vibrations or undulations of the ethereal medium of different
lengths and velocities. The whole body of vibrations caused by the
sun is termed radiation, or, more commonly, rays; and consists of
sets of waves which vary considerably in their dimensions and rate
of recurrence, but of which the middle portion only is capable of
exciting in us sensations of light and colour. Beginning with the
largest waves, which recur at the longest intervals, we have first
those which produce heat-sensations only; as they get smaller and
recur quicker, we perceive a dull red colour; and as the waves
increase in rapidity and diminish in size, we get successively
sensations of orange, yellow, green, blue, indigo, and violet, all
fading imperceptibly into each other. Then come more invisible
rays, of shorter wave-length and quicker recurrence, which produce,
solely or chiefly, chemical effects. The red rays, which first
become visible, have been ascertained to recur at the rate of 458
millions of millions of times in a second, the length of each wave
being ¹⁄₃₆₉₀₀th of an inch; while the violet rays, which last remain
visible, recur 727 millions of millions of times per second, and
have a wave-length of ¹⁄₆₄₅₁₆th of an inch. Although the waves recur
at different rates, they are all propagated through the ether with
the same velocity (192,000 miles per second); just as different
musical sounds, which are produced by waves of _air_ of different
lengths and rates of recurrence, travel at the same speed, so that
a tune played several hundred yards off reaches the ear in correct
time. There are, therefore, an almost infinite number of different
colour-producing undulations, and these may be combined in an almost
infinite variety of ways, so as to excite in us the sensation of
all the varied colours and tints we are capable of perceiving. When
all the different kinds of rays reach us in the proportion in which
they exist in the light of the sun, they produce the sensation of
white. If the rays which excite the sensation of any one colour are
prevented from reaching us, the remaining rays in combination produce
a sensation of colour often very far removed from white. Thus green
rays being abstracted leave purple light; blue, orange-red light;
violet, yellowish-green light, and so on. These pairs are termed
complementary colours. And if portions of differently coloured
lights are abstracted in various degrees, we have produced all
those infinite gradations of colours, and all those varied tints
and hues which are of such use to us in distinguishing external
objects, and which form one of the great charms of our existence.
Primary colours would therefore be as numerous as the different
wave-lengths of the visible radiations, if we could appreciate all
their differences; while secondary or compound colours, caused by
the simultaneous action of any combination of rays of different
wave-lengths, must be still more numerous.
In order to account for the fact that all colours appear to us to
be produced by combinations of three primary colours--red, green,
and violet--it is believed that we have three sets of nerve-fibres
in the retina, each of which is capable of being excited by all
rays, but that one set is excited most by the larger or red waves,
another by the medium or green waves, and the third set chiefly by
the violet or smallest waves of light; and when all three sets are
excited together in proper proportions we see white. This view is
supported by the phenomena of colour-blindness, which are explicable
on the theory that one of these sets of nerve-fibres (usually that
adapted to perceive red) has lost its sensibility, causing all
colours to appear as if the red rays were abstracted from them.
It is a property of these various radiations, that they are unequally
refracted or bent in passing obliquely through transparent bodies,
the longer waves being least refracted, the shorter most. Hence
it becomes possible to analyse white or any other light into its
component rays. A small ray of sunlight, for example, which would
produce a round white spot on a wall, if passed through a prism is
lengthened out into a band of coloured light, exactly corresponding
to the colours of the rainbow. Any one colour can thus be isolated
and separately examined; and by means of reflecting mirrors the
separate colours can be again compounded in various ways, and the
resulting colours observed. This band of coloured light is called
a _spectrum_, and the instrument by which the _spectra_ of various
kinds of light are examined is called a _spectroscope_. This branch
of the subject has, however, no direct bearing on the mode in which
the colours of living things are produced, and it has only been
alluded to in order to complete our sketch of the nature of colour.
The colours which we perceive in material substances are produced
either by the absorption or by the interference of some of the
rays which form white light. Pigmental or absorption-colours are
the most frequent, comprising all the opaque tints of flowers and
insects, and all the colours of dyes and pigments. They are caused
by rays of certain wave-lengths being absorbed, while the remaining
rays are reflected and give rise to the sensation of colour. When
all the colour-producing rays are reflected in due proportion, the
colour of the object is white; when all are absorbed the colour
is black. If blue rays only are absorbed the resulting colour is
orange-red; and generally, whatever colour an object appears to us,
it is because the complementary colours are absorbed by it. The
reason why rays of only certain refrangibilities are reflected,
and the rest of the incident light absorbed by each substance,
is supposed to depend upon the molecular structure of the body.
Chemical action almost always implies change of molecular structure,
hence chemical action is the most potent cause of change of colour.
