of the humming birds, made the objection that "A crest of topaz is no
better in the struggle for existence than a crest of sapphire. A frill
ending in spangles of the emerald is no better in the battle of life
than a frill ending in spangles of the ruby. A tail is not affected for
the purposes of flight, whether its marginal or its central feathers are
decorated with white;" and he goes on to urge that mere beauty and
variety for their own sake are the only causes of these differences.
But, on the principles here suggested, the divergence itself is useful,
and must have been produced _pari passu_ with the structural differences
on which the differentiation of species depends; and thus we have
explained the curious fact that prominent differences of colour often
distinguish species otherwise very closely allied to each other.

Among insects, the principle of distinctive coloration for recognition
has probably been at work in the production of the wonderful diversity
of colour and marking we find everywhere, more especially among the
butterflies and moths; and here its chief function may have been to
secure the pairing together of individuals of the same species. In some
of the moths this has been secured by a peculiar odour, which attracts
the males to the females from a distance; but there is no evidence that
this is universal or even general, and among butterflies, especially,
the characteristic colour and marking, aided by size and form, afford
the most probable means of recognition. That this is so is shown by the
fact that "the common white butterfly often flies down to a bit of paper
on the ground, no doubt mistaking it for one of its own species;" while,
according to Mr. Collingwood, in the Malay Archipelago, "a dead
butterfly pinned upon a conspicuous twig will often arrest an insect of
the same species in its headlong flight, and bring it down within easy
reach of the net, especially if it be of the opposite sex."[87] In a
great number of insects, no doubt, form, motions, stridulating sounds,
or peculiar odours, serve to distinguish allied species from each other,
and this must be especially the case with nocturnal insects, or with
those whose colours are nearly uniform and are determined by the need of
protection; but by far the larger number of day-flying and active
insects exhibit varieties of colour and marking, forming the most
obvious distinction between allied species, and which have, therefore,
in all probability been acquired in the process of differentiation for
the purpose of checking the intercrossing of closely allied forms.[88]

Whether this principle extends to any of the less highly organised
animals is doubtful, though it may perhaps have affected the higher
mollusca. But in marine animals it seems probable that the colours,
however beautiful, varied, and brilliant they may often be, are in most
cases protective, assimilating them to the various bright-coloured
seaweeds, or to some other animals which it is advantageous for them to
imitate.[89]


_Summary of the Preceding Exposition._

Before proceeding to discuss some of the more recondite phenomena of
animal coloration, it will be well to consider for a moment the extent
of the ground we have already covered. Protective coloration, in some of
its varied forms, has not improbably modified the appearance of one-half
of the animals living on the globe. The white of arctic animals, the
yellowish tints of the desert forms, the dusky hues of crepuscular and
nocturnal species, the transparent or bluish tints of oceanic creatures,
represent a vast host in themselves; but we have an equally numerous
body whose tints are adapted to tropical foliage, to the bark of trees,
or to the soil or dead leaves on or among which they habitually live.
Then we have the innumerable special adaptations to the tints and forms
of leaves, or twigs, or flowers; to bark or moss; to rock or pebble; by
which such vast numbers of the insect tribes obtain protection; and we
have seen that these various forms of coloration are equally prevalent
in the waters of the seas and oceans, and are thus coextensive with the
domain of life upon the earth. The comparatively small numbers which
possess "terrifying" or "alluring" coloration may be classed under the
general head of the protectively coloured.

But under the next head - colour for recognition - we have a totally
distinct category, to some extent antagonistic or complementary to the
last, since its essential principle is visibility rather than
concealment. Yet it has been shown, I think, that this mode of
coloration is almost equally important, since it not only aids in the
preservation of existing species and in the perpetuation of pure races,
but was, perhaps, in its earlier stages, a not unimportant factor in
their development. To it we owe most of the variety and much of the
beauty in the colours of animals; it has caused at once bilateral
symmetry and general permanence of type; and its range of action has
been perhaps equally extensive with that of coloration for concealment.


