account given by Mr. Darwin of the colours and habits of male and
female birds (_Descent of Man_, Chapters xiii. and xiv.), proves
that in most, if not in all cases, the male birds fully display
their ornamental plumage before the females or in rivalry with each
other; but on the essential point of whether the female’s choice
is determined by minute differences in these ornaments or in their
colours, there appears to be an entire absence of evidence. In the
section on “_Preference for particular Males by the Females_,” the
facts quoted show indifference to colour, except that some colour
similar to their own seems to be preferred. But in the case of
the hen canary, who chose a greenfinch in preference to either
chaffinch or goldfinch, gay colours had evidently no preponderating
attraction. There is some evidence adduced that female birds may,
and probably do, choose their mates; but none whatever that the
choice is determined by difference of colour; and no less than three
eminent breeders informed Mr. Darwin that they “did not believe that
the females prefer certain males on account of the beauty of their
plumage.” Again, Mr. Darwin himself says: “As a general rule colour
appears to have little influence on the pairing of pigeons.” The
oft-quoted case of Sir R. Heron’s pea-hens which preferred an “old
pied cock” to those normally coloured, is a very unfortunate one;
because pied birds are just those that are not favoured in a state
of nature, or the breeds of wild animals would become as varied
and mottled as our domestic varieties. If such irregular fancies
were not rare exceptions, the production of definite colours and
patterns by the choice of the female birds, or in any other way,
would be impossible.
There remains, however, to be accounted for, the remarkable fact of
the display by the male of each species of its peculiar beauties of
plumage and colour,--a display which Mr. Darwin evidently considers
his strongest argument in favour of conscious selection by the
female. This display is, no doubt, a very interesting and important
phenomenon; but it may, I believe, be satisfactorily explained on
the general principles here laid down, without calling to our aid
a purely hypothetical choice exerted by the female bird.
At pairing-time, the male is in a state of excitement, and full
of exuberant energy. Even unornamental birds flutter their wings
or spread them out, erect their tails or crests, and thus give
vent to the nervous excitability with which they are overcharged.
It is not improbable that crests and other erectile feathers may
be primarily of use in frightening away enemies, since they are
generally erected when angry or during combat. Those individuals who
were most pugnacious and defiant, and who brought these erectile
plumes most frequently and most powerfully into action, would tend
to increase them by use, and to leave them further developed in
some of their descendants. If, in the course of this development,
colour appeared--and we have already shown that such developments of
plumage are a very probable cause of colour--we have every reason
to believe it would be most vivid in these most pugnacious and
energetic individuals; and as these would always have the advantage
in the rivalry for mates (to which advantage the excess of colour and
plumage might sometimes conduce), there seems nothing to prevent a
progressive development of these ornaments in _all dominant races_;
that is, wherever there was such a surplus of vitality, and such
complete adaption to conditions, that the inconvenience or danger
produced by such ornaments was so comparatively small as not to
affect the superiority of the race over its nearest allies.
But if those portions of the plumage, which were originally erected
under the influence of anger or fear, became largely developed
and brightly coloured, the actual display, under the influence
of jealousy or sexual excitement becomes quite intelligible. The
males, in their rivalry with each other, would see what plumes were
most effective; and each would endeavour to excel his enemy as far
as voluntary exertion would enable him, just as they endeavour to
rival each other in song, even sometimes to the point of causing
their own destruction.
_Natural Selection as Neutralizing Sexual Selection._--There is also
a general argument against Mr. Darwin’s views on this question,
founded on the nature and potency of “natural” as opposed to “sexual”
selection, which appears to me to be of itself almost conclusive as
to the whole matter at issue. Natural selection, or the survival
of the fittest, acts perpetually and on an enormous scale. Taking
the offspring of each pair of birds as, on the average, only six
annually, one-third of these at most will be preserved, while the
two-thirds which are least fitted will die. At intervals of a
few years, whenever unfavourable conditions occur, five-sixths,
nine-tenths, or even a greater proportion of the whole yearly
production are weeded out, leaving only the most perfect and best
adapted to survive. Now unless these survivors are, on the whole,
the most ornamental, this rigid natural selection must neutralise
and destroy any influence that may be exerted by female selection.
The utmost that can be claimed for the latter is, that a small
fraction of the least ornamented do not obtain mates, while a few
of the most ornamented may leave more than the average number of
offspring. Unless, therefore, there is the strictest correlation
between ornament and general perfection, the more brightly coloured
or ornamented varieties can obtain no permanent advantage; and
if there is (as I maintain) such a correlation, then the sexual
selection of colour or ornament, for which there is little or no
evidence, becomes needless, because natural selection which is an
admitted _vera causa_, will itself produce all the results.
