, cannot have been
differently affected by external conditions.* In nine of these
twelve species the males rank amongst the most brilliant of all
butterflies, and differ so greatly from the comparatively plain
females that they were formerly placed in distinct genera. The females
of these nine species resemble each other in their general type of
coloration; and they likewise resemble both sexes of the species in
several allied genera found in various parts of the world. Hence we
may infer that these nine species, and probably all the others of
the genus, are descended from an ancestral form which was coloured
in nearly the same manner. In the tenth species the female still
retains the same general colouring, but the male resembles her, so
that he is coloured in a much less gaudy and contrasted manner than
the males of the previous species. In the eleventh and twelfth
species, the females depart from the usual type, for they are gaily
decorated almost like the males, but in a somewhat less degree.
Hence in these two latter species the bright colours of the males seem
to have been transferred to the females; whilst in the tenth species
the male has either retained or recovered the plain colours of the
female, as well as of the parent-form of the genus. The sexes in these
three cases have thus been rendered nearly alike, though in an
opposite manner. In the allied genus Eubagis, both sexes of some of
the species are plain-coloured and nearly alike; whilst with the
greater number the males are decorated with beautiful metallic tints
in a diversified manner, and differ much from their females. The
females throughout the genus retain the same general style of
colouring, so that they resemble one another much more closely than
they resemble their own males.

  * See also Mr. Bates's paper in Proc. Ent. Soc. of Philadelphia,
1865, p. 206. Also Mr. Wallace on the same subject, in regard to
Diadema, in Transactions, Entomological Society, London, 1869, p. 278.

  In the genus Papilio, all the species of the Aeneas group are
remarkable for their conspicuous and strongly contrasted colours,
and they illustrate the frequent tendency to gradation in the amount
of difference between the sexes. In a few species, for instance in
P. ascanius, the males and females are alike; in others the males
are either a little brighter, or very much more superb than the
females. The genus Junonia, allied to our Vanessae, offers a nearly
parallel case, for although the sexes of most of the species
resemble each other, and are destitute of rich colours, yet in certain
species, as in J. oenone, the male is rather more bright-coloured than
the female, and in a few (for instance J. andremiaja) the male is so
different from the female that he might be mistaken for an entirely
distinct species.
  Another striking case was pointed out to me in the British Museum by
Mr. A. Butler, namely, one of the tropical American Theclae, in
which both sexes are nearly alike and wonderfully splendid; in another
species the male is coloured in a similarly gorgeous manner, whilst
the whole upper surface of the female is of a dull uniform brown.
Our common little English blue butterflies of the genus Lycaena,
illustrate the various differences in colour between the sexes, almost
as well, though not in so striking a manner, as the above exotic
genera. In Lycaena agestis both sexes have wings of a brown colour,
bordered with small ocellated orange spots, and are thus alike. In
L. oegon the wings of the males are of a fine blue, bordered with
black, whilst those of the female are brown, with a similar border,
closely resembling the wings of L. agestis. Lastly, in L. arion both
sexes are of a blue colour and are very like, though in the female the
edges of the wings are rather duskier, with the black spots plainer;
and in a bright blue Indian species both sexes are still more alike.
  I have given the foregoing details in order to show, in the first
place, that when the sexes of butterflies differ, the male as a
general rule is the more beautiful, and departs more from the usual
type of colouring of the group to which the species belongs. Hence
in most groups the females of the several species resemble each
other much more closely than do the males. In some cases, however,
to which I shall hereafter allude, the females are coloured more
splendidly than the males. In the second place, these details have
been given to bring clearly before the mind that within the same
genus, the two sexes frequently present every gradation from no
difference in colour, to so great a difference that it was long before
the two were placed by entomologists in the same genus. In the third
place, we have seen that when the sexes nearly resemble each other,
this appears due either to the male having transferred his colours
to the female, or to the male having retained, or perhaps recovered,
the primordial colours of the group. It also deserves notice that in
those groups in which the sexes differ, the females usually somewhat
resemble the males, so that when the males are beautiful to an
extraordinary degree, the females almost invariably exhibit some
degree of beauty. From the many cases of gradation in the amount of
difference between the sexes, and from the prevalence of the same
general type of coloration throughout the whole of the same group,
we may conclude that the causes have generally been the same which
have determined the brilliant colouring of the males alone of some
species, and of both sexes of other species.
  As so many gorgeous butterflies inhabit the tropics, it has often
been supposed that they owe their colours to the great heat and
moisture of these zones; but Mr. Bates* has shown by the comparison of
various closely-allied groups of insects from the temperate and
tropical regions, that this view cannot be maintained; and the
evidence becomes conclusive when brilliantly-coloured males and
plain-coloured females of the same species inhabit the same
district, feed on the same food, and follow exactly the same habits of
life. Even when the sexes resemble each other, we can hardly believe
that their brilliant and beautifully arranged colours are the
purposeless result of the nature of the tissues and of the action of
the surrounding conditions.

  * The Naturalist on the Amazons, vol. i., 1863, p. 19.

  With animals of all kinds, whenever colour has been modified for
some special purpose, this has been, as far as we can judge, either
for direct or indirect protection, or as an attraction between the
sexes. With many species of butterflies the upper surfaces of the
wings are obscure; and this in all probability leads to their escaping
observation and danger. But butterflies would be particularly liable
to be attacked by their enemies when at rest; and most kinds whilst
resting raise their wings vertically over their backs, so that the
lower surface alone is exposed to view. Hence it is this side which is
often coloured so as to imitate the objects on which these insects
commonly rest. Dr. Rossler, I believe, first noticed the similarity of
the closed wings of certain Vanessae and other butterflies to the bark
of trees. Many analogous and striking facts could be given. The most
interesting one is that recorded by Mr. Wallace* of a common Indian
and Sumatran butterfly (Kallima) which disappears like magic when it
settles on a bush; for it hides its head and antennae between its
closed wings, which, in form, colour and veining, cannot be
distinguished from a withered leaf with its footstalk. In some other
cases the lower surfaces of the wings are brilliantly coloured, and
yet are protective; thus in Thecla rubi the wings when closed are of
an emerald green, and resemble the young leaves of the bramble, on
which in spring this butterfly may often be seen seated. It is also
remarkable that in very many species in which the sexes differ greatly
in colour on their upper surface, the lower surface is closely similar
or identical in both sexes, and serves as a protection.*(2)

  * See the interesting article in the Westminster Review, July, 1867,
p. 10. A woodcut of the Kallima is given by Mr. Wallace in Hardwicke's
Science Gossip, September 1867, p. 196.
  *(2) Mr. G. Fraser, in Nature, April, 1871, p. 489.