_Influence of Locality or of Climate on Colour._

Certain relations between locality and coloration have long been
noticed. Mr. Gould observed that birds from inland or continental
localities were more brightly coloured than those living near the
sea-coast or on islands, and he supposed that the more brilliant
atmosphere of the inland stations was the explanation of the
phenomenon.[90] Many American naturalists have observed similar facts,
and they assert that the intensity of the colours of birds and mammals
increases from north to south, and also with the increase of humidity.
This change is imputed by Mr. J.A. Allen to the direct action of the
environment. He says: "In respect to the correlation of intensity of
colour in animals with the degree of humidity, it would perhaps be more
in accordance with cause and effect to express the law of correlation as
a _decrease_ of intensity of colour with a _decrease_ of humidity, the
paleness evidently resulting from exposure and the blanching effect of
intense sunlight, and a dry, often intensely heated atmosphere. With the
decrease of the aqueous precipitation the forest growth and the
protection afforded by arborescent vegetation gradually also decreases,
as of course does also the protection afforded by clouds, the
excessively humid regions being also regions of extreme cloudiness,
while the dry regions are comparatively cloudless districts."[91] Almost
identical changes occur in birds, and are imputed by Mr. Allen to
similar causes.

It will be seen that Mr. Gould and Mr. Allen impute opposite effects to
the same cause, brilliancy or intensity of colour being due to a
brilliant atmosphere according to the former, while paleness of colour
is imputed by the latter to a too brilliant sun. According to the
principles which have been established by the consideration of arctic,
desert, and forest animals respectively, we shall be led to conclude
that there has been no direct action in this case, but that the effects
observed are due to the greater or less need of protection. The pale
colour that is prevalent in arid districts is in harmony with the
general tints of the surface; while the brighter tints or more intense
coloration, both southward and in humid districts, are sufficiently
explained by the greater shelter due to a more luxuriant vegetation and
a shorter winter. The advocates of the theory that intensity of light
directly affects the colours of organisms, are led into perpetual
inconsistencies. At one time the brilliant colours of tropical birds and
insects are imputed to the intensity of a tropical sun, while the same
intensity of sunlight is now said to have a "bleaching" effect. The
comparatively dull and sober hues of our northern fauna were once
supposed to be the result of our cloudy skies; but now we are told that
cloudy skies and a humid atmosphere intensify colour.

In my _Tropical Nature_ (pp. 257-264) I have called attention to what is
perhaps the most curious and decided relation of colour to locality
which has yet been observed - the prevalence of white markings in the
butterflies and birds of islands.

So many cases are adduced from so many different islands, both in the
eastern and western hemisphere, that it is impossible to doubt the
existence of some common cause; and it seems probable to me now, after a
fuller consideration of the whole subject of colour, that here too we
have one of the almost innumerable results of the principle of
protective coloration. White is, as a rule, an uncommon colour in
animals, but probably only because it is so conspicuous. Whenever it
becomes protective, as in the case of arctic animals and aquatic birds,
it appears freely enough; while we know that white varieties of many
species occur occasionally in the wild state, and that, under
domestication, white or parti-coloured breeds are freely produced. Now
in all the islands in which exceptionally white-marked birds and
butterflies have been observed, we find two features which would tend to
render the conspicuous white markings less injurious - a luxuriant
tropical vegetation, and a decided scarcity of rapacious mammals and
birds. White colours, therefore, would not be eliminated by natural
selection; but variations in this direction would bear their part in
producing the recognition marks which are everywhere essential, and
which, in these islands, need not be so small or so inconspicuous as
elsewhere.


_Concluding Remarks._

On a review of the whole subject, then, we must conclude that there is
no evidence of the individual or prevalent colours of organisms being
directly determined by the amount of light, or heat, or moisture, to
which they are exposed; while, on the other hand, the two great
principles of the need of concealment from enemies or from their prey,
and of recognition by their own kind, are so wide-reaching in their
application that they appear at first sight to cover almost the whole
ground of animal coloration. But, although they are indeed wonderfully
general and have as yet been very imperfectly studied, we are acquainted
with other modes of coloration which have a different origin. These
chiefly appertain to the very singular class of warning colours, from
which arise the yet more extraordinary phenomena of mimicry; and they
open up so curious a field of inquiry and present so many interesting
problems, that a chapter must be devoted to them. Yet another chapter
will be required by the subject of sexual differentiation of colour and
ornament, as to the origin and meaning of which I have arrived at
different conclusions from Mr. Darwin. These various forms of coloration
having been discussed and illustrated, we shall be in a position to
attempt a brief sketch of the fundamental laws which have determined the
general coloration of the animal world.

