cultivated _varieties_, far surpassing in colour the original stock,
while the tropical plants are mostly unvaried wild _species_.
But this does not really much affect the question at issue. For
our florists’ gorgeous varieties have all been produced under
the influence of our cloudy skies, and with even a still further
deficiency of light, owing to the necessity of protecting them
under glass from our sudden changes of temperature; so that
they are themselves an additional proof that tropical light and
heat are not needed for the production of intense and varied
colour. Another important consideration is, that these cultivated
_varieties_ in many cases displace a number of wild _species_
which are hardly, if at all, cultivated. Thus there are scores of
_species_ of wild hollyhocks varying in colour almost as much as the
cultivated varieties, and the same may be said of the pentstemons,
rhododendrons, and many other flowers; and if these were all brought
together in well-grown specimens, they would produce a grand effect.
But it is far easier, and more profitable for our nurserymen to
grow _varieties_ of one or two species, which all require a similar
culture, rather than fifty distinct _species_, most of which would
require special treatment; the result being that the varied beauty
of the temperate flora is even now hardly known, except to botanists
and to a few amateurs.
But we may go further, and say that the hardy plants of our cold
temperate zone equal, if they do not surpass, the productions of
the tropics. Let us only remember such gorgeous tribes of flowers
as the Roses, Pæonies, Hollyhocks, and Antirrhinums; the Laburnum,
Wistaria, and Lilac; the Lilies, Irises, and Tulips; the Hyacinths,
Anemones, Gentians, and Poppies; and even our humble Gorse, Broom,
and Heather; and we may defy any tropical country to produce masses
of floral colour in greater abundance and variety. It may be true
that individual tropical shrubs and flowers do surpass everything
in the rest of the world; but that is to be expected, because the
tropical zone comprises a much greater land area than the two
temperate zones, while, owing to its more favourable climate, it
produces a still larger proportion of species of plants, and a
greater number of peculiar natural orders.
Direct observation in tropical forests, plains, and mountains,
fully supports this view. Occasionally we are startled by some
gorgeous mass of colour, but as a rule we gaze upon an endless
expanse of green foliage, only here and there enlivened by not very
conspicuous flowers. Even the orchids, whose superb blossoms adorn
our stoves, form no exception to this rule. It is only in favoured
spots that we find them in abundance; the species with small and
inconspicuous flowers greatly preponderate; and the flowering
season of each kind being of short duration, they rarely produce
any marked effect of colour amid the vast masses of foliage which
surround them. An experienced collector in the Eastern tropics once
told me, that although a single mountain in Java had produced three
hundred species of Orchideæ, only about two per cent. of the whole
were sufficiently ornamental or showy to be worth sending home
as a commercial speculation. The Alpine meadows and rock-slopes,
the open plains of the Cape of Good Hope or of Australia, and the
flower-prairies of North America, offer an amount and variety of
floral colour which can certainly not be surpassed, even if it can
be equalled, between the tropics.
It appears, therefore, that we may dismiss the theory that the
development of colour in nature is directly dependent on, and in any
way proportioned to the amount of solar heat and light, as entirely
unsupported by facts. Strange to say, however, there are some rare
and little-known phenomena which prove, that in exceptional cases,
light does directly affect the colours of natural objects; and
it will be as well to consider these before passing on to other
matters.
_Changes of Colour in Animals produced by Coloured Light._--A few
years ago Mr. T. W. Wood called attention to the curious changes in
the colour of the chrysalis of the small cabbage-butterfly (_Pontia
rapæ_) when the caterpillars, just before their change, were confined
in boxes lined with different tints. Thus in black boxes they were
very dark, in white boxes nearly white; and he further showed that
similar changes occurred in a state of nature, chrysalises fixed
against a white-washed, wall being nearly white; against a red brick
wall, reddish; against a pitched paling, nearly black. It has also
been observed that the cocoon of the emperor moth is either white
or brown, according to the colours surrounding it. But the most
extraordinary example of this kind of change is that furnished by
the chrysalis of an African butterfly (_Papilio Nireus_), observed
at the Cape by Mrs. Barber, and described (with a coloured plate)
in the _Transactions of the Entomological Society_, 1874, p. 519.
