or the other of the already mentioned male distinguishing characters
some which do not directly lead to gaining possession of the female,
but do so only by sexually exciting her, can we doubt that the same
principle has been operative, that here too processes of selection
are fundamental, depending on the fact that in the wooing of the
female the successful male is the one who most strongly excites her?
There is no question of æsthetic pleasure in this, as the opponents
of the theory of sexual selection have often urged, but only of
sexual excitement, which may be aroused by very different means, by
colours and shapes, but also by love-calls, songs, or odours. There
are a few tropical birds (_Chasmorhynchus_) which have as the only
distinguishing character of the male sex a hollow and soft appendage
several inches long borne on the head. Usually it hangs down limply at
the side of the head, but during the breeding season it is inflated
from the mouth-cavity, and then stands erect like a spur. One species
of this genus has as many as three of these horns, one of which is
upright, while the other two stand out laterally from the head. Can it
be supposed that these remarkable horns satisfy the female's 'sense
of beauty'? To human beings they appear rather ugly than beautiful,
both when limp and when inflated, but at any rate they are striking,
and will be regarded by the female bird as something out of the
common, and, since they are only fully displayed during the breeding
season, that is, when the male is sexually excited, they will have an
exciting effect on the female too. These inflated horns are symptoms
of excitement, and they arouse it in the female. In exactly the same
way the decorative feathers, the ruby-red and emerald-green feather
collars of the humming-birds and birds of Paradise, are only erected
and displayed when the males are wooing, and they, too, act as signs
of excitement. This is not to say that the gorgeousness of colour,
the eye-spots on the train of the peacock and the Argus pheasant, and
the hundreds of different kinds of beautiful feathers, do not also
exercise a fascinating influence; on the contrary, we cannot avoid
assuming this, since otherwise we could find no sufficient reason for
their origin. But the primary effect in wooing is not due to the mere
pleasure in the sight, or in the odour, or in the song, but to the
contagious excitement which these express. The females do not behave as
dispassionate judges, but as excitable persons which fall to the lot of
the male who is able to excite them most strongly. It may be, however,
that a sense of æsthetic satisfaction in perceiving such symptoms of
excitement may also have been evolved as an accessory effect, at least
in the higher and more intelligent animals.
In the lower animals, which are lacking not only in intelligence but
also in the higher and more complex differentiation of the sensory
system, the development of such secondary sex characters is rare or
altogether absent. Animals which have no sense of hearing can develop
no song, and animals which do not see cannot acquire gorgeous colours
as a means of exciting one sex through the other. But distinctive sex
coloration may arise even in lowly animals, though there can be no
question of æsthetic pleasure associated therewith; if the animals are
able to see the colours at all, sexual excitement may be associated
with these.
We need not wonder, therefore, that in the somewhat stupid fishes, in
the butterflies, and in the lower crustaceans, like the Daphnids, we
still find brilliant colours, which we can hardly interpret otherwise
than as the results of sexual selection. On the other hand, the absence
of such characters in animals of a still lower order, with still
simpler sense-organs, like the Polyps, Medusæ, Echinoderms, most Worms,
and the Sponges, affords an indirect confirmation of the correctness
of our view as to the reality of a sexual selection in the more highly
organized animals.
We see, then, that numerous peculiarities which distinguish the
males of a species from the females depend on the process of sexual
selection. This may be said of ornamental outgrowths, colours,
remarkable feathers and feather-groups, peculiar odoriferous organs,
vocal organs, artistic instincts, and also weapons, like antlers,
tusks, and spurs, notable size and strength of body, and protective
devices like manes; and again, the various organs for catching and
holding the females, or for finding them out by sight or smell, must
also be referred, at least in part, to sexual selection. The diversity
of the male sexual characters is so great that I cannot give more than
a faint idea of them without entering on a long catalogue; whoever
wishes a complete survey has only to consult Darwin's _Descent of Man_.
But the significance of sexual selection is by no means exhausted with
the production of the male sexual characters, for these characters are
often more or less completely transferred to the females, and thus give
rise to a transformation of the whole species, and not only of the male
section of it. This is obviously a very important consequence of sexual
selection, one which, as we shall see, materially deepens our insight
into the mode of origin of new species.