FOOTNOTES:

[Footnote 65: _Proceedings of the Royal Society_, No. 243, 1886;
_Transactions of the Royal Society_, vol. clxxviii. B. pp. 311-441.]

[Footnote 66: _A Naturalist's Wanderings in the Eastern Archipelago_, p.
460.]

[Footnote 67: _Trans. Phil. Soc._ (? _of S. Africa_), 1878, part iv, p.
27.]

[Footnote 68: _Proc. Zool. Soc._, 1862 p. 357.]

[Footnote 69: With reference to this general resemblance of insects to
their environment the following remarks by Mr. Poulton are very
instructive. He says: "Holding the larva of Sphinx ligustri in one hand
and a twig of its food-plant in the other, the wonder we feel is, not at
the resemblance but at the difference; we are surprised at the
difficulty experienced in detecting so conspicuous an object. And yet
the protection is very real, for the larvae will be passed over by those
who are not accustomed to their appearance, although the searcher may be
told of the presence of a large caterpillar. An experienced entomologist
may also fail to find the larvae till after a considerable search. This
is general protective resemblance, and it depends upon a general harmony
between the appearance of the organism and its whole environment. It is
impossible to understand the force of this protection for any larva,
without seeing it on its food-plant and in an entirely normal condition.
The artistic effect of green foliage is more complex than we often
imagine; numberless modifications are wrought by varied lights and
shadows upon colours which are in themselves far from uniform. In the
larva of Papilio machaon the protection is very real when the larva is
on the food-plant, and can hardly be appreciated at all when the two are
apart." Numerous other examples are given in the chapter on "Mimicry and
other Protective Resemblances among Animals," in my _Contributions to
the Theory of Natural Selection_.]

[Footnote 70: _The Naturalist in Nicaragua_, p. 19.]

[Footnote 71: R. Meldola, in _Proc. Zool. Soc._, 1873, p. 155.]

[Footnote 72: _Nature_, vol. iii. p. 166.]

[Footnote 73: _Trans. Ent. Soc. Lond._, 1878, p. 185.]

[Footnote 74: _Ibid._ (_Proceedings_, p. xlii.)]

[Footnote 75: Wallace's _Malay Archipelago_, vol. i. p. 204 (fifth
edition, p. 130), with figure.]

[Footnote 76: Moseley's _Notes by a Naturalist on the Challenger_.]

[Footnote 77: _Proceedings of the Boston Soc. of Nat. Hist._, vol. xiv.
1871.]

[Footnote 78: _Nature_, 1870, p. 376.]

[Footnote 79: _A Naturalist's Wanderings in the Eastern Archipelago_, p.
63.]

[Footnote 80: A beautiful drawing of this rare insect, Hymenopus
bicornis (in the nymph or active pupa state), was kindly sent me by Mr.
Wood-Mason, Curator of the Indian Museum at Calcutta. A species, very
similar to it, inhabits Java, where it is said to resemble a pink
orchid. Other Mantidae, of the genus Gongylus, have the anterior part of
the thorax dilated and coloured either white, pink, or purple; and they
so closely resemble flowers that, according to Mr. Wood-Mason, one of
them, having a bright violet-blue prothoracic shield, was found in Pegu
by a botanist, and was for a moment mistaken by him for a flower. See
_Proc. Ent. Soc. Lond._, 1878, p. liii.]

[Footnote 81: C. Dixon, in Seebohm's _History of British Birds_, vol.
ii. Introduction, p. xxvi. Many of the other examples here cited are
taken from the same valuable work.]

[Footnote 82: See A.H.S. Lucas, in _Proceedings of Royal Society of
Victoria_, 1887, p. 56.]