This caterpillar feeds upon the orange tree, and also upon a
forest-tree (_Vepris lanceolata_) which has a lighter green leaf;
and its colour corresponds with that of the leaves it feeds upon,
being of a darker green when it feeds on the orange. The chrysalis
is usually found suspended among the leafy twigs of its food-plant,
or of some neighbouring tree, but it is probably often attached
to larger branches; and Mrs. Barber has discovered that it has
the property of acquiring the colour, more or less accurately,
of any natural object it may be in contact with. A number of the
caterpillars were placed in a case with a glass cover, one side of
the case being formed by a red brick wall, the other sides being of
yellowish wood. They were fed on orange leaves, and a branch of the
bottle-brush tree (_Banksia_, _sp._) was also placed in the case.
When fully fed, some attached themselves to the orange twigs, others
to the bottle-brush branch; and these all changed to green pupæ; but
each corresponded exactly in tint to the leaves around it, the one
being dark, the other a pale faded green. Another attached itself to
the wood, and the pupa became of the same yellowish colour; while
one fixed itself just where the wood and brick joined, and became
one side red, the other side yellow! These remarkable changes would
perhaps not have been credited had it not been for the previous
observations of Mr. Wood; but the two support each other, and oblige
us to accept them as actual phenomena. It is a kind of natural
photography, the particular coloured rays to which the fresh pupa
is exposed in its soft, semi-transparent condition, effecting such
a chemical change in the organic juices as to produce the same tint
in the hardened skin. It is interesting however to note, that the
range of colour that can be acquired seems to be limited to those
of natural objects to which the pupa is likely to be attached; for
when Mrs. Barber surrounded one of the caterpillars with a piece
of scarlet cloth no change of colour at all was produced, the pupa
being of the usual green tint, but the small red spots with which
it is marked were brighter than usual.
Many other cases are known among insects in which the same species
acquires a different tint according to its surroundings; this being
particularly marked in some South African locusts, which correspond
with the colour of the soil wherever they are found. There are also
many caterpillars which feed on two or more plants, and which vary
in colour accordingly. A number of such changes are quoted by Mr.
R. Meldola, in a paper on Variable Protective Colouring in Insects
(_Proceedings of the Zoological Society of London_, 1873, p. 153),
and some of them may perhaps be due to a photographic action of the
reflected light. In other cases, however, it has been shown that
green chlorophyll remains unchanged in the tissues of leaf-eating
insects, and being discernible through the transparent integument,
produces the same colour as that of the food plant.
In the case of all these insects, as well as in the great majority of
cases in which a change of colour occurs in animals, the action is
quite involuntary; but among some of the higher animals the colour
of the integument can be modified at the will of the individual,
or at all events by a reflex action dependent on sensation. The
most remarkable case of this kind occurs with the chameleon, which
has the power of changing its colour from dull white to a variety
of tints. This singular power has been traced to two layers of
movable pigment-cells deeply seated in the skin, but capable of
being brought near to the surface. The pigment-layers are bluish
and yellowish, and by the pressure of suitable muscles these can
be forced upwards either together or separately. When no pressure
is exerted the colour is dirty white, which changes to various
tints of bluish, green, yellow, or brown, as more or less of either
pigment is forced up and rendered visible. The animal is excessively
sluggish and defenceless, and its power of changing its colour so as
to harmonise with surrounding objects is essential to its safety.
Here too, as with the pupa of _Papilio Nireus_, colours, such as
scarlet or blue, which do not occur in the immediate environment of
the animal, cannot be produced. Somewhat similar changes of colour
occur in some prawns and flat-fish, according to the colour of the
bottom on which they rest. This is very striking in the chameleon
shrimp (_Mysis Chamæleon_), which is grey when on sand, but brown or
green when among sea-weed of these two colours. Experiment shows,
however, that when blinded the change does not occur; so that here
too we probably have a voluntary or reflex sense-action.
