THE ORIGIN AND USES OF COLOUR IN ANIMALS
The Darwinian theory threw new light on organic colour--The
problem to be solved--The constancy of animal colour indicates
utility--Colour and environment--Arctic animals
white--Exceptions prove the rule--Desert, forest, nocturnal, and
oceanic animals--General theories of animal colour--Variable
protective colouring--Mr. Poulton's experiments--Special or
local colour adaptations--Imitation of particular objects--How
they have been produced--Special protective colouring of
butterflies--Protective resemblance among marine
animals--Protection by terrifying enemies--Alluring
coloration--The coloration of birds' eggs--Colour as a means of
recognition--Summary of the preceding exposition--Influence of
locality or of climate on colour--Concluding remarks.
Among the numerous applications of the Darwinian theory in the
interpretation of the complex phenomena presented by the organic world,
none have been more successful, or are more interesting, than those
which deal with the colours of animals and plants. To the older school
of naturalists colour was a trivial character, eminently unstable and
untrustworthy in the determination of species; and it appeared to have,
in most cases, no use or meaning to the objects which displayed it. The
bright and often gorgeous coloration of insect, bird, or flower, was
either looked upon as having been created for the enjoyment of mankind,
or as due to unknown and perhaps undiscoverable laws of nature.
But the researches of Mr. Darwin totally changed our point of view in
this matter. He showed, clearly, that some of the colours of animals are
useful, some hurtful to them; and he believed that many of the most
brilliant colours were developed by sexual choice; while his great
general principle, that all the fixed characters of organic beings have
been developed under the action of the law of utility, led to the
inevitable conclusion that so remarkable and conspicuous a character as
colour, which so often constitutes the most obvious distinction of
species from species, or group from group, must also have arisen from
survival of the fittest, and must, therefore, in most cases have some
relation to the wellbeing of its possessors. Continuous observation and
research, carried on by multitudes of observers during the last thirty
years, have shown this to be the case; but the problem is found to be
far more complex than was at first supposed. The modes in which colour
is of use to different classes of organisms is very varied, and have
probably not yet been all discovered; while the infinite variety and
marvellous beauty of some of its developments are such as to render it
hopeless to arrive at a complete and satisfactory explanation of every
individual case. So much, however, has been achieved, so many curious
facts have been explained, and so much light has been thrown on some of
the most obscure phenomena of nature, that the subject deserves a
prominent place in any account of the Darwinian theory.
_The Problem to be Solved._
Before dealing with the various modifications of colour in the animal
world it is necessary to say a few words on colour in general, on its
prevalence in nature, and how it is that the colours of animals and
plants require any special explanation. What we term colour is a
subjective phenomenon, due to the constitution of our mind and nervous
system; while, objectively, it consists of light-vibrations of different
wave-lengths emitted by, or reflected from, various objects. Every
visible object must be coloured, because to be visible it must send rays
of light to our eye. The kind of light it sends is modified by the
molecular constitution or the surface texture of the object. Pigments
absorb certain rays and reflect the remainder, and this reflected
portion has to our eyes a definite colour, according to the portion of
the rays constituting white light which are absorbed. Interference
colours are produced either by thin films or by very fine striae on the
surfaces of bodies, which cause rays of certain wave-lengths to
neutralise each other, leaving the remainder to produce the effects of
colour. Such are the colours of soap-bubbles, or of steel or glass on
which extremely fine lines have been ruled; and these colours often
produce the effect of metallic lustre, and are the cause of most of the
metallic hues of birds and insects.