Sometimes simple solution in water effects a marvellous change, as
in the case of the well-known aniline dyes; the magenta and violet
dyes exhibiting, when in the solid form, various shades of golden
or bronzy metallic green.
Heat alone often produces change of colour without effecting
any chemical change. Mr. Ackroyd has recently investigated this
subject,[18] and has shown that a large number of bodies are changed
by heat, returning to their normal colour when cooled, and that this
change is almost always in the direction of the less refrangible
rays or longer wave-lengths; and he connects the change with the
molecular expansion caused by heat. As examples may be mentioned
mercuric oxide, which is orange yellow, but which changes to orange,
red, and brown when heated; chromic-oxide, which is green, and
changes to yellow; cinnabar, which is scarlet, and changes to puce;
and metaborate of copper, which is blue, and changes to green and
greenish yellow.
[18] “Metachromatism, or Colour-Change,” _Chemical News_, August,
1876.
_How Animal Colours are Produced._--The colouring matters of animals
are very varied. Copper has been found in the red pigment of the
wing of the turaco, and Mr. Sorby has detected no less than seven
distinct colouring matters in birds’ eggs, several of which are
chemically related to those of blood and bile. The same colours are
often produced by quite different substances in different groups, as
shown by the red of the wing on the burnet-moth changing to yellow
with muriatic acid, while the red of the red-admiral-butterfly
undergoes no such change.
These pigmental colours have a different character in animals
according to their position in the integument. Following Dr. Hagen’s
classification, epidermal colours are those which exist in the
external chitinised skin of insects, in the hairs of mammals, and,
partially, in the feathers of birds. They are often very deep and
rich, and do not fade after death. The hypodermal colours are those
which are situated in the inferior soft layer of the skin. These
are often of lighter and more vivid tints, and usually fade after
death. Many of the reds and yellows of butterflies and birds belong
to this class, as well as the intensely vivid hues of the naked
skin about the heads of many birds. These colours sometimes exude
through the pores, forming an evanescent bloom on the surface.
Interference colours are less frequent in the organic world. They
are caused in two ways: either by reflection from the two surfaces
of transparent films, as seen in the soap-bubble and in thin films
of oil on water; or by fine striæ which produce colours either by
reflected or transmitted light, as seen in mother-of-pearl and in
finely-ruled metallic surfaces. In both cases colour is produced
by light of one wave-length being neutralised, owing to one set of
such waves being caused to be half a wave length behind the other
set, as may be found explained in any treatise on physical optics.
The result is, that the complementary colour of that neutralised
is seen; and, as the thickness of the film or the fineness of the
striæ undergo slight changes, almost any colour can be produced.
This is believed to be the origin of many of the glossy or metallic
tints of insects, as well as those of the feathers of some birds.
The iridescent colours of the wings of dragon-flies are caused by
the superposition of two or more transparent lamellæ; while the
shining blue of the Purple-Emperor and other butterflies, and the
intensely metallic colours of humming-birds, are probably due to
fine striæ.
_Colour a Normal Product of Organization._--This outline sketch of
the nature of colour in the animal world, however imperfect, will at
least serve to show us how numerous and varied are the causes which
perpetually tend to the production of colour in animal tissues. If
we consider, that in order to produce white, all the rays which
fall upon an object must be reflected in the same proportions as
they exist in solar light--whereas, if rays of any one or more
kinds are absorbed or neutralised, the resultant reflected light
will be coloured; and that this colour may be infinitely varied
according to the proportions in which different rays are reflected
or absorbed--we should expect that white would be, as it really is,
comparatively rare and exceptional in nature. The same observation
will apply to black, which arises from the absorption of all the
different rays. Many of the complex substances which exist in animals
and plants are subject to changes of colour under the influence of
light, heat, or chemical change, and we know that chemical changes
are continually occurring during the physiological processes of
development and growth. We also find that every external character
is subject to minute changes, which are generally perceptible to
us in closely allied species; and we can therefore have no doubt
that the extension and thickness of the transparent lamellæ, and
the fineness of the striæ or rugosities of the integuments, must
be undergoing constant minute changes; and these changes will very
frequently produce changes of colour. These considerations render it
probable that colour is a normal and even necessary result of the
complex structure of animals and plants; and that those parts of an
organism which are undergoing continual development and adaptation
to new conditions, and are also continually subject to the action of
light and heat, will be the parts in which changes of colour will
most frequently appear. Now there is little doubt that the external
changes of animals and plants in adaptation to the environment are
much more numerous than the internal changes; as seen in the varied
character of the integuments and appendages of animals--hair, horns,
scales, feathers, &c. &c.--and in plants, the leaves, bark, flowers,
and fruit, with their various modifications--as compared with the
great uniformity in the texture and composition of their internal
tissues; and this accords with the uniformity of the tints of blood,
muscle, nerve, and bone throughout extensive groups, as compared
with the great diversity of colour of their external organs. It
seems a fair conclusion that colour _per se_ may be considered to
be normal, and to need no special accounting for; while the absence
of colour (that is, either _white_ or _black_), or the prevalence
of certain colours to the constant exclusion of others, must be
traced, like other modifications in the economy of living things,
to the needs of the species. Or, looking at it in another aspect,
we may say, that amid the constant variations of animals and plants
colour is ever tending to vary and to appear where it is absent;
and that natural selection is constantly eliminating such tints as
are injurious to the species, or preserving and intensifying such
as are useful.