  Although the obscure tints both of the upper and under sides of many
butterflies no doubt serve to conceal them, yet we cannot extend
this view to the brilliant and conspicuous colours on the upper
surface of such species as our admiral and peacock Vanessae, our white
cabbage-butterflies (Pieris), or the great swallowtail Papilio which
haunts the open fens- for these butterflies are thus rendered
visible to every living creature. In these species both sexes are
alike; but in the common brimstone butterfly (Gonepteryx rhamni),
the male is of an intense yellow, whilst the female is much paler; and
in the orange-tip (Anthocharis cardamines) the males alone have
their wings tipped with bright orange. Both the males and females in
these cases are conspicuous, and it is not credible that their
difference in colour should stand in any relation to ordinary
protection. Prof. Weismann remarks,* that the female of one of the
Lycaenae expands her brow wings when she settles on the ground, and is
then almost invisible; the male, on the other hand, as if aware of the
danger incurred from the bright blue of the upper surface of his
wings, rests with them closed; and this shows that the blue colour
cannot be in any way protective. Nevertheless, it is probable that
conspicuous colours are indirectly beneficial to many species, as a
warning that they are unpalatable. For in certain other cases,
beauty has been gained through the imitation of other beautiful
species, which inhabit the same district and enjoy an immunity from
attack by being in some way offensive to their enemies; but then we
have to account for the beauty of the imitated species.

  * Einfluss der Isolirung auf die Artbildung, 1872, p. 58.

  As Mr. Walsh has remarked to me, the females of our orange-tip
butterfly, above referred to, and of an American species (Anth.
genutia) probably show us the primordial colours of the parent-species
of the genus; for both sexes of four or five widely-distributed
species are coloured in nearly the same manner. As in several previous
cases, we may here infer that it is the males of Anth. cardamines
and genutia which have departed from the usual type of the genus. In
the Anth. sara from California, the orange-tips to the wings have been
partially developed in the female; but they are paler than in the
male, and slightly different in some other respects. In an allied
Indian form, the Iphias glaucippe, the orange-tips are fully developed
in both sexes. In this Iphias, as pointed out to me by Mr. A.
Butler, the under surface of the wings marvellously resembles a
pale-coloured leaf; and in our English orange-tip, the under surface
resembles the flower-head of the wild parsley, on which the
butterfly often rests at night.* The same reason which compels us to
believe that the lower surfaces have here been coloured for the sake
of protection, leads us to deny that the wings have been tipped with
bright orange for the same purpose, especially when this character
is confined to the males.

  * See the interesting observations by T. W. Wood, the Student,
Sept., 1868, p. 81.

  Most moths rest motionless during the whole or greater part of the
day with their wings depressed; and the whole upper surface shaded and
coloured in an admirable manner, as Mr. Wallace has remarked, for
escaping detection. The front-wings of the Bombycidae,* when at
rest, generally overlap and conceal the hind-wings; so that the latter
might be brightly coloured without much risk; and they are in fact
often thus coloured. During flight, moths would often be able to
escape from their enemies; nevertheless, as the hind-wings are then
fully exposed to view, their bright must generally have been
acquired at some little risk. But the following fact shews how
cautious we ought to be in drawing conclusions on this head. The
common yellow under-wings (Triphoena) often fly about during the day
or early evening, and are then conspicuous from the colour of their
hind-wings. It would naturally be thought that this would be a
source of danger; but Mr. J. Jenner Weir believes that it actually
serves them as a means of escape, for birds strike at these brightly
coloured and fragile surfaces, instead of at the body. For instance,
Mr. Weir turned into his aviary a vigorous specimen of Triphoena
pronuba, which was instantly pursued by a robin; but the bird's
attention being caught by the coloured wings, the moth was not
captured until after about fifty attempts, and small portions of the
wings were repeatedly broken off. He tried the same experiment, in the
open air, with a swallow and T. fimbria; but the large size of this
moth probably interfered with its capture.*(2) We are thus reminded of
a statement made by Mr. Wallace,*(3) namely, that in the Brazilian
forests and Malayan islands, many common and highly-decorated
butterflies are weak flyers, though furnished with a broad expanse
of wing; and they "are often captured with pierced and broken wings,
as if they had been seized by birds, from which they had escaped: if
the wings had been much smaller in proportion to the body, it seems
probable that the insect would more frequently have been struck or
pierced in a vital part, and thus the increased expanse of the wings
may have been indirectly beneficial."

  * Mr. Wallace in Harwicke's Science Gossip, September, 1867, p. 193.
  *(2) See also, on this subject, Mr. Weir's paper in Transactions,
Entomological Society, 1869, p. 23.
  *(3) Westminster Review, July, 1867, p. 16.

  Display.- The bright colours of many butterflies and of some moths
are specially arranged for display, so that they may be readily
seen. During the night colours are not visible, and there can be no
doubt that the nocturnal moths, taken as a body, are much less gaily
decorated than butterflies, all of which are diurnal in their
habits. But the moths of certain families, such as the Zygaenidae,
several Sphingidae, Uraniidae, some Arctiidae and Saturniidae, fly
about during the day or early evening, and many of these are extremely
beautiful, being far brighter coloured than the strictly nocturnal
kinds. A few exceptional cases, however, of bright-coloured
nocturnal species have been recorded.*

  * For instance, Lithosia; but Prof. Westwood (Modern Class. of
Insects, vol. ii., p. 390) seems surprised at this case. On the
relative colours of diurnal and nocturnal Lepidoptera, see ibid.,
pp. 333 and 392; also Harris, Treatise on the Insects of New
England, 1842, p. 315.

