brightness or intensity of colour, even when the general coloration
is the same in both sexes, now seems to me to be, primarily, due
to the greater vigour and activity and the higher vitality of the
male. The colours of an animal usually fade during disease or
weakness, while robust health and vigour adds to their intensity.
This is a most important and suggestive fact, and one that appears
to hold universally. In all quadrupeds a “dull coat” is indicative
of ill-health or low condition; while a glossy coat and sparkling
eye are the invariable accompaniments of health and energy. The
same rule applies to the feathers of birds, whose colours are only
seen in their purity during perfect health; and a similar phenomenon
occurs even among insects, for the bright hues of caterpillars begin
to fade as soon as they become inactive preparatory to undergoing
their transformation. Even in the vegetable kingdom we see the same
thing; for the tints of foliage are deepest, and the colours of
flowers and fruits richest, on those plants which are in the most
healthy and vigorous condition.
This intensity of coloration becomes most developed in the male
during the breeding season, when the vitality is at a maximum. It
is also very general in those cases in which the male is smaller
than the female, as in the hawks and in most butterflies and moths.
The same phenomena occur, though in a less marked degree, among
mammalia. Whenever there is a difference of colour between the sexes
the male is the darker or more strongly marked, and the difference
of intensity is most visible during the breeding season (_Descent
of Man_, p. 533). Numerous cases among domestic animals also
prove, that there is an inherent tendency in the male to special
developments of dermal appendages and colour, quite independently
of sexual or any other form of selection. Thus,--“the hump on the
male zebu cattle of India, the tail of fat-tailed rams, the arched
outline of the forehead in the males of several breeds of sheep,
and the mane, the long hairs on the hind legs, and the dew-lap of
the male of the Berbura goat,”--are all adduced by Mr. Darwin as
instances of characters peculiar to the male, yet not derived from
any parent ancestral form. Among domestic pigeons the character of
the different breeds is often most strongly manifested in the male
birds; the wattles of the carriers and the eye-wattles of the barbs
are largest in the males, and male pouters distend their crops to
a much greater extent than do the females, while the cock fantails
often have a greater number of tail-feathers than the females. There
are also some varieties of pigeons of which the males are striped or
spotted with black while the females are never so spotted (_Animals
and Plants under Domestication_, I. 161); yet in the parent stock
of these pigeons there are no differences between the sexes either
of plumage or colour, and artificial selection has not been applied
to produce them.
The greater intensity of coloration in the male--which may be termed
the normal sexual difference, would be further developed by the
combats of the males for the possession of the females. The most
vigorous and energetic usually being able to rear most offspring,
intensity of colour, if dependent on, or correlated with vigour,
would tend to increase. But as differences of colour depend upon
minute chemical or structural differences in the organism, increasing
vigour acting unequally on different portions of the integument,
and often producing at the same time abnormal developments of hair,
horns, scales, feathers, &c., would almost necessarily lead also
to variable distribution of colour, and thus to the production of
new tints and markings. These acquired colours would, as Mr. Darwin
has shown, be transmitted to both sexes or to one only, according
as they first appeared at an early age, or in adults of one sex;
and thus we may account for some of the most marked differences
in this respect. With the exception of butterflies, the sexes are
almost alike in the great majority of insects. The same is the
case in mammals and reptiles; while the chief departure from the
rule occurs in birds, though even here in very many cases the law
of sexual likeness prevails. But in all cases where the increasing
development of colour became disadvantageous to the female, it would
be checked by natural selection; and thus produce those numerous
instances of protective colouring in the female only, which occur
in these two groups, birds and butterflies.
_Colour as a Means of Recognition._--There is also, I believe, a
very important purpose and use of the varied colours of the higher
animals, in the facility it affords for recognition by the sexes or
by the young of the same species; and it is this use which probably
fixes and determines the coloration in many cases. When differences
in size and form are very slight, colour affords the only means of
recognition at a distance, or while in motion; and such a distinctive
character must therefore be of especial value to flying insects
which are continually in motion, and encounter each other, as it
were, by accident. This view offers us an explanation of the curious
fact, that among butterflies the females of closely-allied species
in the same locality sometimes differ considerably, while the males
are much alike; for, as the males are the swiftest and by far the
highest fliers, and seek out the females, it would evidently be
advantageous for them to be able to recognise their true partners
at some distance off. This peculiarity occurs with many species
of _Papilio_, _Diadema_, _Adolias_, and _Colias_; and these are
all genera, the males of which are strong on the wing and mount
high in the air. In birds such marked differences of colour are
not required, owing to their higher organization and more perfect
senses, which render recognition easy by means of a combination of
very slight differential characters.