This view is in accordance with the well-known fact, of colours
which rarely or never appear in the species in a state of nature,
continually occurring among domesticated animals and cultivated
plants; showing us that the capacity to develop colour is ever
present, so that almost any required tint can be produced which may,
under changed conditions, be useful, in however small a degree.
Let us now see how these principles will enable us to understand
and explain the varied phenomena of colour in nature, taking them
in the order of our functional classification of colours.
_Theory of Protective Colours._--We have seen that obscure or
protective tints in their infinitely varied degrees are present in
every part of the animal kingdom, whole families or genera being
often thus coloured. Now the various brown, earthy, ashy, and
other neutral tints are those which would be most readily produced,
because they are due to an irregular mixture of many kinds of rays;
while pure tints require either rays of one kind only, or definite
mixtures in proper proportions of two or more kinds of rays. This
is well exemplified by the comparative difficulty of producing
definite pure tints by the mixture of two or more pigments; while
a haphazard mixture of a number of these will be almost sure to
produce browns, olives, or other neutral or dingy colours. An
indefinite or irregular absorption of some rays and reflection of
others would, therefore, produce obscure tints; while pure and
vivid colours would require a perfectly definite absorption of one
portion of the coloured rays, leaving the remainder to produce the
true complementary colour. This being the case we may expect these
brown tints to occur when the need of protection is very slight or
even when it does not exist at all; always supposing that bright
colours are not in any way useful to the species. But whenever a
pure colour is protective,--as green in tropical forests or white
among arctic snows, there is no difficulty in producing it, by
natural selection acting on the innumerable slight variations of
tint which are ever occurring. Such variations may, as we have seen,
be produced in a great variety of ways; either by chemical changes
in the secretions, or by molecular changes in surface structure;
and may be brought about by change of food, by the photographic
action of light, or by the normal process of generative variation.
Protective colours therefore, however curious and complex they may
be in certain cases, offer no real difficulties.
_Theory of Warning Colours._--These differ greatly from the last
class, inasmuch as they present us with a variety of brilliant hues,
often of the greatest purity, and combined in striking contrasts
and conspicuous patterns. Their use depends upon their boldness and
visibility, not on the presence of any one colour; hence we find
among these groups some of the most exquisitely-coloured objects in
nature. Many of the uneatable caterpillars are strikingly beautiful;
while the Danaidæ, Heliconidæ, and protected groups of Papilionidæ,
comprise a series of butterflies of the most brilliant and contrasted
colours. The bright colours of many of the sea-anemones and sea-slugs
will probably be found to be in this sense protective, serving
as a warning of their uneatableness. On our theory none of these
colours offer any difficulty. Conspicuousness being useful, every
variation tending to brighter and purer colours was selected; the
result being the beautiful variety and contrast we find.
_Imitative Warning Colours:--The Theory of Mimicry._--We now come
to those groups which gain protection solely by being mistaken for
some of these brilliantly coloured but uneatable creatures, and here
a difficulty really exists, and to many minds is so great as to be
insuperable. It will be well therefore to endeavour to explain how
the resemblance in question may have been brought about.
The most difficult case, and the one which may be taken as a type
of the whole class, is that of the genus Leptalis (a group of South
American butterflies allied to our common white and yellow kinds),
many of the larger species of which are still white or yellow, and
which are all eatable by birds and other insectivorous creatures.