  There is evidence of another kind in regard to display. Butterflies,
as before remarked, elevate their wings when at rest, but whilst
basking in the sunshine often alternately raise and depress them, thus
exposing both surfaces to full view; and although the lower surface is
often coloured in an obscure manner as a protection, yet in many
species it is as highly decorated as the upper surface, and
sometimes in a very different manner. In some tropical species the
lower surface is even more brilliantly coloured than the upper.* In
the English fritillaries (Argynnis) the lower surface alone is
ornamented with shining silver. Nevertheless, as a general rule, the
upper surface, which is probably more fully exposed, is coloured
more brightly and diversely than the lower. Hence the lower surface
generally affords to entomologists the more useful character for
detecting the affinities of the various species. Fritz Muller
informs me that three species of Castnia are found near his house in
S. Brazil: of two of them the hind-wings are obscure, and are always
covered by the front-wings when these butterflies are at rest; but the
third species has black hind-wings, beautifully spotted with red and
white, and these are fully expanded and displayed whenever the
butterfly rests. Other such cases could be added.

  * Such differences between the upper and lower surfaces of the wings
of several species of Papilio may be seen in the beautiful plates to
Mr. Wallace's "Memoir on the Papilionidae of the Malayan Region," in
Transactions of the Linnean Society, vol. xxv., part i., 1865.

  If we now turn to the enormous group of moths, which, as I hear from
Mr. Stainton, do not habitually expose the under surface of their
wings to full view, we find this side very rarely coloured with a
brightness greater than, or even equal to, that of the upper side.
Some exceptions to the rule, either real or apparent, must be noticed,
as the case of Hypopyra.* Mr. Trimen informs me that in Guenee's great
work, three moths are figured, in which the under surface is much
the more brilliant. For instance, in the Australian Gastrophora the
upper surface of the fore -wing is pale greyish-ochreous, while the
lower surface is magnificently ornamented by an ocellus of
cobalt-blue, placed in the midst of a black mark, surrounded by
orange-yellow, and this by bluish-white. But the habits of these three
moths are unknown; so that no explanation can be given of their
unusual style of colouring. Mr. Trimen also informs me that the
lower surface of the wings in certain other Geometrae*(2) and
quadrifid Noctuae are either more variegated or more brightly-coloured
than the upper surface; but some of these species have the habit of
"holding their wings quite erect over their backs, retaining them in
this position for a considerable time," and thus exposing the under
surface to view. Other species, when settled on the ground or herbage,
now and then suddenly and slightly lift up their wings. Hence the
lower surface of the wings being brighter than the upper surface in
certain moths is not so anomalous as it at first appears. The
Saturniidae include some of the most beautiful of all moths, their
wings being decorated, as in our British emperor moth, with fine
ocelli; and Mr. T. W. Wood*(3) observes that they resemble butterflies
in some of their movements; "for instance, in the gentle waving up and
down of the wings as if for display, which is more characteristic of
diurnal than of nocturnal Lepidoptera."

  * See Mr. Wormald on this moth: Proceedings of the Entomological
Society, March 2, 1868.
  *(2) See also an account of the S. American genus Erateina (one of
the Geometrae) in Transactions, Ent., Soc., new series, vol. v.,
pls. xv. and xvi.
  *(3) Proc Ent. Soc. of London, July 6, 1868, p. xxvii.

  It is a singular fact that no British moths which are brilliantly
coloured, and, as far as I can discover, hardly any foreign species,
differ much in colour according to sex; though this is the case with
many brilliant butterflies. The male, however, of one American moth,
the Saturnia io, is described as having its forewings deep yellow,
curiously marked with purplish-red spots; whilst the wings of the
female are purple-brown, marked with grey lines.* The British moths
which differ sexually in colour are all brown, or of various dull
yellow tints, or nearly white. In several species the males are much
darker than the females,*(2) and these belong to groups which
generally fly about during the afternoon. On the other hand, in many
genera, as Mr. Stainton informs me, the males have the hind-wings
whiter than those of the female- of which fact Agrotis exclamationis
offers a good instance. In the ghost-moth (Hepialus humuli) the
difference is more strongly marked; the males being white, and the
females yellow with darker markings.*(3) It is probable that in
these cases the males are thus rendered more conspicuous, and more
easily seen by the females whilst flying about in the dusk.

  * Harris, Treatise, &c., edited by Flint, 1862, p. 395.
  *(2) For instance, I observe in my son's cabinet that the males
are darker than the females in the Lasiocampa quercus Odonestis
potatoria, Hypogymna dispar, Dasychira pudibunda, and Cycnia
mendica. In this latter species the difference in colour between the
two sexes is strongly marked; and Mr. Wallace informs me that we
here have, as he believes, an instance of protective mimicry
confined to one sex, as will hereafter be more fully explained. The
white female of the Cycnia resembles the very common Spilosoma
menthrasti, both sexes of which are white; and Mr. Stainton observed
that this latter moth was rejected with utter disgust by a whole brood
of young turkeys, which were fond of eating other moths; so that if
the Cycnia was commonly mistaken by British birds for the Spilosoma,
it would escape being devoured, and its white deceptive colour would
thus be highly beneficial.
  *(3) It is remarkable, that in the Shetland Islands the male of this
moth, instead of differing widely from the female, frequently
resembles her closely in colour (see Mr. MacLachlan, Transactions,
Entomological Society, vol. ii., 1866, p. 459). Mr. G. Fraser suggests
(Nature, April, 1871, p. 489) that at the season of the year when
the ghost-moth appears in these northern islands, the whiteness of the
males would not be needed to render them visible to the females in the
twilight night.