In the case of butterflies the argument becomes even stronger,
because the fertility is so much greater than in birds, and the
weeding-out of the unfit takes place, to a great extent, in the
egg and larva state. Unless the eggs and larvæ which escaped to
produce the next generation were those which would produce the more
highly-coloured butterflies, it is difficult to perceive how the
slight preponderance of colour sometimes selected by the females,
should not be wholly neutralized by the extremely rigid selection
for other qualities to which the offspring in every stage are
exposed. The only way in which we can account for the observed
facts is, by the supposition that colour and ornament are strictly
correlated with health, vigour, and general fitness to survive. We
have shown that there is reason to believe that this is the case,
and if so, conscious sexual selection becomes as unnecessary as it
would certainly be ineffective.
_Greater Brilliancy of some Female Birds._--There is one other
very curious case of sexual colouring among birds--that, namely,
in which the female is decidedly brighter or more strongly marked
than the male; as in the fighting quails (_Turnix_), painted snipe
(_Rhynchæa_), two species of phalarope (_Phalaropus_), and the
common cassowary (_Casuarius galeatus_). In all these cases, it is
known that the males take charge of and incubate the eggs, while
the females are almost always larger and more pugnacious.
In my “Theory of Birds’ Nests” (_Natural Selection_, p. 251), I
imputed this difference of colour to the greater need for protection
by the male bird while incubating; to which Mr. Darwin has
objected that the difference is not sufficient, and is not always
so distributed as to be most effective for this purpose; and he
believes that it is due to reversed sexual selection, that is, to
the female taking the usual _rôle_ of the male, and being chosen
for her brighter tints. We have already seen reason for rejecting
this latter theory in every case; and I also admit that Mr. Darwin’s
criticism is sound, and that my theory of protection is, in this
case, only partially, if at all, applicable. But the theory now
advanced, of intensity of colour being due to general vital energy,
is quite applicable; and the fact that the superiority of the female
in this respect is quite exceptional, and is therefore probably not
in any case of very ancient date, will account for the difference
of colour thus produced being always very slight.
_Colour-development as Illustrated by Humming-birds._--Of the mode
of action of the general principles of colour-development among
animals, we have an excellent example in the humming-birds. Of all
birds these are at once the smallest, the most active, and the
fullest of vital energy. When poised in the air their wings are
invisible, owing to the rapidity of their motion, and when startled
they dart away with the rapidity of a flash of light. Such active
creatures would not be an easy prey to any rapacious bird; and if
one at length was captured, the morsel obtained would hardly repay
the labour. We may be sure, therefore, that they are practically
unmolested. The immense variety they exhibit in structure, plumage,
and colour, indicates a high antiquity for the race; while their
general abundance in individuals shows that they are a dominant
group, well adapted to all the conditions of their existence. Here
we find everything necessary for the development of colour and
accessory plumes. The surplus vital energy shown in their combats
and excessive activity, has expended itself in ever-increasing
developments of plumage, and greater and greater intensity of colour,
regulated only by the need for specific identification which would
be especially required in such small and mobile creatures. Thus
may be explained those remarkable differences of colour between
closely-allied species, one having a crest like the topaz, while
in another it resembles the sapphire. The more vivid colours and
more developed plumage of the males, I am now inclined to think may
be wholly due to their greater vital energy, and to those general
laws which lead to such superior developments even in domestic
breeds; but in some cases the need of protection by the female while
incubating, to which I formerly imputed the whole phenomenon, may
have suppressed a portion of the ornament which she would otherwise
have attained.
The extreme pugnacity of humming-birds has been noticed by all
observers, and it seems to be to some extent proportioned to the
degree of colour and ornament in the species. Thus Mr. Salvin
observes of Eugenes fulgens, that it is “a most pugnacious bird,”
and that “hardly any species shows itself more brilliantly on
the wing.” Again of Campylopterus hemileucurus,--“the pugnacity
of this species is remarkable. It is very seldom that two males
meet without an aërial battle,”--and “the large and showy tail
of this humming-bird makes it one of the most conspicuous on the
wing.” Again, the elegant frill-necked Lophornis ornatus “is
very pugnacious, erecting its crest, throwing out its whiskers
and attacking every humming-bird that may pass within its range
of vision;” and of another species L. magnificus, it is said
that “it is so bold that the sight of man creates no alarm.” The
beautifully-coloured Thaumastura Cora “rarely permits any other
humming-bird to remain in its neighbourhood, but wages a continual
and terrible war upon them.” The magnificent bar-tail, Cometes
sparganurus, one of the most imposing of all the humming-birds,
is extremely fierce and pugnacious, “the males chasing each other
through the air with surprising perseverance and acrimony.” These
are all the species I find noticed as being especially pugnacious,
and every one of them is exceptionally coloured or ornamented;
while not one of the small, plain, and less ornamental species are
so described, although many of them are common and well observed
species. It is also to be noticed that the remarkable pugnacity of
these birds is not confined to one season or even to birds of the
same species, as is usual in sexual combats, but extends to any
other species that may be encountered, while they are said even
to attack birds of prey that approach too closely to their nests.