But there are also a number of species of Leptalis, which are
brilliantly red, yellow, and black, and which, band for band and
spot for spot, resemble some one of the Danaidæ or Heliconidæ which
inhabit the same district and which are nauseous and uneatable. Now
the usual difficulty is, that a slight approach to one of these
protected butterflies would be of no use, while a greater sudden
variation is not admissible on the theory of gradual change by
indefinite slight variations. This objection depends almost wholly
on the supposition that, when the first steps towards mimicry
occurred, the South American Danaidæ were what they are now; while
the ancestors of the Leptalides were like the ordinary white or
yellow Pieridæ to which they are allied. But the danaioid butterflies
of South America are so immensely numerous and so greatly varied,
not only in colour but in structure, that we may be sure they are
of vast antiquity and have undergone great modification. A large
number of them, however, are still of comparatively plain colours,
often rendered extremely elegant by the delicate transparency of the
wing membrane, but otherwise not at all conspicuous. Many have only
dusky or purplish bands or spots; others have patches of reddish or
yellowish brown--perhaps the commonest colour among butterflies;
while a considerable number are tinged or spotted with yellow, also
a very common colour, and one especially characteristic of the
Pieridæ, the family to which Leptalis belongs. We may therefore
reasonably suppose that in the early stages of the development of the
Danaidæ, when they first began to acquire those nauseous secretions
which are now their protection, their colours were somewhat plain;
either dusky with paler bands and spots, or yellowish with dark
borders, and sometimes with reddish bands or spots. At this time
they had probably shorter wings and a more rapid flight, just like
the other unprotected families of butterflies. But as soon as they
became decidedly unpalatable to any of their enemies, it would
be an advantage to them to be readily distinguished from all the
eatable kinds; and as butterflies were no doubt already very varied
in colour, while all probably had wings adapted for rather quick
or jerking flight, the best distinction might have been found in
outline and habits; whence would arise the preservation of those
varieties whose longer wings, bodies, and antennæ, as well as
their slower flight, rendered them noticeable--characters which
now distinguish the whole group in every part of the world.
Now it would be at this stage, that some of the weaker-flying Pieridæ
which happened to resemble some of the Danaidæ around them in their
yellow and dusky tints and in the general outline of their wings,
would be sometimes mistaken for them by the common enemy, and would
thus gain an advantage in the struggle for existence. Admitting
this one step to be made, and all the rest must inevitably follow
from simple variation and survival of the fittest. So soon as the
nauseous butterfly varied in form or colour to such an extent that
the corresponding eatable butterfly no longer closely resembled it,
the latter would be exposed to attacks, and only those variations
would be preserved which kept up the resemblance. At the same time
we may well suppose the enemies to become more acute and able to
detect smaller differences than at first. This would lead to
the destruction of all adverse variations, and thus keep up in
continually increasing complexity the outward mimicry which now
so amazes us. During the long ages in which this process has been
going on, and the Danaidæ have been acquiring those specialities
of colour which aid in their preservation, many a _Leptalis_ may
have become extinct from not varying sufficiently in the right
direction and at the right time to keep up a protective resemblance
to its neighbour; and this well accords with the comparatively small
number of cases of true mimicry, as compared with the frequency of
those protective resemblances to vegetable or inorganic objects
whose forms are less definite and colours less changeable. About a
dozen other genera of butterflies and moths mimic the Danaidæ in
various parts of the world, and exactly the same explanation will
apply to all of them. They represent those species of each group
which, at the time when the Danaidæ first acquired their protective
secretions, happened outwardly to resemble some of them, and which
have, by concurrent variation aided by a rigid selection, been able
to keep up that resemblance to the present day.[19]
[19] For fuller information on this subject the reader should consult
Mr. Bates’s original paper, “Contributions to an Insect-fauna of
the Amazon Valley,” in _Transactions of the Linnean Society_, vol.
xxiii. p. 495; Mr. Trimen’s paper in vol. xxvi. p. 497; the author’s
essay on “Mimicry,” &c., already referred to; and in the absence of
collections of butterflies, the plates of Heliconidæ and Leptalidæ,
in Hewitson’s _Exotic Butterflies_, and Felder’s _Voyage of the
“Novara,”_ may be examined.
_Theory of Sexual Colours._--In Mr. Darwin’s celebrated work, _The
Descent of Man and Selection in Relation to Sex_, he has treated
of sexual colour in combination with other sexual characters, and
has arrived at the conclusion that all or almost all the colours
of the higher animals (including among these insects and all
vertebrates) are due to voluntary or conscious sexual selection;
and that diversity of colour in the sexes is due, primarily, to
the transmission of colour-variations either to one sex only or to
both sexes; the difference depending on some unknown law, and not
being due to natural selection.
I have long held this portion of Mr. Darwin’s theory to be erroneous;
and have argued that the primary cause of sexual diversity of colour
was the need of protection, repressing in the female those bright
colours which are normally produced in both sexes by general laws;
and I have attempted to explain many of the more difficult cases on
this principle. (“A Theory of Birds’ Nests,” in _Contributions, &c._,
p. 231.) As I have since given much thought to this subject, and
have arrived at some views which appear to me to be of considerable
importance, it will be well to sketch briefly the theory I now
hold, and afterwards show its application to some of the detailed
cases adduced in Mr. Darwin’s work.