This principle may perhaps, however, account for some anomalies of
coloration among the higher animals. Thus, while admitting that the
hare and the rabbit are coloured protectively, Mr. Darwin remarks
that the latter while running to its burrow, is made conspicuous to
the sportsman, and no doubt to all beasts of prey, by its upturned
white tail. But this very conspicuousness while running away, may
be useful as a signal and guide to the young, who are thus enabled
to escape danger by following the older rabbits, directly and
without hesitation, to the safety of the burrow; and this may be
the more important from the semi-nocturnal habits of the animal.
If this explanation is correct, and it certainly seems probable,
it may serve as a warning of how impossible it is, without exact
knowledge of the habits of an animal and a full consideration of all
the circumstances, to decide that any particular coloration cannot
be protective or in any way useful. Mr. Darwin himself is not free
from such assumptions. Thus, he says:--“The zebra is conspicuously
striped, and stripes cannot afford any protection on the open plains
of South Africa.” But the zebra is a very swift animal, and, when in
herds, by no means void of means of defence. The stripes therefore
_may_ be of use by enabling stragglers to distinguish their fellows
at a distance, and they _may_ be even protective when the animal is
at rest among herbage--the only time when it would need protective
colouring. Until the habits of the zebra have been observed with
special reference to these points, it is surely somewhat hasty to
declare that the stripes “cannot afford any protection.”
_Colour Proportionate to Integumentary Development._--The wonderful
display and endless variety of colour in which butterflies and
birds so far exceed all other animals, seems primarily due to the
excessive development and endless variations of the integumentary
structures. No insects have such widely-expanded wings in proportion
to their bodies as butterflies and moths; in none do the wings vary
so much in size and form, and in none are they clothed with such a
beautiful and highly-organized coating of scales. According to the
general principles of the production of colour already explained,
these long-continued expansions of membranes and developments of
surface structures, must have led to numerous colour-changes;
which have been sometimes checked, sometimes fixed and utilised,
sometimes intensified, by natural selection, according to the needs
of the animal. In birds, too, we have the wonderful clothing of
plumage--the most highly organized, the most varied, and the most
expanded of all dermal appendages. The endless processes of growth
and change during the development of feathers, and the enormous
extent of this delicately-organized surface, must have been highly
favourable to the production of varied colour-effects; which, when
not injurious, have been merely fixed for purposes of specific
identification, but have often been modified or suppressed whenever
different tints were needed for purposes of protection.
_Selection by Females not a Cause of Colour._--To conscious sexual
selection, that is, the actual choice by the females of the more
brilliantly-coloured males, I believe very little if any effect is
directly due. It is undoubtedly proved that in birds the females do
sometimes exert a choice; but the evidence of this fact collected by
Mr. Darwin (_Descent of Man_, chap. xiv.) does not prove that colour
determines that choice, while much of the strongest evidence is
directly opposed to this view. All the facts appear to be consistent
with the choice depending on a variety of male characteristics,
with some of which colour is often correlated. Thus it is the
opinion of some of the best observers that vigour and liveliness
are most attractive, and these are no doubt usually associated with
intensity of colour. Again, the display of the various ornamental
appendages of the male during courtship may be attractive; but
these appendages, with their bright colours or shaded patterns, are
due probably to general laws of growth, and to that superabundant
vitality which we have seen to be a cause of colour. But there are
many considerations which seem to show that the possession of these
ornamental appendages and bright colours in the male is not an
important character functionally, and that it has not been produced
by the action of conscious sexual selection. Amid the copious mass
of facts and opinions collected by Mr. Darwin as to the display of
colour and ornaments by the male birds, there is a total absence of
any evidence that the females admire or even notice this display.
The hen, the turkey, and the pea-fowl go on feeding while the male
is displaying his finery; and there is reason to believe that it is
his persistency and energy rather than his beauty which wins the
day. Again, evidence collected by Mr. Darwin himself proves that
each bird finds a mate under any circumstances. He gives a number of
cases of one of a pair of birds being shot, and the survivor being
always found paired again almost immediately. This is sufficiently
explained on the assumption that the destruction of birds by various
causes is continually leaving widows and widowers in nearly equal
proportions, and thus each one finds a fresh mate; and it leads to
the conclusion that permanently unpaired birds are very scarce; so
that, speaking broadly, every bird finds a mate and breeds. But this
would almost or quite neutralize any effect of sexual selection
of colour or ornament, since the less highly-coloured birds would
be at no disadvantage as regards leaving healthy offspring. If,
however, heightened colour is correlated with health and vigour; and
if these healthy and vigorous birds provide best for their young,
and leave offspring which, being equally healthy and vigorous, can
best provide for themselves--which cannot be denied; then natural
selection becomes a preserver and intensifier of colour.