[Footnote 83: Professor Wm.H. Brewer of Yale College has shown that the
white marks or the spots of domesticated animals are rarely symmetrical,
but have a tendency to appear more frequently on the left side. This is
the case with horses, cattle, dogs, and swine. Among wild animals the
skunk varies considerably in the amount of white on the body, and this
too was found to be usually greatest on the left side. A close
examination of numerous striped or spotted species, as tigers, leopards,
jaguars, zebras, etc., showed that the bilateral symmetry was not exact,
although the general effect of the two sides was the same. This is
precisely what we should expect if the symmetry is not the result of a
general law of the organisation, but has been, in part at least,
produced and preserved for the useful purpose of recognition by the
animal's fellows of the same species, and especially by the sexes and
the young. See _Proc. of the Am. Ass. for Advancement of Science_, vol.
xxx. p. 246.]

[Footnote 84: _Descent of Man_, p. 542.]

[Footnote 85: It may be thought that such extremely conspicuous markings
as those of the zebra would be a great danger in a country abounding
with lions, leopards, and other beasts of prey; but it is not so. Zebras
usually go in bands, and are so swift and wary that they are in little
danger during the day. It is in the evening, or on moonlight nights,
when they go to drink, that they are chiefly exposed to attack; and Mr.
Francis Galton, who has studied these animals in their native haunts,
assures me, that in twilight they are not at all conspicuous, the
stripes of white and black so merging together into a gray tint that it
is very difficult to see them at a little distance. We have here an
admirable illustration of how a glaringly conspicuous style of marking
for recognition may be so arranged as to become also protective at the
time when protection is most needed; and we may also learn how
impossible it is for us to decide on the inutility of any kind of
coloration without a careful study of the habits of the species in its
native country.]

[Footnote 86: The principle of colouring for recognition was, I believe,
first stated in my article on "The Colours of Animals and Plants" in
Macmillan's _Magazine_, and more fully in my volume on _Tropical
Nature_. Subsequently Mrs. Barber gave a few examples under the head of
"Indicative or Banner Colours," but she applied it to the distinctive
colours of the males of birds, which I explain on another principle,
though this may assist.]

[Footnote 87: Quoted by Darwin in _Descent of Man_, p. 317.]

[Footnote 88: In the _American Naturalist_ of March 1888, Mr. J.E. Todd
has an article on "Directive Coloration in Animals," in which he
recognises many of the cases here referred to, and suggests a few
others, though I think he includes many forms of coloration - as
"paleness of belly and inner side of legs" - which do not belong to this
class.]

[Footnote 89: For numerous examples of this protective colouring of
marine animals see Moseley's _Voyage of the Challenger_, and Dr. E.S.
Morse in _Proc. of Bost. Soc. of Nat. Hist._, vol. xiv. 1871.]

[Footnote 90: See _Origin of Species_, p. 107.]

[Footnote 91: The "Geographical Variation of North American Squirrels,"
_Proc. Bost. Soc. of Nat. Hist._, 1874, p. 284; and _Mammals and Winter
Birds of Florida_, pp. 233-241.]


CHAPTER IX

WARNING COLORATION AND MIMICRY


The skunk as an example of warning coloration - Warning colours
among insects - Butterflies - Caterpillars - Mimicry - How mimicry
has been produced - Heliconidae - Perfection of the
imitation - Other cases of mimicry among Lepidoptera - Mimicry
among protected groups - Its explanation - Extension of the
principle - Mimicry in other orders of insects - Mimicry among the
vertebrata - Snakes - The rattlesnake and the cobra - Mimicry among
birds - Objections to the theory of mimicry - Concluding remarks
on warning colours and mimicry.


We have now to deal with a class of colours which are the very opposite
of those we have hitherto considered, since, instead of serving to
conceal the animals that possess them or as recognition marks to their
associates, they are developed for the express purpose of rendering the
species conspicuous. The reason of this is that the animals in question
are either the possessors of some deadly weapons, as stings or poison
fangs, or they are uneatable, and are thus so disagreeable to the usual
enemies of their kind that they are never attacked when their peculiar
powers or properties are known. It is, therefore, important that they
should not be mistaken for defenceless or eatable species of the same
class or order, since in that case they might suffer injury, or even
death, before their enemies discovered the danger or the uselessness of
the attack. They require some signal or danger-flag which shall serve as
a warning to would-be enemies not to attack them, and they have usually
obtained this in the form of conspicuous or brilliant coloration, very
distinct from the protective tints of the defenceless animals allied to
them.