First let us try to determine the facts. Many male characters are not
represented in the female in any degree, and therefore have never
been transmitted to them at all. Such are the mane of the lion, the
grasping antennæ of _Moina_, the turban eyes of the Ephemerides, the
intensification of the sense of smell in _Leptodora_, the lasso-like
antennæ of the Copepods, the scent-scales of the butterflies, and the
musk glands of the alligators and stags. But in other cases there has
been transmission, though only to a slight extent. Thus many female
humming-birds have a faint indication of the magnificent metallic
colouring of the males; many female blue butterflies have a tinge of
the beautiful blue of their mates; the females of the stag-beetle
(_Lucanus cervus_) possess a diminutive suggestion of the antler-like
jaws of the male, and the female crickets, although they do not chirp,
have a slight indication of the 'musical' mechanism of the male on the
wing-coverts, and some of them even produce feeble notes at certain
times.
It can be proved, however, that such transmissions may, in the
course of many successive generations, become intensified until the
characters are exhibited by the females in the same degree as in the
males. I know no better example of this than that afforded by the
beautiful butterflies of the genus _Lycæna_. In this genus, which is
rich in species and widely distributed over the whole earth, and must
therefore be an old one, the upper surface of the wing is blue in by
far the greater number of species, at least in the male sex. But there
are three or four species which are dark-brown, and quite or nearly
alike in the two sexes; such are the species _Lycæna agestis_, _L.
eumedon_, _L. admetus_, and others. Everything indicates that this is
the primitive colour of the genus. Moreover, there are some species
with brown females, in which the males are not completely blue, but
which have a slight bluish tinge, like _L. alsus_, the smallest of
our indigenous Blues. Then follows a host of beautiful species, like
_L. alexis_, _L. adonis_, _L. damon_, _L. corydon_, and many others,
with brown females, and among these there occasionally occur females
more or less tinged with blue. These lead on to _L. meleager_, which
has two forms of female, a common brown and a rarer blue; and thus
we reach _L. tiresias_, _L. optilete_, and _L. argiolus_, in which
all the females are blue, although less intensely and completely so
than their mates. The climax of this evolutionary series is reached
by some species like _L. beatica_, belonging to tropical or at least
warm countries, in which both sexes are of an equally intense blue. As
we know that, in species with an excess of males, sexual characters
always begin in the males, there can be no doubt as to the direction
of evolution--from brown to blue--in this series. Furthermore, the
entire absence of scent-scales in most of the species with brown males
indicates the great age of these species, for, as far as I have been
able to investigate, all the males of the blue species possess them.
Darwin regarded this transferring of the male characters to the females
as due to inheritance, and it really seems as if it were simply a case
of transmission by inheritance to one sex of what has been acquired
by the other. Yet we have to ask whether we can continue to regard
the facts in this light. In any case this 'transmission' is not an
inevitable physiological process, necessarily resulting from the
intrinsic conditions of inheritance, for we see that it often does
not occur, even in many cases in which we can see no external reasons
why it should not do so, though in other cases the failure may be
presumably correlated with the external conditions of life. Thus, for
instance, the persistent retention of the brown colour in the majority
of our female Lycænidæ has probably its reason in the greater need
of protection on the part of the much rarer females, and this must
be so also in the case of many birds in which the brilliant colours
of the males have not been transferred to the females. Wallace first
pointed out that all birds whose females brood in exposed nests are
inconspicuously coloured in the female sex, even if the males are
brightly coloured, while those whose nests are concealed in holes of
trees or the like, or which build domes over them, not rarely exhibit
brilliant colouring in both sexes. This is the case in woodpeckers and
parrots, while the gallinaceous birds, which brood in the open, have
usually inconspicuously coloured females, for the most part very well
adapted to their surroundings.
If we grasp the fact that a transference of the characters which have
arisen through sexual selection can take place, we have a valuable
aid in the interpretation of many phenomena which would otherwise
remain quite inexplicable. What is the meaning of the gay colours of
the parrots, which occur in such incredibly diverse combinations
in this large and widely distributed family? Or of the marvellously
complex markings and colour-patterns of the butterflies? In some cases
they may be protective, as is the green of many parrots; in others,
warning signs of unpalatability, like the bright colours and contrasted
markings of many Heliconiidæ and Eusemiidæ and other butterflies with
a nauseous taste; but there remain a great many cases to which neither
of these explanations applies, which could only be regarded as pure
freaks of nature if we did not know that male sexual characters can
be transferred to the females, and that thus all the individuals of a
species can be totally altered in their colouring.
Thus the occurrence not only of conspicuous, but of complicated,
coloration is explained.
Darwin has shown that, in the equipment developed by the males in their
competition for the possession of the females, it is by no means only
those characters which may be considered 'beautiful' in themselves that
have to be considered; it is rather the striking characteristics which
mark their possessor and distinguish it from others that are primarily
important. In fact, it is the principle of 'mode' or 'fashion' which is
operative; something new is demanded, and as far as possible something
quite different from that which was previously considered beautiful.