  From the several foregoing facts it is impossible to admit that
the brilliant colours of butterflies, and of some few moths, have
commonly been acquired for the sake of protection. We have seen that
their colours and elegant patterns are arranged and exhibited as if
for display. Hence I am led to believe that the females prefer or
are most excited by the more brilliant males; for on any other
supposition the males would, as far as we can see, be ornamented to no
purpose. We know that ants and certain lamellicorn beetles are capable
of feeling an attachment for each other, and that ants recognise their
fellows after an interval of several months. Hence there is no
abstract improbability in the Lepidoptera, which probably stand nearly
or quite as high in the scale as these insects, having sufficient
mental capacity to admire bright colours. They certainly discover
flowers by colour. The humming-bird sphinx may often be seen to
swoop down from a distance on a bunch of flowers in the midst of green
foliage; and I have been assured by two persons abroad, that these
moths repeatedly visit flowers painted on the walls of a room, and
vainly endeavour to insert their proboscis into them. Fritz Muller
informs me that several kinds of butterflies in S. Brazil shew an
unmistakable preference for certain colours over others: he observed
that they very often visited the brilliant red flowers of five or
six genera of plants, but never the white or yellow flowering
species of the same and other genera, growing in the same garden;
and I have received other accounts to the same effect. As I hear
from Mr. Doubleday, the common white butterfly often flies down to a
bit of paper on the ground, no doubt mistaking it for one of its own
species. Mr. Collingwood* in speaking of the difficulty in
collecting certain butterflies in the Malay Archipelago, states that
"a dead specimen pinned upon a conspicuous twig will often arrest an
insect of the same species in its headlong flight, and bring it down
within easy reach of the net, especially if it be of the opposite
sex."

  * Rambles of a Naturalist in the Chinese Seas, 1868, p. 182.

  The courtship of butterflies is, as before remarked, a prolonged
affair. The males sometimes fight together in rivalry; and many may be
seen pursuing or crowding round the same female. Unless, then, the
females prefer one male to another, the pairing must be left to mere
chance, and this does not appear probable. If, on the other band,
the females habitually, or even occasionally, prefer the more
beautiful males, the colours of the latter will have been rendered
brighter by degrees, and will have been transmitted to both sexes or
to one sex, according to the law of inheritance which has prevailed.
The process of sexual selection will have been much facilitated, if
the conclusion can be trusted, arrived at from various kinds of
evidence in the supplement to the ninth chapter; namely, that the
males of many Lepidoptera, at least in the imago state, greatly exceed
the females in number.
  Some facts, however, are opposed to the belief that female
butterflies prefer the more beautiful males; thus, as I have been
assured by several collectors, fresh females may frequently be seen
paired with battered, faded, or dingy males; but this is a
circumstance which could hardly fail often to follow from the males
emerging from their cocoons earlier than the females. With moths of
the family of the Bombycidae, the sexes pair immediately after
assuming the imago state; for they cannot feed, owing to the
rudimentary condition of their mouths. The females, as several
entomologists have remarked to me, lie in an almost torpid state,
and appear not to evince the least choice in regard to their partners.
This is the case with the common silk-moth (B. mori), as I have been
told by some continental and English breeders. Dr. Wallace, who has
had great experience in breeding Bombyx cynthia, is convinced that the
females evince no choice or preference. He has kept above 300 of these
moths together, and has often found the most vigorous females mated
with stunted males. The reverse appears to occur seldom; for, as he
believes, the more vigorous males pass over the weakly females, and
are attracted by those endowed with most vitality. Nevertheless, the
Bombycidae, though obscurely-coloured, are often beautiful to our eyes
from their elegant and mottled shades.
  I have as yet only referred to the species in which the males are
brighter coloured than the females, and I have attributed their beauty
to the females for many generations having chosen and paired with
the more attractive males. But converse cases occur, though rarely, in
which the females are more brilliant than the males; and here, as I
believe, the males have selected the more beautiful females, and
have thus slowly added to their beauty. We do not know why in
various classes of animals the males of some few species have selected
the more beautiful females instead of having gladly accepted any
female, as seems to be the general rule in the animal kingdom: but if,
contrary to what generally occurs with the Lepidoptera, the females
were much more numerous than the males, the latter would be likely
to pick out the more beautiful females. Mr. Butler shewed me several
species of Callidryas in the British Museum, in some of which the
females equalled, and in others greatly surpassed the males in beauty;
for the females alone have the borders of their wings suffused with
crimson and orange, and spotted with black. The plainer males of these
species closely resemble each other, shewing that here the females
have been modified; whereas in those cases, where the males are the
more ornate, it is these which have been modified, the females
remaining closely alike.
  In England we have some analogous cases, though not so marked. The
females alone of two species of Thecla have a bright-purple or
orange patch on their fore-wings. In Hipparchia the sexes do not
differ much; but it is the female of H. janira which has a conspicuous
light-brown patch on her wings; and the females of some of the other
species are brighter coloured than their males. Again, the females
of Colias edusa and hyale have "orange or yellow spots on the black
marginal border, represented in the males only by thin streaks"; and
in Pieris it is the females which "are ornamented with black spots
on the fore-wings, and these are only partially present in the males."
Now the males of many butterflies are known to support the females
during their marriage flight; but in the species just named it is
the females which support the males; so that the part which the two
sexes play is reversed, as is their relative beauty. Throughout the
animal kingdom the males commonly take the more active share in
wooing, and their beauty seems to have been increased by the females
having accepted the more attractive individuals; but with these
butterflies, the females take the more active part in the final
marriage ceremony, so that we may suppose that they likewise do so
in the wooing; and in this case we can understand how it is that
they have been rendered the more beautiful. Mr. Meldola, from whom the
foregoing statements have been taken, says in conclusion: "Though I am
not convinced of the action of sexual selection in producing the
colours of insects, it cannot be denied that these facts are
strikingly corroborative of Mr. Darwin's views."*

  * Nature, April 27, 1871, p. 508. Mr. Meldola quotes Donzel, in Soc.
Ent. de France, 1837, p. 77, on the flight of butterflies whilst
pairing. See also Mr. G. Fraser, in Nature, April 20, 1871, p. 489, on
the sexual differences of several British butterflies.