As colour thus depends on molecular or chemical constitution or on the
minute surface texture of bodies, and, as the matter of which organic
beings are composed consists of chemical compounds of great complexity
and extreme instability, and is also subject to innumerable changes
during growth and development, we might naturally expect the phenomena
of colour to be more varied here than in less complex and more stable
compounds. Yet even in the inorganic world we find abundant and varied
colours; in the earth and in the water; in metals, gems, and minerals;
in the sky and in the ocean; in sunset clouds and in the many-tinted
rainbow. Here we can have no question of _use_ to the coloured object,
and almost as little perhaps in the vivid red of blood, in the brilliant
colours of red snow and other low algae and fungi, or even in the
universal mantle of green which clothes so large a portion of the
earth's surface. The presence of some colour, or even of many brilliant
colours, in animals and plants would require no other explanation than
does that of the sky or the ocean, of the ruby or the emerald--that is,
it would require a purely physical explanation only. It is the wonderful
individuality of the colours of animals and plants that attracts our
attention--the fact that the colours are localised in definite patterns,
sometimes in accordance with structural characters, sometimes altogether
independent of them; while often differing in the most striking and
fantastic manner in allied species. We are thus compelled to look upon
colour not merely as a physical but also as a biological characteristic,
which has been differentiated and specialised by natural selection, and
must, therefore, find its explanation in the principle of adaptation or
utility.
_The Constancy of Animal Colour indicates Utility._
That the colours and markings of animals have been acquired under the
fundamental law of utility is indicated by a general fact which has
received very little attention. As a rule, colour and marking are
constant in each species of wild animal, while, in almost every
domesticated animal, there arises great variability. We see this in our
horses and cattle, our dogs and cats, our pigeons and poultry. Now, the
essential difference between the conditions of life of domesticated and
wild animals is, that the former are protected by man, while the latter
have to protect themselves. The extreme variations in colour that
immediately arise under domestication indicate a tendency to vary in
this way, and the occasional occurrence of white or piebald or other
exceptionally coloured individuals of many species in a state of nature,
shows that this tendency exists there also; and, as these exceptionally
coloured individuals rarely or never increase, there must be some
constant power at work to keep it in check. This power can only be
natural selection or the survival of the fittest, which again implies
that some colours are useful, some injurious, in each particular case.
With this principle as our guide, let us see how far we can account both
for the general and special colours of the animal world.
_Colour and Environment._
The fact that first strikes us in our examination of the colours of
animals as a whole, is the close relation that exists between these
colours and the general environment. Thus, white prevails among arctic
animals; yellow or brown in desert species; while green is only a common
colour in tropical evergreen forests. If we consider these cases
somewhat carefully we shall find, that they afford us excellent
materials for forming a judgment on the various theories that have been
suggested to account for the colours of the animal world.
In the arctic regions there are a number of animals which are wholly
white all the year round, or which only turn white in winter. Among the
former are the polar bear and the American polar hare, the snowy owl and
the Greenland falcon; among the latter the arctic fox, the arctic hare,
the ermine, and the ptarmigan. Those which are permanently white remain
among the snow nearly all the year round, while those which change their
colour inhabit regions which are free from snow in summer. The obvious
explanation of this style of coloration is, that it is protective,
serving to conceal the herbivorous species from their enemies, and
enabling carnivorous animals to approach their prey unperceived. Two
other explanations have, however, been suggested. One is, that the
prevalent white of the arctic regions has a direct effect in producing
the white colour in animals, either by some photographic or chemical
action on the skin or by a reflex action through vision. The other is,
that the white colour is chiefly beneficial as a means of checking
radiation and so preserving animal heat during the severity of an arctic
winter. The first is part of the general theory that colour is the
effect of coloured light on the objects--a pure hypothesis which has, I
believe, no facts whatever to support it. The second suggestion is also
an hypothesis merely, since it has not been proved by experiment that a
white colour, _per se_, independently of the fur or feathers which is so
coloured, has any effect whatever in checking the radiation of low-grade
heat like that of the animal body. But both alike are sufficiently
disproved by the interesting exceptions to the rule of white coloration
in the arctic regions, which exceptions are, nevertheless, quite in
harmony with the theory of protection.
Whenever we find arctic animals which, from whatever cause, do not
require protection by the white colour, then neither the cold nor the
snow-glare has any effect upon their coloration. The sable retains its
rich brown fur throughout the Siberian winter; but it frequents trees at
that season and not only feeds partially on fruits or seeds, but is able
to catch birds among the branches of the fir-trees, with the bark of
which its colour assimilates. Then we have that thoroughly arctic
animal, the musk-sheep, which is brown and conspicuous; but this animal
is gregarious, and its safety depends on its association in small herds.