Another most important consideration is, that male butterflies
rival or even excel the most gorgeous male birds in bright colours
and elegant patterns; and among these there is literally not one
particle of evidence that the female is influenced by colour, or
even that she has any power of choice; while there is much direct
evidence to the contrary (_Descent of Man_, p. 318). The weakness
of the evidence for conscious sexual selection among these insects
is so palpable, that Mr. Darwin is obliged to supplement it by the
singularly inconclusive argument that, “Unless the female prefer
one male to another, the pairing must be left to mere chance, and
this does not appear probable” (_l.c._ p. 317). But he has just
said--“The males sometimes fight together in rivalry, and many
may be seen pursuing or crowding round the same female;” while in
the case of the silk-moths,--“the females appear not to evince
the least choice in regard to their partners.” Surely the plain
inference from all this is, that males fight and struggle for the
almost passive female; and that the most vigorous and energetic,
the strongest-winged or the most persevering, wins her. How can
there be chance in this? Natural selection would here act, as in
birds, in perpetuating the strongest and most vigorous males; and
as these would usually be the more highly coloured of their race,
the same results would be produced as regards the intensification
and variation of colour in the one case as in the other.
Let us now see how these principles will apply to some of the cases
adduced by Mr. Darwin in support of his theory of conscious sexual
selection.
In _Descent of Man_, 2nd ed., pp. 307-316, we find an elaborate
account of the various modes of colouring of butterflies and moths,
proving that the coloured parts are always more or less displayed,
and that they have some evident relation to an observer. Mr. Darwin
then says: “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” (_l.c._, p. 316). I am not aware that any
one has ever maintained that the brilliant colours of butterflies
have “commonly been acquired for the sake of protection,” yet Mr.
Darwin has himself referred to cases in which the brilliant colour is
so placed as to serve for protection; as for example, the eye-spots
on the hind wings of moths, which are pierced by birds and so
save the vital parts of the insect; while the bright patch on the
orange-tip butterflies which Mr. Darwin denies are protective, may
serve the same purpose. It is in fact somewhat remarkable how very
generally the black spots, ocelli, or bright patches of colour are
on the tips, margins, or discs of the wings; and as the insects are
necessarily visible while flying, and this is the time when they
are most subject to attacks by insectivorous birds, the position of
the more conspicuous parts at some distance from the body may be a
real protection to them. Again, Mr. Darwin admits that the white
colour of the male ghost-moth may render it more easily seen by the
female while flying about in the dusk; and if to this we add that
it will be also more readily distinguished, from allied species,
we have a reason for diverse ornamentation in these insects quite
sufficient to account for most of the facts, without believing in
the selection of brilliant males by the females, for which there
is not a particle of evidence.[20]
[20] See M. Fabre’s testimony on this point, _Descent of Man_, p.
291.
_Probable use of the Horns of Beetles._--A somewhat analogous case
is furnished by the immense horns of some beetles of the families
Copridæ and Dynastidæ, which Mr. Darwin admits are not used for
fighting, and therefore concludes are ornaments, developed through
selection of the larger-horned males by the females. But it has been
overlooked that these horns may be protective. The males probably
fly about most, as is usually the case with male insects; and as
they generally fly at dusk they are subject to the attacks of
large-mouthed goatsuckers and podargi, as well as of insect-eating
owls. Now the long, pointed or forked horns, often divergent, or
movable with the head, would render it very difficult for these
birds to swallow such insects, and would therefore be an efficient
protection; just as are the hooked spines of some stingless ants
and the excessively hard integuments of many beetles, against the
smaller insectivorous birds.
_Cause of the greater Brilliancy of some Female Insects._--The facts
given by Mr. Darwin to show that butterflies and other insects
can distinguish colours and are attracted by colours similar
to their own, are quite consistent with the view that colour,
which continually tends to appear, is utilised for purposes of
identification and distinction, when not required to be modified or
suppressed for the purpose of protection. The cases of the females of
some species of _Thecla_, _Callidryas_, _Colias_, and _Hipparchia_,
which have more conspicuous markings than the male, may be due to
several causes: to obtain greater distinction from other species;
for protection from birds, as in the case of the yellow-underwing
moths; while sometimes--as in _Hipparchia_--the lower intensity of
colouring in the female may lead to more contrasted markings. Mr.
Darwin thinks that here the males have selected the more beautiful
females; although one chief fact in support of his theory of
conscious sexual selection is, that throughout the whole animal
kingdom the males are usually so ardent that they will accept any
female, while the females are coy, and choose the handsomest males,
whence it is believed the general brilliancy of males as compared
with females has arisen.
Perhaps the most curious cases of sexual difference of colour are
those in which the female is very much more gaily coloured than
the male. This occurs most strikingly in some species of _Pieris_
in South America, and of _Diadema_ in the Malay islands; and in
both cases the females resemble species of the uneatable Danaidæ
and Heliconidæ, and thus gain a protection. In the case of _Pieris
pyrrha_, _P. malenka_, and _P. lorena_, the males are plain white
and black, while the females are orange, yellow, and black, and so
banded and spotted as exactly to resemble species of Heliconidæ.