  As sexual selection primarily depends on variability, a few words
must be added on this subject. In respect to colour there is no
difficulty, for any number of highly variable Lepidoptera could be
named. One good instance will suffice. Mr. Bates shewed me a whole
series of specimens of Papilio sesostris and P. childrenae; in the
latter the males varied much in the extent of the beautifully
enamelled green patch on the fore-wings, and in the size of the
white mark, and of the splendid crimson stripe on the hind-wings; so
that there was a great contrast amongst the males between the most and
the least gaudy. The male of Papilio sesostris is much less
beautiful than of P. childrenae; and it likewise varies a little in
the size of the green patch on the fore-wings, and in the occasional
appearance of the small crimson stripe on the hind-wings, borrowed, as
it would seem, from its own female; for the females of this and of
many other species in the Aeneas group possess this crimson stripe.
Hence between the brightest specimens of P. sesostris and the
dullest of P. childrenae, there was but a small interval; and it was
evident that as far as mere variability is concerned, there would be
no difficulty in permanently increasing the beauty of either species
by means of selection. The variability is here almost confined to
the male sex; but Mr. Wallace and Mr. Bates have shewn* that the
females of some species are extremely variable, the males being nearly
constant. In a future chapter I shall have occasion to shew that the
beautiful eye-like spots, or ocelli, found on the wings of many
Lepidoptera, are eminently variable. I may here add that these
ocelli offer a difficulty on the theory of sexual selection; for
though appearing to us so ornamental, they are never present in one
sex and absent in the other, nor do they ever differ much in the two
sexes.*(2) This fact is at present inexplicable; but if it should
hereafter be found that the formation of an ocellus is due to some
change in the tissues of the wings, for instance, occurring at a
very early period of development, we might expect, from what we know
of the laws of inheritance, that it would be transmitted to both
sexes, though arising and perfected in one sex alone.

  * Wallace on the "Papilionidae of the Malayan Region," in
Transact. Linn. Soc., vol. xxv., 1865, pp. 8, 36. A striking case of a
rare variety, strictly intermediate between two other well-marked
female varieties, is given by Mr. Wallace. See also Mr. Bates, in
Proc. Entomolog. Soc., Nov. 19, 1866, p. xl.
  *(2) Mr. Bates was so kind as to lay this subject before the
Entomological Society, and I have received answers to this effect from
several entomologists.

  On the whole, although many serious objections may be urged, it
seems probable that most of the brilliantly-coloured species of
Lepidoptera owe their colours to sexual selection, excepting in
certain cases, presently to be mentioned, in which conspicuous colours
have been gained through mimicry as a protection. From the ardour of
the male throughout the animal kingdom, he is generally willing to
accept any female; and it is the female which usually exerts a choice.
Hence, if sexual selection has been efficient with the Lepidoptera,
the male, when the sexes differ, ought to be the more brilliantly
coloured, and this undoubtedly is the case. When both sexes are
brilliantly coloured and resemble each other, the characters
acquired by the males appear to have been transmitted to both. We
are led to this conclusion by cases, even within the same genus, of
gradation from an extraordinary amount of difference to identity in
colour between the two sexes.
  But it may be asked whether the difference in colour between the
sexes may not be accounted for by other means besides sexual
selection. Thus the males and females of the same species of butterfly
are in several cases known* to inhabit different stations, the
former commonly basking in the sunshine, the latter haunting gloomy
forests. It is therefore possible that different conditions of life
may have acted directly on the two sexes; but this is not probable*(2)
as in the adult state they are exposed to different conditions
during a very short period; and the larvae of both are exposed to
the same conditions. Mr. Wallace believes that the difference
between the sexes is due not so much to the males having been
modified, as to the females having in all or almost all cases acquired
dull colours for the sake of protection. It seems to me, on the
contrary, far more probable that it is the males which have been
chiefly modified through sexual selection, the females having been
comparatively little changed. We can thus understand how it is that
the females of allied species generally resemble one another so much
more closely than do the males. They thus shew us approximately the
primordial colouring of the parent-species of the group to which
they belong. They have, however, almost always been somewhat
modified by the transfer to them of some of the successive variations,
through the accumulation of which the males were rendered beautiful.
But I do not wish to deny that the females alone of some species may
have been specially modified for protection. In most cases the males
and females of distinct species will have been exposed during their
prolonged larval state to different conditions, and may have been thus
affected; though with the males any slight change of colour thus
caused will generally have been masked by the brilliant tints gained
through sexual selection. When we treat of birds, I shall have to
discuss the whole question, as to how far the differences in colour
between the sexes are due to the males having been modified through
sexual selection for ornamental purposes, or to the females having
been modified through natural selection for the sake of protection, so
that I will here say but little on the subject.

  * H. W. Bates, The Naturalist on the Amazons, vol. ii., 1863, p.
228. A. R. Wallace, in Transactions, Linnean Society, vol. xxv., 1865,
p. 10.
  *(2) On this whole subject see The Variation of Animals and Plants
under Domestication, 1868, vol. ii., chap. xxiii.