It is, therefore, of more importance for it to be able to recognise its
kind at a distance than to be concealed from its enemies, against which
it can well protect itself so long as it keeps together in a compact
body. But the most striking example is that of the common raven, which
is a true arctic bird, and is found even in mid-winter as far north as
any known bird or mammal. Yet it always retains its black coat, and the
reason, from our point of view, is obvious. The raven is a powerful bird
and fears no enemy, while, being a carrion-feeder, it has no need for
concealment in order to approach its prey. The colour of the raven and
of the musk-sheep are, therefore, both inconsistent with any other
theory than that the white colour of arctic animals has been acquired
for concealment, and to that theory both afford a strong support. Here
we have a striking example of the exception proving the rule.
In the desert regions of the earth we find an even more general
accordance of colour with surroundings. The lion, the camel, and all the
desert antelopes have more or less the colour of the sand or rock among
which they live. The Egyptian cat and the Pampas cat are sandy or earth
coloured. The Australian kangaroos are of similar tints, and the
original colour of the wild horse is supposed to have been sandy or clay
coloured. Birds are equally well protected by assimilative hues; the
larks, quails, goatsuckers, and grouse which abound in the North African
and Asiatic deserts are all tinted or mottled so as closely to resemble
the average colour of the soil in the districts they inhabit. Canon
Tristram, who knows these regions and their natural history so well,
says, in an often quoted passage: "In the desert, where neither trees,
brushwood, nor even undulations of the surface afford the slightest
protection to its foes, a modification of colour which shall be
assimilated to that of the surrounding country is absolutely necessary.
Hence, without exception, the upper plumage of every bird, whether lark,
chat, sylvain, or sand-grouse, and also the fur of all the smaller
mammals, and the skin of all the snakes and lizards, is of one uniform
isabelline or sand colour."
Passing on to the tropical regions, it is among their evergreen forests
alone that we find whole groups of birds whose ground colour is green.
Parrots are very generally green, and in the East we have an extensive
group of green fruit-eating pigeons; while the barbets, bee-eaters,
turacos, leaf-thrushes (Phyllornis), white-eyes (Zosterops), and many
other groups, have so much green in their plumage as to tend greatly to
their concealment among the dense foliage. There can be no doubt that
these colours have been acquired as a protection, when we see that in
all the temperate regions, where the leaves are deciduous, the ground
colour of the great majority of birds, especially on the upper surface,
is a rusty brown of various shades, well corresponding with the bark,
withered leaves, ferns, and bare thickets among which they live in
autumn and winter, and especially in early spring when so many of them
build their nests.
Nocturnal animals supply another illustration of the same rule, in the
dusky colours of mice, rats, bats, and moles, and in the soft mottled
plumage of owls and goatsuckers which, while almost equally
inconspicuous in the twilight, are such as to favour their concealment
in the daytime.
An additional illustration of general assimilation of colour to the
surroundings of animals, is furnished by the inhabitants of the deep
oceans. Professor Moseley of the Challenger Expedition, in his British
Association lecture on this subject, says: "Most characteristic of
pelagic animals is the almost crystalline transparency of their bodies.
So perfect is this transparency that very many of them are rendered
almost entirely invisible when floating in the water, while some, even
when caught and held up in a glass globe, are hardly to be seen. The
skin, nerves, muscles, and other organs are absolutely hyaline and
transparent, but the liver and digestive tract often remain opaque and
of a yellow or brown colour, and exactly resemble when seen in the water
small pieces of floating seaweed." Such marine organisms, however, as
are of larger size, and either occasionally or habitually float on the
surface, are beautifully tinged with blue above, thus harmonising with
the colour of the sea as seen by hovering birds; while they are white
below, and are thus invisible against the wave-foam and clouds as seen
by enemies beneath the surface. Such are the tints of the beautiful
nudibranchiate mollusc, Glaucus atlanticus, and many others.