Thus the starting-point for such processes of selection may have been
afforded by white spots on a black ground, or, indeed, by any light
spots on a dark ground, which may have been the primitive colour in
most cases. If in the course of a long series of generations these
spots became the common property of all the males, a possibility of
further change was opened up as soon as a new contrast cropped up as
a chance variation, which would then, under favourable conditions, be
the starting-point of a new process of selection. Darwin has cited
some cases in which, from a comparison of the dress of the young bird
with that of the adult, we may conclude that a transformation of the
colouring of the whole plumage must have taken place in the course of
the phylogenetic history.
In other cases the course of the process of selection has been such
that, though the general colouring has not been changed, variations
have appeared in particular regions of the body--spots or stripes which
accumulated through the ages and co-operated to form an increasingly
diverse and complex colour-scheme, such a 'marking' of the animal as we
may observe to-day, especially in butterflies, but also in birds.
It is a fine corroboration of the origin of bright colours through
sexual selection that, even in those groups of the animal kingdom which
are in general sexually monomorphic, there always occur some species
in which male and female are quite different, and a host of species in
which both sexes are alike in the main, yet with differences in certain
minor points. Among the parrots similarity of colouring prevails as
a general rule, but in New Guinea there lives a parrot the female of
which is a gorgeous blood-red and the male a beautiful light-green;
minor differences occur in many species, for instance, the female
of the horned parrot (_Cyanorhamphus cornutus_ Gm.) lacks the two
long black and red feathers on the head, that of the grass-parakeet
(_Melopsittacus undulatus_) is a slightly paler green and has not
the beautiful blue spots on the cheeks which the male possesses.
Innumerable similar instances might be cited, serving to show that all
these distinguishing characters of the males have been acquired step by
step and piece by piece, and are slowly and independently transferred
to the females--if, indeed, at all.
In yet another way the correctness of the Darwinian theory of sexual
selection may be deduced from the markings and coloration of birds and
butterflies.
It has frequently struck me, during the long period in which I have
been studying brightly coloured birds and butterflies, that those
colour-patterns which are referable to sexual selection are much
simpler than those which must be referred to species-selection,
especially in the case of what we call 'sympathetic coloration.' How
crude is the decorative pattern of most parrots, notwithstanding all
the brilliance of their colour. Large tracts of the body are red,
others green, yellow, blue, and occasionally one finds a red and
blue striped feather collar, a head which is red above and yellow
underneath, but it is seldom that the colours vary enough in a small
space to give rise to a delicate decorative pattern. The gayest of
parrots are the Brush Tongues (_Trichoglossus_), and even among them
subtlety of coloration does not go further than the combination of
three colours on one of the long tail-feathers, or the production of
a double band round the neck, and so forth. If we compare with this
the complex markings of the inconspicuously coloured females of the
pheasants, of the partridges, or that of the upper surface of the
many birds in mingled grey, blackish-brown and white, which resemble
the ground or the dried leaves when they crouch, we find that the
colour-pattern in these cases is infinitely finer and more complex.
This seems to me quite intelligible when we remember, on the one hand,
that species-selection must operate far more intensively than sexual
selection, and that in the production of a protective colouring it is
a question of deceiving the eye of a sharp-sighted enemy, while the
aim of sexual selection is to secure the approval of others of the
same species. As long as the enemy on the search for prey perceives
the difference between the markings of its victim and those of the
surroundings, so long will the gradual and steady improvement of the
protective coloration continue, so long will new shades and new lines
be added. We can thus understand how there would be gradually reached
a complexity of marking to which sexual selection can never attain,
or at least only in regard to a few specially favourable points.
The eye-spots on the train feathers of the Argus pheasant and the
peacock are such points, and these occur among polygamous birds in
which sexual selection must be very intense; they are placed, too,
on a part of the body, the wheel-shaped train, which is peculiarly
suited for communicating the excitement of the male to the female, and
must therefore be especially influenced by the latter. In general,
however, we may say on _a priori_ grounds that the intensity of
species-selection is greater than that of sexual selection, because the
former ceaselessly and pitilessly eliminates the less perfect, while
the claims of the latter are in any case less imperative, and are also
often mollified by a variety of chance circumstances.