These peculiar powers of change of colour and adaptation are,
however, rare and quite exceptional. As a rule, there is no direct
connection between the colours of organisms and the kind of light
to which they are usually exposed. This is well seen in most fishes
and in such marine animals as porpoises, whose backs are always
dark, although this part is exposed to the blue and white light of
the sky and clouds, while their bellies are very generally white,
although these are constantly subjected to the deep blue or dusky
green light from the bottom. It is evident, however, that these two
tints have been acquired for concealment and protection. Looking
_down_ on the dark back of a fish it is almost invisible, while,
to an enemy looking _up_ from below, the light under-surface would
be equally invisible against the light of the clouds and sky.
Again, the gorgeous colours of the butterflies which inhabit the
depths of tropical forests bear no relation to the kind of light
that falls upon them, coming as it does almost wholly from green
foliage, dark brown soil, or blue sky; and the bright underwings of
many moths, which are only exposed at night, contrast remarkably
with the sombre tints of the upper wings, which are more or less
exposed to the various colours of surrounding nature.
_Classification of Organic Colours._--We find, then, that neither the
general influence of solar light and heat, nor the special action
of variously tinted rays, are adequate causes for the wonderful
variety, intensity, and complexity of the colours that everywhere
meet us in the animal and vegetable worlds. Let us therefore take a
wider view of these colours, grouping them into classes determined
by what we know of their actual uses or special relations to the
habits of their possessors. This, which may be termed the functional
and biological classification of the colours of living organisms,
seems to be best expressed by a division into five groups, as
follows:--
{1. Protective colours.
{
{2. Warning colours. {_a._ Of creatures specially protected.
Animals. { {_b._ Of defenceless creatures,
{ mimicking _a_.
{
{3. Sexual colours.
{
{4. Typical colours.
Plants. 5. Attractive colours.
It is now proposed, firstly, to point out the nature of the phenomena
presented under each of these heads; then to explain the general
laws of the production of colour in nature; and, lastly, to show
how far the varied phenomena of animal coloration can be explained
by means of those laws, acting in conjunction with the laws of
evolution and natural selection.
_Protective Colours._--The nature of the two first groups, Protective
and Warning colours, has been so fully detailed and illustrated
in my chapter on “Mimicry and other Protective Resemblances among
Animals,” (_Contributions to the Theory of Natural Selection_, p.
45), that very little need be added here except a few words of
general explanation. Protective colours are exceedingly prevalent
in nature, comprising those of all the white arctic animals, the
sandy-coloured desert forms, and the green birds and insects of
tropical forests. It also comprises thousands of cases of special
resemblance--of birds to the surroundings of their nests, and
especially of insects to the bark, leaves, flowers, or soil, on or
amid which they dwell. Mammalia, fishes, and reptiles, as well as
mollusca and other marine invertebrates, present similar phenomena;
and the more the habits of animals are investigated, the more
numerous are found to be the cases in which their colours tend
to conceal them, either from their enemies or from the creatures
they prey upon. One of the last-observed and most curious of these
protective resemblances has been communicated to me by Sir Charles
Dilke. He was shown in Java a pink-coloured _Mantis_ which, when
at rest, exactly resembled a pink orchis-flower. The mantis is a
carnivorous insect which lies in wait for its prey; and, by its
resemblance to a flower, the insects it feeds on would be actually
attracted towards it. This one is said to feed especially on
butterflies, so that it is really a living trap, and forms its own
bait!
All who have observed animals, and especially insects, in their
native haunts and attitudes, can understand how it is that an insect
which in a cabinet looks exceedingly conspicuous, may yet when
alive, in its peculiar attitude of repose and with its habitual
surroundings, be perfectly well concealed. We can hardly ever tell by
the mere inspection of an animal, whether its colours are protective
or not. No one would imagine the exquisitely beautiful caterpillar
of the emperor-moth, which is green with pink star-like spots, to
be protectively coloured; yet, when feeding on the heather, it so
harmonises with the foliage and flowers as to be almost invisible.
Every day fresh cases of protective colouring are being discovered,
even in our own country; and it is becoming more and more evident
that the need of protection has played a very important part in
determining the actual coloration of animals.