  In all the cases in which the more common form of equal
inheritance by both sexes has prevailed, the selection of
bright-coloured males would tend to make the females
bright-coloured; and the selection of dull-coloured females would tend
to make the males dull. If both processes were carried on
simultaneously, they would tend to counteract each other; and the
final result would depend on whether a greater number of females
from being well protected by obscure colours, or a greater number of
males by being brightly-coloured and thus finding partners,
succeeded in leaving more numerous offspring.
  In order to account for the frequent transmission of characters to
one sex alone, Mr. Wallace expresses his belief that the more common
form of equal inheritance by both sexes can be changed through natural
selection into inheritance by one sex alone, but in favour of this
view I can discover no evidence. We know from what occurs under
domestication that new characters often appear, which from the first
are transmitted to one sex alone; and by the selection of such
variations there would not be the slightest difficulty in giving
bright colours to the males alone, and at the same time or
subsequently, dull colours to the females alone. In this manner the
females of some butterflies and moths have, it is probable, been
rendered inconspicuous for the sake of protection, and widely
different from their males.
  I am, however, unwilling without distinct evidence to admit that two
complex processes of selection, each requiring the transference of new
characters to one sex alone, have been carried on with a multitude
of species,- that the males have been rendered more brilliant by
beating their rivals, and the females more dull-coloured by having
escaped from their enemies. The male, for instance, of the common
brimstone butterfly (Gonepteryx), is of a far more intense yellow than
the female, though she is equally conspicuous; and it does not seem
probable that she specially acquired her pale tints as a protection,
though it is probable that the male acquired his bright colours as a
sexual attraction. The female of Anthocharis cardamines does not
possess the beautiful orange wing-tips of the male; consequently she
closely resembles the white butterflies (Pieris) so common in our
gardens; but we have no evidence that this resemblance is beneficial
to her. As, on the other hand, she resembles both sexes of several
other species of the genus inhabiting various quarters of the world,
it is probable that she has simply retained to a large extent her
primordial colours.
  Finally, as we have seen, various considerations lead to the
conclusion that with the greater number of brilliantly-coloured
Lepidoptera it is the male which has been chiefly modified through
sexual selection; the amount of difference between the sexes mostly
depending on the form of inheritance which has prevailed.
Inheritance is govemed by so many unknown laws or conditions, that
it seems to us to act in a capricious manner;* and we can thus, to a
certain extent, understand how it is that with closely allied
species the sexes either differ to an astonishing degree, or are
identical in colour. As all the successive steps in the process of
variation are necessarily transmitted through the female, a greater or
less number of such steps might readily become developed in her; and
thus we can understand the frequent gradations from an extreme
difference to none at all between the sexes of allied species. These
cases of gradation, it may be added, are much too common to favour the
supposition that we here see females actually undergoing the process
of transition and losing their brightness for the sake of
protection; for we have every reason to conclude that at any one
time the greater number of species are in a fixed condition.

  * The Variation of Animals and Plants under Domestication, vol. ii.,
chap. xii., p. 17.

  Mimicry.- This principle was first made clear in an admirable
paper by Mr. Bates,* who thus threw a flood of light on many obscure
problems. It had previously been observed that certain butterflies
in S. America belonging to quite distinct families, resembled the
Heliconidae so closely in every stripe and shade of colour, that
they could not be distinguished save by an experienced entomologist.
As the Heliconidae are coloured in their usual manner, whilst the
others depart from the usual colouring of the groups to which they
belong, it is clear that the latter are the imitators, and the
Heliconidae the imitated. Mr. Bates further observed that the
imitating species are comparatively rare, whilst the imitated
abound, and that the two sets live mingled together. From the fact
of the Heliconidae being conspicuous and beautiful insects, yet so
numerous in individuals and species, he concluded that they must be
protected from the attacks of enemies by some secretion or odour;
and this conclusion has now been amply confirmed,*(2) especially by
Mr. Belt. Hence Mr. Bates inferred that the butterflies which
imitate the protected species have acquired their present marvellously
deceptive appearance through variation and natural selection, in order
to be mistaken for the protected kinds, and thus to escape being
devoured. No explanation is here attempted of the brilliant colours of
the imitated, but only of the imitating butterflies. We must account
for the colours of the former in the same general manner, as in the
cases previously discussed in this chapter. Since the publication of
Mr. Bates's paper, similar and equally striking facts have been
observed by Mr. Wallace in the Malayan region, by Mr. Trimen in
South Africa, and by Mr. Riley in the United States.*(3)

  * Transact. Linn. Soc., vol. xxiii., 1862, p. 495.
  *(2) Proc. Entomological Soc., Dec. 3, 1866, p. xlv.
  *(3) Wallace, Transact. Linn. Soc., vol. xxv., 1865 p. i.; also,
Transact. Ent. Soc., vol. iv., 3rd series: 1867, p. 301. Trimen, Linn.
Transact., vol. xxvi., 1869, p. 497. Riley, Third Annual Report on the
Noxious Insects of Missouri, 1871, pp. 163-168. This latter essay is
valuable, as Mr. Riley here discusses all the objections which have
been raised against Mr. Bates's theory.

  As some writers have felt much difficulty in understanding how the
first steps in the process of mimicry could have been effected through
natural selection, it may be well to remark that the process
probably commenced long ago between forms not widely dissimilar in
colour. In this case even a slight variation would be beneficial, if
it rendered the one species more like the other; and afterwards the
imitated species might be modified to an extreme degree through sexual
selection or other means, and if the changes were gradual, the
imitators might easily be led along the same track, until they
differed to an equally extreme degree from their original condition;
and they would thus ultimately assume an appearance or colouring
wholly unlike that of the other members of the group to which they
belonged. It should also be remembered that many species of
Lepidoptera are liable to considerable and abrupt variations in
colour. A few instances have been given in this chapter; and many more
may be found in the papers of Mr. Bates and Mr. Wallace.
  With several species the sexes are alike, and imitate the two
sexes of another species. But Mr. Trimen gives, in the paper already
referred to, three cases in which the sexes of the imitated form
differ from each other in colour, and the sexes of the imitating
form differ in a like manner. Several cases have also been recorded
where the females alone imitate brilliantly-coloured and protected
species, the males retaining "the normal aspect of their immediate
congeners." It is here obvious that the successive variations by which
the female has been modified have been transmitted to her alone. It
is, however, probable that some of the many successive variations
would have been transmitted to, and developed in, the males had not
such males been eliminated by being thus rendered less attractive to
the females; so that only those variations were preserved which were
from the first strictly limited in their transmission to the female
sex. We have a partial illustration of these remarks in a statement by
Mr. Belt;* that the males of some of the Leptalides, which imitate
protected species, still retain in a concealed manner some of their
original characters. Thus in the males "the upper half of the lower
wing is of a pure white, whilst all the rest of the wings is barred
and spotted with black, red and yellow, like the species they mimic.
The females have not this white patch, and the males usually conceal
it by covering it with the upper wing, so that I cannot imagine its
being of any other use to them than as an attraction in courtship,
when they exhibit it to the females, and thus gratify their
deep-seated preference for the normal colour of the Order to which the
Leptalides belong."