But in the case of insects, in particular, we have to add that the
protective colours and the decorative colours have been, so to speak,
painted by different artists--the former by birds, lizards, and other
persecutors endowed with well-developed eyes, the latter by the insects
themselves, whose eyes can hardly possess, for objects not quite near,
that acuteness of vision which the bird's eye has. Thus we find that
the protective coloration of butterflies has often a very complex
marking, while the same butterfly may exhibit only a rather crude
though brilliant pattern on its upper surface, where the coloration is
due to sexual selection. Thus the famous _Kallima_ has on its under
surface the likeness of a dry or decayed leaf composed of a number
of colour-tones--quite a complex painting. But if we look at the
upper surface we see a deep brown with a shimmer of steel blue as the
ground-colour of the wings, and on it a broad yellow band and a white
spot: that is the whole pattern. We find a similar state of things
among many of the forest butterflies of Brazil, and also among our
indigenous butterflies. The pattern of our gayest diurnal butterflies,
the red Admiral and the tortoiseshell butterfly (_Vanessa atalanta_ and
_Vanessa cardui_), is somewhat crude on the upper surface, and very
simple compared with the protective colouring of the under surface,
which is made up of hundreds of points, spots, strokes, and lines of
every shape and colour. On the other hand, the upper surface of the
anterior wings in the hawk-moths and the Noctuidæ exhibits protective
coloration, and is made up of curious zigzag complex lines, strokes,
and spots, so that it resembles the bark of a tree or a bit of an old
wooden fence--a painting, like the modern impressionist work, which,
with an apparently meaningless confusion of colour splashes, conveys a
perfect impression even of the details of a landscape. In the owl-moths
(Noctuidæ) the wing surfaces, which are brightly coloured, are simple,
almost crude, in pattern, as in the moths of the genus _Catocala_, with
their red, blue, or yellow posterior wings, traversed by a large black
band; while in the Geometer-moths, whose wings are spread out flat when
at rest, the protective upper surface of all four wings is covered
with a complex pattern of lines, spots, and streaks in different
shades of grey, yellow, white, and black, so that it bears a deceptive
resemblance to the bark of a tree or the side of a wall. For a long
time I could not understand how such a definite and constant pattern
could arise through natural selection if it was a case of mimicking the
impression of bark or of any other irregularly covered surface, the
colours of which are not mingled in exactly the same way everywhere.
But now I think I understand it; for in the apparently meaningless
colour-splashes of an 'impressionist' landscape the different splashes
must be exactly where they are, otherwise on stepping back from the
picture one would see, not a Haarlem hyacinth-field, or an avenue of
poplars with their golden autumn leaves, but a mere unintelligible
daub. It is the _type_ of the colour-pattern that must be attained,
and in nature this is attained very slowly, step by step, spot after
spot, and therefore, obviously, no correct stroke once attained will
be given up again, since, in combination with the rest, it secures
the proper type of colour-pattern. Only thus, it seems to me, can we
understand how apparently meaningless lines, like the figures 1840 on
the under surface of _Vanessa atalanta_, could have become a constant
characteristic of the species.
To sum up briefly, we may say that sexual selection is a much more
powerful factor in transformation than we should at first be inclined
to believe. It cannot, of course, have been operative in the case of
plants, nor can it be taken into consideration in regard to the lower
animals, for these, like the plants, do not pair, or, at any rate,
do so without any possibility of choice. Animals which live on the
sea-floor, or which are attached there, must simply liberate their
reproductive cells into the water, and cannot secure that they unite
with those of this or that individual. This is the case among sponges,
corals, and Hydroid polyps. In some other classes the sense organs are
too poorly developed, and the eyes in particular too imperfect to be
excited in different degrees by any peculiarities in the appearance
or behaviour of the males. This is what Darwin meant when he ascribed
to these animals 'too imperfect senses and much too low intelligence'
'to estimate the beauty or other attractive points of the opposite
sex, or to feel anything like rivalry.' Accordingly, in the Protozoa,
Echinoderms, Medusæ, and Ctenophores, secondary sexual characters are
entirely absent, as pairing also is.
In those worms that pair we first meet with secondary sexual
characters, and from this level upwards they are never quite absent
from any large group, and gradually play an increasingly important rôle.
But the significance of sexual selection lies, as we have seen,
not only in the fact that one sex of a species, usually the male,
is modified, but in the possibility of the transference of this
modification to the females, and further, in the fact that the process
of variation may start afresh at any time, and thus one variation may
be developed upon or alongside of another. In this way we can explain
certain complex and often fantastic forms and colourings which we could
not otherwise understand; thus the extraordinary number of nearly
related species in some animal groups, such as butterflies and birds,
in which the differences mainly concern the colour-patterns.
Darwin has shown convincingly that a surprising number of characters
in animals, from worms upwards, have their roots in sexual selection,
and has pointed out the probability that this process has played an
important part in the evolution of the human race also, though, in this
case, all is not yet so clearly and certainly known as among animals.