_Warning Colours._--The second class--the warning colours--are
exceedingly interesting, because the object and effect of these is,
not to conceal the object, but to make it conspicuous. To these
creatures it is _useful_ to be seen and recognized; the reason
being that they have a means of defence which, if known, will
prevent their enemies from attacking them, though it is generally
not sufficient to save their lives if they are actually attacked.
The best examples of these specially protected creatures consist
of two extensive families of butterflies, the Danaidæ and Acræidæ,
comprising many hundreds of species inhabiting the tropics of all
parts of the world. These insects are generally large, are all
conspicuously and often most gorgeously coloured, presenting almost
every conceivable tint and pattern; they all fly slowly, and they
never attempt to conceal themselves; yet no bird, spider, lizard,
or monkey (all of which eat other butterflies) ever touches them.
The reason simply is that they are not fit to eat, their juices
having a powerful odour and taste that is absolutely disgusting to
all these animals. Now we see the reason of their showy colours and
slow flight. It is good for them to be seen and recognised, for
then they are never molested; but if they did not differ in form
and colouring from other butterflies, or if they flew so quickly
that their peculiarities could not be easily noticed, they would
be captured, and though not eaten would be maimed or killed.
As soon as the cause of the peculiarities of these butterflies was
clearly recognised, it was seen that the same explanation applied
to many other groups of animals. Thus, bees and wasps and other
stinging insects are showily and distinctively coloured; many soft
and apparently defenceless beetles, and many gay-coloured moths,
were found to be as nauseous as the above-named butterflies; other
beetles, whose hard and glossy coats of mail render them unpalatable
to insect-eating birds, are also sometimes showily coloured; and
the same rule was found to apply to caterpillars, all the brown and
green (or protectively coloured species) being greedily eaten by
birds, while showy kinds which never hide themselves--like those of
the magpie-, mullein-, and burnet-moths--were utterly refused by
insectivorous birds, lizards, frogs, and spiders. (_Contributions
to the Theory of Natural Selection_, p. 117.) Some few analogous
examples are found among vertebrate animals. I will only mention
here a very interesting case not given in my former work. In his
delightful book entitled, _The Naturalist in Nicaragua_, Mr. Belt
tells us that there is in that country a frog which is very abundant;
which hops about in the daytime; which never hides himself; and
which is gorgeously coloured with red and blue. Now frogs are
usually green, brown, or earth-coloured; feed mostly at night;
and are all eaten by snakes and birds. Having full faith in the
theory of protective and warning colours, to which he had himself
contributed some valuable facts and observations, Mr. Belt felt
convinced that this frog must be uneatable. He therefore took one
home, and threw it to his ducks and fowls; but all refused to touch
it except one young duck, which took the frog in its mouth, but
dropped it directly, and went about jerking its head as if trying
to get rid of something nasty. Here the uneatableness of the frog
was predicted from its colours and habits, and we can have no more
convincing proof of the truth of a theory than such previsions.
The universal avoidance by carnivorous animals of all these specially
protected groups, which are thus entirely free from the constant
persecution suffered by other creatures not so protected, would
evidently render it advantageous for any of these latter which were
subjected to extreme persecution to be mistaken for the former;
and for this purpose it would be necessary that they should have
the same colours, form, and habits. Now, strange to say, wherever
there is a large group of directly-protected forms (division _a_
of animals with warning colours), there are sure to be found a few
otherwise defenceless creatures which resemble them externally so
as to be mistaken for them, and which thus gain protection, as it
were, on false pretences (division _b_ of animals with warning
colours). This is what is called “mimicry,” and it has already been
very fully treated of by Mr. Bates (its discoverer), by myself, by
Mr. Trimen, and others. Here it is only necessary to state that
the uneatable Danaidæ and Acræidæ are accompanied by a few species
of other groups of butterflies (Leptalidæ, Papilios, Diademas, and
Moths) which are all really eatable, but which escape attack by
their close resemblance to some species of the uneatable groups
found in the same locality. In like manner there are a few eatable
beetles which exactly resemble species of uneatable groups; and
others, which are soft, imitate those which are uneatable through
their hardness. For the same reason wasps are imitated by moths,
and ants by beetles; and even poisonous snakes are mimicked by
harmless snakes, and dangerous hawks by defenceless cuckoos. How
these curious imitations have been brought about, and the laws which
govern them, have been discussed in the work already referred to.