_General Theories of Animal Colour._
We are now in a position to test the general theories, or, to speak more
correctly, the popular notions, as to the origin of animal coloration,
before proceeding to apply the principle of utility to the explanation
of some among the many extraordinary manifestations of colour in the
animal world. The most generally received theory undoubtedly is, that
brilliancy and variety of colour are due to the direct action of light
and heat; a theory no doubt derived from the abundance of
bright-coloured birds, insects, and flowers which are brought from
tropical regions. There are, however, two strong arguments against this
theory. We have already seen how generally bright coloration is wanting
in desert animals, yet here heat and light are both at a maximum, and if
these alone were the agents in the production of colour, desert animals
should be the most brilliant. Again, all naturalists who have lived in
tropical regions know that the proportion of bright to dull coloured
species is little if any greater there than in the temperate zone, while
there are many tropical groups in which bright colours are almost
entirely unknown. No part of the world presents so many brilliant birds
as South America, yet there are extensive families, containing many
hundreds of species, which are as plainly coloured as our average
temperate birds. Such are the families of the bush-shrikes and
ant-thrushes (Formicariidae), the tyrant-shrikes (Tyrannidae), the
American creepers (Dendrocolaptidae), together with a large proportion
of the wood-warblers (Mniotiltidae), the finches, the wrens, and some
other groups. In the eastern hemisphere, also, we have the
babbling-thrushes (Timaliidae), the cuckoo-shrikes (Campephagidae), the
honey-suckers (Meliphagidae), and several other smaller groups which are
certainly not coloured above the average standard of temperate birds.
Again, there are many families of birds which spread over the whole
world, temperate and tropical, and among these the tropical species
rarely present any exceptional brilliancy of colour. Such are the
thrushes, goatsuckers, hawks, plovers, and ducks; and in the last-named
group it is the temperate and arctic zones that afford the most
brilliant coloration.
The same general facts are found to prevail among insects. Although
tropical insects present some of the most gorgeous coloration in the
whole realm of nature, yet there are thousands and tens of thousands of
species which are as dull coloured as any in our cloudy land. The
extensive family of the carnivorous ground-beetles (Carabidae) attains
its greatest brilliancy in the temperate zone; while by far the larger
proportion of the great families of the longicorns and the weevils, are
of obscure colours even in the tropics. In butterflies, there is
undoubtedly a larger proportion of brilliant colour in the tropics; but
if we compare families which are almost equally developed over the
globe--as the Pieridae or whites and yellows, and the Satyridae or
ringlets--we shall find no great disproportion in colour between those
of temperate and tropical regions.
The various facts which have now briefly been noticed are sufficient to
indicate that the light and heat of the sun are not the direct causes of
the colours of animals, although they may favour the production of
colour when, as in tropical regions, the persistent high temperature
favours the development of the maximum of life. We will now consider the
next suggestion, that light reflected from surrounding coloured objects
tends to produce corresponding colours in the animal world.
This theory is founded on a number of very curious facts which prove,
that such a change does sometimes occur and is directly dependent on the
colours of surrounding objects; but these facts are comparatively rare
and exceptional in their nature, and the same theory will certainly not
apply to the infinitely varied colours of the higher animals, many of
which are exposed to a constantly varying amount of light and colour
during their active existence. A brief sketch of these dependent changes
of colour may, however, be advantageously given here.
_Variable Protective Colouring._
There are two distinct kinds of change of colour in animals due to the
colouring of the environment. In one case the change is caused by reflex
action set up by the animal _seeing_ the colour to be imitated, and the
change produced can be altered or repeated as the animal changes its
position. In the other case the change occurs but once, and is probably
not due to any conscious or sense action, but to some direct influence
on the surface tissues while the creature is undergoing a moult or
change to the pupa form.