It must be admitted that these facts agree well with the theory
that colour and ornament are due to surplus vital energy and a long
course of unchecked development. We have also direct evidence that
the males are more active and energetic than the females. Mr. Gosse
says that the whirring made by the male Polytmus humming-bird is
shriller than that produced by the female; and he also informs us
that the male flies higher and frequents mountains while the female
keeps to the lowlands.
_Theory of Typical Colours._--The remaining kinds of animal colours,
those which can neither be classed as protective, warning, or sexual,
are for the most part readily explained on the general principles of
the development of colour which we have now laid down. It is a most
suggestive fact, that, in cases where colour is required only as a
warning, as among the uneatable caterpillars, we find, not one or
two glaring tints only, but every kind of colour disposed in elegant
patterns, and exhibiting almost as much variety and beauty as among
insects and birds. Yet here, not only is sexual selection out of the
question, but the need for recognition and identification by others
of the same species, seems equally unnecessary. We can then only
impute this variety to the normal production of colour in organic
forms, when fully exposed to light and air and undergoing great
and rapid developmental modification. Among more perfect animals,
where the need for recognition has been added, we find intensity and
variety of colour at its highest pitch among the South American
butterflies of the families Heliconidæ and Danaidæ, as well as among
the Nymphalidæ and Erycinidæ, many of which obtain the necessary
protection in other ways. Among birds also, wherever the habits
are such that no special protection is needed for the females, and
where the species frequent the depths of tropical forests, and are
thus naturally protected from the swoop of birds of prey, we find
almost equally intense coloration; as in the trogons, barbets, and
gapers.
_Local Causes of Colour-development._--Another real, though as
yet inexplicable cause of diversity of colour, is to be found in
the influence of locality. It is observed that species of totally
distinct groups are coloured alike in one district, while in another
district the allied species all undergo the same change of colour.
Cases of this kind have been adduced by Mr. Bates, by Mr. Darwin, and
by myself, and I have collected all the more curious and important
examples in my Address to the Biological Section of the British
Association, at Glasgow in 1876 (Chap. VII. of this volume). The
most probable cause for these simultaneous variations would seem
to be the presence of peculiar elements or chemical compounds in
the soil, the water, or the atmosphere, or of special organic
substances in the vegetation; and a wide field is thus offered for
chemical investigation in connection with this interesting subject.
Yet, however we may explain it the fact remains, of the same vivid
colours in definite patterns being produced in quite unrelated
groups, which only agree, so far as we yet know, in inhabiting the
same locality.
_Summary on Colour-development in Animals._--Let us now sum up the
conclusion at which we have arrived, as to the various modes in
which colour is produced or modified in the animal kingdom.
The various causes of colour in the animal world are, molecular
and chemical change of the substance of their integuments, or the
action on it of heat, light or moisture. It is also produced by
interference of light in superposed transparent lamellæ, or by
excessively fine surface-striæ. These elementary conditions for the
production of colour are found everywhere in the surface-structures
of animals, so that its presence must be looked upon as normal,
its absence as exceptional.
Colours are fixed or modified in animals by natural selection for
various purposes; obscure or imitative colours for concealment;
gaudy colours as a warning; and special markings, either for easy
recognition by strayed individuals, females, or young, or to direct
attack from a vital part, as in the large brilliantly-marked wings
of some butterflies and moths.
Colours are produced or intensified by processes of
development,--either where the integument or its appendages undergo
great extension or modification, or where there is a surplus of
vital energy, as in male animals generally, and more especially at
the breeding season.
Colours are also more or less influenced by a variety of causes,
such as the nature of the food, the photographic action of light,
and also by some unknown local action probably dependent on chemical
peculiarities in the soil or vegetation.
These various causes have acted and reacted in a variety of ways,
and have been modified by conditions dependent on age or on sex, on
competition with new forms, or on geographical or climatic changes.
In so complex a subject, for which experiment and systematic inquiry
has done so little, we cannot expect to explain every individual
case, or solve every difficulty; but it is believed that all the
great features of animal coloration and many of the details become
explicable on the principles we have endeavoured to lay down.