_Sexual Colours._--The third class comprises all cases in which the
colours of the two sexes differ. This difference is very general,
and varies greatly in amount, from a slight divergence of tint up
to a radical change of coloration. Differences of this kind are
found among all classes of animals in which the sexes are separated,
but they are much more frequent in some groups than in others. In
mammalia, reptiles, and fishes, they are comparatively rare, and
not great in amount, whereas among birds they are very frequent
and very largely developed. So among insects, they are abundant
in butterflies, while they are comparatively uncommon in beetles,
wasps, and hemiptera.
The phenomena of sexual variations of colour, as well as of colour
generally, are wonderfully similar in the two analogous yet totally
unrelated groups of birds and butterflies; and as they both offer
ample materials, we shall confine our study of the subject chiefly
to them. The most common case of difference of colour between the
sexes, is for the male to have the same general hue as the females,
but deeper and more intensified; as in many thrushes, finches,
and hawks; and among butterflies in the majority of our British
species. In cases where the male is smaller the intensification of
colour is especially well pronounced; as in many of the hawks and
falcons, and in most butterflies and moths in which the coloration
does not materially differ. In another extensive series we have spots
or patches of vivid colour in the male, which are represented in
the female by far less brilliant tints or are altogether wanting;
as exemplified in the gold-crest warbler, the green woodpecker,
and most of the orange-tip butterflies (_Anthocharis_). Proceeding
with our survey, we find greater and greater differences of colour
in the sexes, till we arrive at such extreme cases as some of the
pheasants, the chatterers, tanagers, and birds-of-paradise, in
which the male is adorned with the most gorgeous and vivid colours,
while the female is usually dull brown, or olive green, and often
shows no approximation whatever to the varied tints of her partner.
Similar phenomena occur among butterflies; and in both these groups
there are also a considerable number of cases in which both sexes
are highly coloured in a different way. Thus many woodpeckers have
the head in the male red, in the female yellow; while some parrots
have red spots in the male, replaced by blue in the female, as in
_Psittacula diopthalma_. In many South American Papilios, green
spots on the male are represented by red on the female; and in
several species of the genus _Epicalia_, orange bands in the male
are replaced by blue in the female, a similar change of colour to
that in the small parrot above referred to. For fuller details of
the varieties of sexual coloration we refer our readers to Mr.
Darwin’s _Descent of Man_, chapters x. to xviii., and to chapters
iii., iv. and vii. of my _Contributions to the Theory of Natural
Selection_.
_Typical Colours._--The fourth group--of Typically-coloured
animals--includes all species which are brilliantly or conspicuously
coloured in both sexes, and for whose particular colours we can
assign no function or use. It comprises an immense number of
showy birds, such as Kingfishers, Barbets, Toucans, Lories, Tits,
and Starlings; among insects most of the largest and handsomest
butterflies, innumerable bright-coloured beetles, locusts,
dragon-flies, and hymenoptera; a few mammalia, as the zebras; a
great number of marine fishes; thousands of striped and spotted
caterpillars; and abundance of mollusca, star-fish, and other marine
animals. Among these we have included some which, like the gaudy
caterpillars, have warning colours; but as that theory does not
explain the particular colours or the varied patterns with which
they are adorned, it is best to include them also in this class. It
is a suggestive fact, that all the brightly-coloured birds mentioned
above build in holes or form covered nests, so that the females do
not need that protection during the breeding season which I believe
to be one of the chief causes of the dull colour of female birds
when their partners are gaily coloured. This subject is fully argued
in my _Contributions_, &c., chapter vii.
As the colours of plants and flowers are very different from those
of animals both in their distribution and functions, it will be
well now to consider how the general facts of colour here sketched
out can be explained. We have first to inquire what is colour,
and how it is produced; what is known of the causes of change of
colour; and what theory best accords with the whole assemblage of
facts.