  * The Naturalist in Nicaragua, 1874, p. 385.

  Bright Colours of Caterpillars.- Whilst reflecting on the beauty
of many butterflies, it occurred to me that some caterpillars were
splendidly coloured; and as sexual selection could not possibly have
here acted, it appeared rash to attribute the beauty of the mature
insect to this agency, unless the bright colours of their larvae could
be somehow explained. In the first place, it may be observed that
the colours of caterpillars do not stand in any close correlation with
those of the mature insect. Secondly, their bright colours do not
serve in any ordinary manner as a protection. Mr. Bates informs me, as
an instance of this, that the most conspicuous caterpillar which he
ever beheld (that of a sphinx) lived on the large green leaves of a
tree on the open llanos of South America; it was about four inches
in length, transversely banded with black and yellow, and with its
head, legs, and tail of a bright red. Hence it caught the eye of any
one who passed by, even at the distance of many yards, and no doubt
that of every passing bird.
  I then applied to Mr. Wallace, who has an innate genius for
solving difficulties. After some consideration he replied: "Most
caterpillars require protection, as may be inferred from some kinds
being furnished with spines or irritating hairs, and from many being
coloured green like the leaves on which they feed, or being
curiously like the twigs of the trees on which they live." Another
instance of protection, furnished me by Mr. J. Mansel Weale, may be
added, namely, that there is a caterpillar of a moth which lives on
the mimosas in South Africa, and fabricates for itself a case quite
indistinguishable from the surrounding thorns. From such
considerations Mr. Wallace thought it probable that conspicuously
coloured caterpillars were protected by having a nauseous taste; but
as their skin is extremely tender, and as their intestines readily
protrude from a wound, a slight peck from the beak of a bird would
be as fatal to them as if they had been devoured. Hence, as Mr.
Wallace remarks, "distastefulness alone would be insufficient to
protect a caterpillar unless some outward sign indicated to its
would-be destroyer that its prey was a disgusting morsel." Under these
circumstances it would be highly advantageous to a caterpillar to be
instantaneously and certainly recognised as unpalatable by all birds
and other animals. Thus the most gaudy colours would be serviceable,
and might have been gained by variation and the survival of the most
easily-recognised individuals.
  This hypothesis appears at first sight very bold, but when it was
brought before the Entomological Society* it was supported by
various statements; and Mr. J. Jenner Weir, who keeps a large number
of birds in an aviary, informs me that he has made many trials, and
finds no exception to the rule, that all caterpillars of nocturnal and
retiring habits with smooth skins, all of a green colour, and all
which imitate twigs, are greedily devoured by his birds. The hairy and
spinose kinds are invariably rejected, as were four
conspicuously-coloured species. When the birds rejected a caterpillar,
they plainly shewed, by shaking their heads, and cleansing their
beaks, that they were disgusted by the taste.*(2) Three conspicuous
kinds of caterpillars and moths were also given to some lizards and
frogs, by Mr. A. Butler, and were rejected, though other kinds were
eagerly eaten. Thus the probability of Mr. Wallace's view is
confirmed, namely, that certain caterpillars have been made
conspicuous for their own good, so as to be easily recognised by their
enemies, on nearly the same principle that poisons are sold in
coloured bottles by druggists for the good of man. We cannot, however,
at present thus explain the elegant diversity in the colours of many
caterpillars; but any species which had at some former period acquired
a dull, mottled, or striped appearance, either in imitation of
surrounding objects, or from the direct action of climate, &c., almost
certainly would not become uniform in colour when its tints were
rendered intense and bright; for in order to make a caterpillar merely
conspicuous, there would be no selection in any definite direction.

  * Proceedings, Entomological Society, Dec. 3, 1866, p. xlv., and
March 4, 1867, p. lxxx.
  *(2) See Mr. J. Jenner Weir's Paper on "Insects and Insectivorus
Birds," in Transact. Ent. Soc., 1869, p. 21; also Mr. Butler's
paper, ibid., p. 27. Mr. Riley has given analogous facts in the
Third Annual Report on the Noxious Insects of Missouri, 1871, p.
148. Some opposed cases are, however, given by Dr. Wallace and M. H.
d'Orville; see Zoological Record, 1869, p. 349.