It will perhaps be considered presumptuous to put forth this sketch
of the subject of colour in animals, as a substitute for one of
Mr. Darwin’s most highly elaborated theories--that of voluntary or
perceptive sexual selection; yet I venture to think that it is more
in accordance with the whole of the facts, and with the theory of
natural selection itself; and I would ask such of my readers as may
be sufficiently interested in the subject, to read again Chapters
XI. to XVI. of the _Descent of Man_, and consider the whole subject
from the point of view here laid down. The explanation of almost
all the ornaments and colours of birds and insects as having been
produced by the perceptions and choice of the females, has, I
believe, staggered many evolutionists, but has been provisionally
accepted because it was the only theory that even attempted to
explain the facts. It may perhaps be a relief to some of them, as
it has been to myself, to find that the phenomena can be shown to
depend on the general laws of development, and on the action of
“natural selection,” which theory will, I venture to think, be
relieved from an abnormal excrescence and gain additional vitality,
by the adoption of the views here imperfectly set forth.
Although we have arrived at the conclusion that tropical light and
heat can in no sense be considered as the cause of colour, there
remains to be explained the undoubted fact that all the more intense
and gorgeous tints are manifested by the animal life of the tropics;
while in some groups, such as butterflies and birds, there is a
marked preponderance of highly-coloured species. This is probably
due to a variety of causes, some of which we can indicate, while
others remain to be discovered. The luxuriant vegetation of the
tropics throughout the entire year affords so much concealment, that
colour may there be safely developed to a much greater extent than
in climates where the trees are bare in winter, during which season
the struggle for existence is most severe, and even the slightest
disadvantage may prove fatal. Equally important, probably, has been
the permanence of favourable conditions in the tropics, allowing
certain groups to continue dominant for long periods, and thus to
carry out in one unbroken line whatever developments of plumage or
colour may once have acquired an ascendency. Changes of climatal
conditions, and pre-eminently the glacial epoch, probably led to
the extinction of a host of highly-developed and finely-coloured
insects and birds in temperate zones; just as we know that it led
to the extinction of the larger and more powerful mammalia which
formerly characterised the temperate zone in both hemispheres;
and this view is supported by the fact that it is amongst those
groups only which are now exclusively tropical that all the more
extraordinary developments of ornament and colour are found. The
obscure local causes of colour to which we have referred will
also have acted most efficiently in regions where the climatal
condition remained constant, and where migration was unnecessary;
while whatever direct effect may be produced by light or heat,
will necessarily have acted more powerfully within the tropics.
And lastly, all these causes have been in action over an actually
greater area in tropical than in temperate zones; while estimated
potentially, in proportion to its life-sustaining power, the lands
which enjoy a practically tropical climate (extending as they do
considerably beyond the geographical tropics) are very much larger
than the temperate regions of the earth.
Combining the effects of all these various causes we are quite able
to understand the superiority of the tropical parts of the globe,
not only in the abundance and variety of their forms of life, but
also as regards the ornamental appendages and vivid coloration
which these forms present.
VI.
THE COLOURS OF PLANTS AND THE ORIGIN OF THE COLOUR-SENSE.
Source of Colouring-matter in Plants--Protective Coloration and
Mimicry among Plants--Attractive Colours of Fruits--Protective
Colours of Fruits--Attractive Colours of Flowers--Attractive Odours
in Flowers--Attractive grouping of Flowers--Why Alpine Flowers are
so Beautiful--Why allied species of Flowers differ in Size and
Beauty--Absence of Colours in Wind-fertilized Flowers--The same
Theory of Colour applicable to Animals and Plants--Relation of the
Colours of Flowers and their Geographical Distribution--Recent
Views as to the Direct Action of Light on the Colours of Flowers
and Fruits--On the Origin of the Colour-sense--Supposed increase
of Colour-perception within the Historical Period--Concluding
Remarks on the Colour-sense.
The colouring of plants is neither so varied nor so complex as
that of animals, and its explanation accordingly offers fewer
difficulties. The colours of foliage are, comparatively, little
varied, and can be traced in almost all cases to a special pigment
termed chlorophyll, to which is due the general green colour of
leaves; but the recent investigations of Mr. Sorby and others have
shown that chlorophyll is not a simple green pigment, but that it
really consists of at least seven distinct substances, varying
in colour from blue to yellow and orange. These differ in their
proportions in the chlorophyll of different plants; they have
different chemical reactions; they are differently affected by
light; and they give distinct spectra. Mr. Sorby further states
that scores of different colouring matters are found in the leaves
and flowers of plants, to some of which appropriate names have been
given, as erythrophyll which is red, and phaiophyll which is brown;
and many of these differ greatly from each other in their chemical
composition. These inquiries are at present in their infancy, but
as the original term chlorophyll seems scarcely applicable under
the present aspect of the subject, it would perhaps be better to
introduce the analogous word _Chromophyll_, as a general term for
the colouring matters of the vegetable kingdom.