Mr. Darwin admits that these bright colours have been acquired
for protection; but as there is no apparent cause for the strict
limitation of the colour to the female, he believes that it has
been kept down in the male by its being _unattractive_ to her. This
appears to me to be a supposition opposed to the whole theory of
sexual selection itself. For this theory is, that minute variations
of colour in the male are _attractive_ to the female, have always
been selected, and that thus the brilliant male colours have been
produced. But in this case he thinks that the female butterfly
had a constant aversion to every trace of colour, even when we
must suppose it was constantly recurring during the successive
variations which resulted in such a marvellous change in herself.
But the case admits of a much more simple interpretation. For if
we consider the fact that the females frequent the forests where
the Heliconidæ abound, while the males fly much in the open and
assemble in great numbers with other white and yellow butterflies
on the banks of rivers; may it not be possible that the appearance
of orange stripes or patches would be as injurious to the male as
it is useful to the female, by making him a more easy mark for
insectivorous birds among his white companions? This seems a more
probable supposition, than the altogether hypothetical choice of
the female, sometimes exercised in favour of and sometimes against
every new variety of colour in her partner.
A strictly analogous case is that of the glow-worm, whose light,
as originally suggested by Mr. Belt, is admitted to be a warning
of its uneatability to insectivorous nocturnal animals. The male,
having wings, does not require this protection. In the tropics
the number of nocturnal insectivorous birds and bats is very much
greater, hence winged species possess the light, as they would
otherwise be eaten by mistake for more savoury insects; and it may
be that the luminous Elateridæ of the tropics really mimic the
true fireflies (Lampyridæ), which are uneatable. This is the more
probable as the Elateridæ, in the great majority of species, have
brown or protective colours, and are therefore certainly palatable
to insectivorous animals.
_Origin of the Ornamental Plumage of Male Birds._--We now come to
such wonderful developments of plumage and colour as are exhibited by
the peacock and the Argus-pheasant; and I may here mention that it
was the case of the latter bird, as fully discussed by Mr. Darwin,
which first shook my belief in “sexual,” or more properly “female”
selection. The long series of gradations, by which the beautifully
shaded ocelli on the secondary wing-feathers of this bird, have
been produced, are clearly traced out; the result being a set of
markings, so exquisitely shaded as to represent “balls lying loose
within sockets”--purely artificial objects of which these birds
could have no possible experience. That this result should have
been attained through thousands and tens of thousands of female
birds all preferring those males whose markings varied slightly in
this one direction, this uniformity of choice continuing through
thousands and tens of thousands of generations, is to me absolutely
incredible. And when, further, we remember that those which did
not so vary, would also, according to all the evidence, find mates
and leave offspring, the actual result seems quite impossible of
attainment by such means.
Without pretending to solve completely so difficult a problem as that
of the origin and uses of the variously coloured plumes and ornaments
so often possessed by male birds, I would point out a few facts
which seem to afford a clue. And first, the most highly-coloured
and most richly-varied markings occur on those parts of the plumage
which have undergone the greatest modification, or have acquired
the most abnormal development. In the peacock, the tail-coverts are
enormously developed, and the “eyes” are situated on the greatly
dilated ends. In the birds-of-paradise, breast, or neck, or head,
or tail-feathers, are greatly developed and highly coloured. The
hackles of the cock, and the scaly breasts of humming-birds are
similar developments; while in the Argus-pheasant the secondary
quills are so enormously lengthened and broadened as to have become
almost useless for flight. Now it is easily conceivable, that during
this process of development, inequalities in the distribution of
colour may have arisen in different parts of the same feather; and
that spots and bands may thus have become broadened out into shaded
spots or ocelli, in the way indicated by Mr. Darwin, much as the
spots and rings on a soap-bubble increase with increasing tenuity.
This is the more probable, because in domestic fowls varieties of
colour tend to become symmetrical, quite independently of sexual
selection. (_Descent of Man_, p. 424.)
If now we accept the evidence of Mr. Darwin’s most trustworthy
correspondents, that the choice of the female, so far as she exerts
any, falls upon the “most vigorous, defiant, and mettlesome male;”
and if we further believe, what is certainly the case, that these
are as a rule the most brightly coloured and adorned with the finest
developments of plumage, we have a real and not a hypothetical cause
at work. For these most healthy, vigorous, and beautiful males will
have the choice of the finest and most healthy females; will have
the most numerous and healthy families; and will be able best to
protect and rear those families. Natural selection, and what may
be termed male selection, will tend to give them the advantage in
the struggle for existence; and thus the fullest plumage and the
finest colours will be transmitted, and tend to advance in each
succeeding generation.