  Summary and Concluding Remarks on Insects.- Looking back to the
several Orders, we see that the sexes often differ in various
characters, the meaning of which is not in the least understood. The
sexes, also, often differ in their organs of sense and means of
locomotion, so that the males may quickly discover and reach the
females. They differ still oftener in the males possessing diversified
contrivances for retaining the females when found. We are, however,
here concerned only in a secondary degree with sexual differences of
these kinds.
  In almost all the Orders, the males of some species, even of weak
and delicate kinds, are known to be highly pugnacious; and some few
are furnished with special weapons for fighting with their rivals. But
the law of battle does not prevail nearly so widely with insects as
with the higher animals. Hence it probably arises, that it is in
only a few cases that the males have been rendered larger and stronger
than the females. On the contrary, they are usually smaller, so that
they may be developed within a shorter time, to be ready in large
numbers for the emergence of the females.
  In two families of the Homoptera and in three of the Orthoptera, the
males alone possess sound-producing organs in an efficient state.
These are used incessantly during the breeding-season, not only for
calling the females, but apparently for charming or exciting them in
rivalry with other males. No one who admits the agency of selection of
any kind, will, after reading the above discussion, dispute that these
musical instruments have been acquired through sexual selection. In
four other Orders the members of one sex, or more commonly of both
sexes, are provided with organs for producing various sounds, which
apparently serve merely as call-notes. When both sexes are thus
provided, the individuals which were able to make the loudest or
most continuous noise would gain partners before those which were less
noisy, so that their organs have probably been gained through sexual
selection. It is instructive to reflect on the wonderful diversity
of the means for producing sound, possessed by the males alone, or
by both sexes, in no less than six Orders. We thus learn how effectual
sexual selection has been in leading to modifications which sometimes,
as with the Homoptera, relate to important parts of the organisation.
  From the reasons assigned in the last chapter, it is probable that
the great horns possessed by the males of many lamellicorn, and some
other beetles, have been acquired as ornaments. From the small size of
insects, we are apt to undervalue their appearance. If we could
imagine a male Chalcosoma (see fig. 16), with its polished bronzed
coat of mail, and its vast complex horns, magnified to the size of a
horse, or even of a dog, it would be one of the most imposing
animals in the world.
  The colouring of insects is a complex and obscure subject. When
the male differs slightly from the female, and neither are
brilliantly-coloured, it is probable that the sexes have varied in a
slightly different manner, and that the variations have been
transmitted by each sex to the same without any benefit or evil thus
accruing. When the male is brilliantly-coloured and differs
conspicuously from the female, as with some dragonflies and many
butterflies, it is probable that he owes his colours to sexual
selection; whilst the female has retained a primordial or very ancient
type of colouring, slightly modified by the agencies before explained.
But in some cases the female has apparently been made obscure by
variations transmitted to her alone, as a means of direct
protection; and it is almost certain that she has sometimes been
made brilliant, so as to imitate other protected species inhabiting
the same district. When the sexes resemble each other and both are
obscurely coloured, there is no doubt that they have been in a
multitude of cases so coloured for the sake of protection. So it is in
some instances when both are brightly-coloured, for they thus
imitate protected species, or resemble surrounding objects such as
flowers; or they give notice to their enemies that they are
unpalatable. In other cases in which the sexes resemble each other and
are both brilliant, especially when the colours are arranged for
display, we may conclude that they have been gained by the male sex as
an attraction, and have been transferred to the female. We are more
especially led to this conclusion whenever the same type of coloration
prevails throughout a whole group, and we find that the males of
some species differ widely in colour from the females, whilst others
differ slightly or not at all with intermediate gradations
connecting these extreme states.
  In the same manner as bright colours have often been partially
transferred from the males to the females, so it has been with the
extraordinary horns of many lamellicorn and some other beetles. So
again, the sound-producing organs proper to the males of the Homoptera
and Orthoptera have generally been transferred in a rudimentary, or
even in a nearly perfect condition, to the females; yet not
sufficiently perfect to be of any use. It is also an interesting fact,
as bearing on sexual selection, that the stridulating organs of
certain male Orthoptera are not fully developed until the last
moult; and that the colours of certain male dragon-flies are not fully
developed until some little time after their emergence from the
pupal state, and when they are ready to breed.
  Sexual selection implies that the more attractive individuals are
preferred by the opposite sex; and as with insects, when the sexes
differ, it is the male which, with some rare exceptions, is the more
ornamented, and departs more from the type to which the species
belongs;- and as it is the male which searches eagerly for the female,
we must suppose that the females habitually or occasionally prefer the
more beautiful males, and that these have thus acquired their
beauty. That the females in most or all the Orders would have the
power of rejecting any particular male, is probable from the many
singular contrivances possessed by the males, such as great jaws,
adhesive cushions, spines, elongated legs, &c., for seizing the
female; for these contrivances shew that there is some difficulty in
the act, so that her concurrence would seem necessary. Judging from
what we know of the perceptive powers and affections of various
insects, there is no antecedent improbability in sexual selection
having come largely into play; but we have as yet no direct evidence
on this head, and some facts are opposed to the belief.
Nevertheless, when we see many males pursuing the same female, we
can hardly believe that the pairing is left to blind chance- that
the female exerts no choice, and is not influenced by the gorgeous
colours or other ornaments with which the male is decorated.
  If we admit that the females of the Homoptera and Orthoptera
appreciate the musical tones of their male partners, and that the
various instruments have been perfected through sexual selection,
there is little improbability in the females of other insects
appreciating beauty in form or colour, and consequently in such
characters having been thus gained by the males. But from the
circumstance of colour being so variable, and from its having been
so often modified for the sake of protection, it is difficult to
decide in how large a proportion of cases sexual selection has
played a part. This is more especially difficult in those Orders, such
as Orthoptera, Hymenoptera, and Coleoptera, in which the two sexes
rarely differ much in colour; for we are then left to mere analogy.
With the Coleoptera, however, as before remarked, it is in the great
lamellicorn group, placed by some authors at the head of the Order,
and in which we sometimes see a mutual attachment between the sexes,
that we find the males of some species possessing weapons for sexual
strife, others furnished with wonderful horns, many with
stridulating organs, and others ornamented with splendid metallic
tints. Hence it seems probable that all these characters have been
gained through the same means, namely sexual selection. With
butterflies we have the best evidence, as the males sometimes take
pains to display their beautiful colours; and we cannot believe that
they would act thus, unless the display was of use to them in their
courtship.
  When we treat of birds, we shall see that they present in their
secondary sexual characters the closest analogy with insects. Thus,
many male birds are highly pugnacious, and some are furnished with
special weapons for fighting with their rivals. They possess organs
which are used during the breeding-season for producing vocal and
instrumental music. They are frequently ornamented with combs,
horns, wattles and plumes of the most diversified kinds, and are
decorated with beautiful colours, all evidently for the sake of
display. We shall find that, as with insects, both sexes in certain
groups are equally beautiful, and are equally provided with
ornaments which are usually confined to the male sex. In other
groups both sexes are equally plain-coloured and unornamented. Lastly,
in some few anomalous cases, the females are more beautiful than the
males. We shall often find, in the same group of birds, every
gradation from no difference between the sexes, to an extreme
difference. We shall see that female birds, like female insects, often
possess more or less plain traces or rudiments of characters which
properly belong to the males and are of use only to them. The analogy,
indeed, in all these respects between birds and insects is curiously
close. Whatever explanation applies to the one class probably
applies to the other; and this explanation, as we shall hereafter
attempt to shew in further detail, is sexual selection.
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