The most striking example of the first class is that of the chameleon,
which changes to white, brown, yellowish, or green, according to the
colour of the object on which it rests. This change is brought about by
means of two layers of pigment cells, deeply seated in the skin, and of
bluish and yellowish colours. By suitable muscles these cells can be
forced upwards so as to modify the colour of the skin, which, when they
are not brought into action, is a dirty white. These animals are
excessively sluggish and defenceless, and the power of changing their
colour to that of their immediate surroundings is no doubt of great
service to them. Many of the flatfish are also capable of changing their
colour according to the colour of the bottom they rest on; and frogs
have a similar power to a limited extent. Some crustacea also change
colour, and the power is much developed in the Chameleon shrimp (Mysis
Chamaeleon) which is gray when on sand, but brown or green when among
brown or green seaweed. It has been proved by experiment that when this
animal is blinded the change does not occur. In all these cases,
therefore, we have some form of reflex or sense action by which the
change is produced, probably by means of pigment cells beneath the skin
as in the chameleon.
The second class consists of certain larvae, and pupae, which undergo
changes of colour when exposed to differently coloured surroundings.
This subject has been carefully investigated by Mr. E.B. Poulton, who
has communicated the results of his experiments to the Royal
Society.[65] It had been noticed that some species of larvae which fed
on several different plants had colours more or less corresponding to
the particular plant the individual fed on. Numerous cases are given in
Professor Meldola's article on "Variable Protective Colouring" (_Proc.
Zool. Soc._, 1873, p. 153), and while the general green coloration was
attributed to the presence of chlorophyll beneath the skin, the
particular change in correspondence to each food-plant was attributed to
a special function which had been developed by natural selection. Later
on, in a note to his translation of Weissmann's _Theory of Descent_,
Professor Meldola seemed disposed to think that the variations of colour
of some of the species might be phytophagic--that is, due to the direct
action of the differently coloured leaves on which the insect fed. Mr.
Poulton's experiments have thrown much light on this question, since he
has conclusively proved that, in the case of the sphinx caterpillar of
Smerinthus ocellatus, the change of colour is not due to the food but to
the coloured light reflected from the leaves.
This was shown by feeding two sets of larvae on the same plant but
exposed to differently coloured surroundings, obtained by sewing the
leaves together, so that in one case only the dark upper surface, in the
other the whitish under surface was exposed to view. The result in each
case was a corresponding change of colour in the larvae, confirming the
experiments on different individuals of the same batch of larvae which
had been supplied with different food-plants or exposed to a different
coloured light.
An even more interesting series of experiments was made on the colours
of pupae, which in many cases were known to be affected by the material
on which they underwent their transformations. The late Mr. T.W. Wood
proved, in 1867, that the pupae of the common cabbage butterflies
(Pieris brassicae and P. rapae) were either light, or dark, or green,
according to the coloured boxes they were kept in, or the colours of the
fences, walls, etc., against which they were suspended. Mrs. Barber in
South Africa found that the pupae of Papilio Nireus underwent a similar
change, being deep green when attached to orange leaves of the same
tint, pale yellowish-green when on a branch of the bottle-brush tree
whose half-dried leaves were of this colour, and yellowish when attached
to the wooden frame of a box. A few other observers noted similar
phenomena, but nothing more was done till Mr. Poulton's elaborate series
of experiments with the larvae of several of our common butterflies were
the means of clearing up several important points. He showed that the
action of the coloured light did not affect the pupa itself but the
larva, and that only for a limited period of time. After a caterpillar
has done feeding it wanders about seeking a suitable place to undergo
its transformation. When this is found it rests quietly for a day or
two, spinning the web from which it is to suspend itself; and it is
during this period of quiescence, and perhaps also the first hour or two
after its suspension, that the action of the surrounding coloured
surfaces determines, to a considerable extent, the colour of the pupa.
By the application of various surrounding colours during this period,
Mr. Poulton was able to modify the colour of the pupa of the common
tortoise-shell butterfly from nearly black to pale, or to a brilliant
golden; and that of Pieris rapae from dusky through pinkish to pale
green. It is interesting to note, that the colours produced were in all
cases such only as assimilated with the surroundings usually occupied by
the species, and also, that colours which did not occur in such
surroundings, as dark red or blue, only produced the same effects as
dusky or black.