_The Skunk as illustrating Warning Coloration._

While staying a few days, in July 1887, at the Summit Hotel on the
Central Pacific Railway, I strolled out one evening after dinner, and on
the road, not fifty yards from the house, I saw a pretty little white
and black animal with a bushy tail coming towards me. As it came on at a
slow pace and without any fear, although it evidently saw me, I thought
at first that it must be some tame creature, when it suddenly occurred
to me that it was a skunk. It came on till within five or six yards of
me, then quietly climbed over a dwarf wall and disappeared under a small
outhouse, in search of chickens, as the landlord afterwards told me.
This animal possesses, as is well known, a most offensive secretion,
which it has the power of ejecting over its enemies, and which
effectually protects it from attack. The odour of this substance is so
penetrating that it taints, and renders useless, everything it touches,
or in its vicinity. Provisions near it become uneatable, and clothes
saturated with it will retain the smell for several weeks, even though
they are repeatedly washed and dried. A drop of the liquid in the eyes
will cause blindness, and Indians are said not unfrequently to lose
their sight from this cause. Owing to this remarkable power of offence
the skunk is rarely attacked by other animals, and its black and white
fur, and the bushy white tail carried erect when disturbed, form the
danger-signals by which it is easily distinguished in the twilight or
moonlight from unprotected animals. Its consciousness that it needs only
to be seen to be avoided gives it that slowness of motion and
fearlessness of aspect which are, as we shall see, characteristic of
most creatures so protected.


_Warning Colours among Insects._

It is among insects that warning colours are best developed, and most
abundant. We all know how well marked and conspicuous are the colours
and forms of the stinging wasps and bees, no one of which in any part of
the world is known to be protectively coloured like the majority of
defenceless insects. Most of the great tribe of Malacoderms among
beetles are distasteful to insect-eating animals. Our red and black
Telephoridae, commonly called "soldiers and sailors," were found, by Mr.
Jenner Weir, to be refused by small birds. These and the allied
Lampyridae (the fireflies and glow-worms) in Nicaragua, were rejected by
Mr. Belt's tame monkey and by his fowls, though most other insects were
greedily eaten by them. The Coccinellidae or lady-birds are another
uneatable group, and their conspicuous and singularly spotted bodies
serve to distinguish them at a glance from all other beetles.

These uneatable insects are probably more numerous than is supposed,
although we already know immense numbers that are so protected. The most
remarkable are the three families of butterflies - Heliconidae, Danaidae,
and Acraeidae - comprising more than a thousand species, and
characteristic respectively of the three great tropical regions - South
America, Southern Asia, and Africa. All these butterflies have
peculiarities which serve to distinguish them from every other group in
their respective regions. They all have ample but rather weak wings, and
fly slowly; they are always very abundant; and they all have conspicuous
colours or markings, so distinct from those of other families that, in
conjunction with their peculiar outline and mode of flight, they can
usually be recognised at a glance. Other distinctive features are, that
their colours are always nearly the same on the under surface of their
wings as on the upper; they never try to conceal themselves, but rest on
the upper surfaces of leaves or flowers; and, lastly, they all have
juices which exhale a powerful scent, so that when one kills them by
pinching the body, the liquid that exudes stains the fingers yellow, and
leaves an odour that can only be removed by repeated washings.

Now, there is much direct evidence to show that this odour, though not
very offensive to us, is so to most insect-eating creatures. Mr. Bates
observed that, when set out to dry, specimens of Heliconidae were less
subject to the attacks of vermin; while both he and I noticed that they
were not attacked by insect-eating birds or dragonflies, and that their
wings were not found in the forest paths among the numerous wings of
other butterflies whose bodies had been devoured. Mr. Belt once observed
a pair of birds capturing insects for their young; and although the
Heliconidae swarmed in the vicinity, and from their slow flight could
have been easily caught, not one was ever pursued, although other
butterflies did not escape. His tame monkey also, which would greedily
munch up other butterflies, would never eat the Heliconidae. It would
sometimes smell them, but always rolled them up in its hand and then
dropped them.

We have also some corresponding evidence as to the distastefulness of
the Eastern Danaidae. The Hon. Mr. Justice Newton, who assiduously
collected and took notes upon the Lepidoptera of Bombay, informed Mr.
Butler of the British Museum that the large and swift-flying butterfly