Careful experiments were made to ascertain whether the effect was
produced through the sight of the caterpillar. The ocelli were covered
with black varnish, but neither this, nor cutting off the spines of the
tortoise-shell larva to ascertain whether they might be sense-organs,
produced any effect on the resulting colour. Mr. Poulton concludes,
therefore, that the colour-action probably occurs over the whole surface
of the body, setting up physiological processes which result in the
corresponding colour-change of the pupa. Such changes are, however, by
no means universal, or even common, in protectively coloured pupae,
since in Papilio machaon and some others which have been experimented
on, both in this country and abroad, no change can be produced on the
pupa by any amount of exposure to differently coloured surroundings. It
is a curious point that, with the small tortoise-shell larva, exposure
to light from gilded surfaces produced pupae with a brilliant golden
lustre; and the explanation is supposed to be that mica abounded in the
original habitat of the species, and that the pupae thus obtained
protection when suspended against micaceous rock. Looking, however, at
the wide range of the species and the comparatively limited area in
which micaceous rocks occur, this seems a rather improbable explanation,
and the occurrence of this metallic appearance is still a difficulty. It
does not, however, commonly occur in this country in a natural state.
The two classes of variable colouring here discussed are evidently
exceptional, and can have little if any relation to the colours of those
more active creatures which are continually changing their position with
regard to surrounding objects, and whose colours and markings are nearly
constant throughout the life of the individual, and (with the exception
of sexual differences) in all the individuals of the species. We will
now briefly pass in review the various characteristics and uses of the
colours which more generally prevail in nature; and having already
discussed those protective colours which serve to harmonise animals with
their general environment, we have to consider only those cases in which
the colour resemblance is more local or special in its character.
_Special or Local Colour Adaptations._
This form of colour adaptation is generally manifested by markings
rather than by colour alone, and is extremely prevalent both among
insects and vertebrates, so that we shall be able to notice only a few
illustrative cases. Among our native birds we have the snipe and
woodcock, whose markings and tints strikingly accord with the dead marsh
vegetation among which they live; the ptarmigan in its summer dress is
mottled and tinted exactly like the lichens which cover the stones of
the higher mountains; while young unfledged plovers are spotted so as
exactly to resemble the beach pebbles among which they crouch for
protection, as beautifully exhibited in one of the cases of British
birds in the Natural History Museum at South Kensington.
In mammalia, we notice the frequency of rounded spots on forest or tree
haunting animals of large size, as the forest deer and the forest cats;
while those that frequent reedy or grassy places are striped vertically,
as the marsh antelopes and the tiger. I had long been of opinion that
the brilliant yellow and black stripes of the tiger were adaptive, but
have only recently obtained proof that it is so. An experienced
tiger-hunter, Major Walford, states in a letter, that the haunts of the
tiger are invariably full of the long grass, dry and pale yellow for at
least nine months of the year, which covers the ground wherever there is
water in the rainy season, and he adds: "I once, while following up a
wounded tiger, failed for at least a minute to see him under a tree in
grass at a distance of about twenty yards--jungle open--but the natives
saw him, and I eventually made him out well enough to shoot him, but
even then I could not see at what part of him I was aiming. There can be
no doubt whatever that the colour of both the tiger and the panther
renders them almost invisible, especially in a strong blaze of light,
when among grass, and one does not seem to notice stripes or spots till
they are dead." It is the black shadows of the vegetation that
assimilate with the black stripes of the tiger; and, in like manner,
the spotty shadows of leaves in the forest so harmonise with the spots
of ocelots, jaguars, tiger-cats, and spotted deer as to afford them a
very perfect concealment.
In some cases the concealment is effected by colours and markings which
are so striking and peculiar that no one who had not seen the creature
in its native haunts would imagine them to be protective. An example of
this is afforded by the banded fruit pigeon of Timor, whose pure white
head and neck, black wings and back, yellow belly, and deeply-curved
black band across the breast, render it a very handsome and conspicuous
bird. Yet this is what Mr. H.O. Forbes says of it: "On the trees the
white-headed fruit pigeon (Ptilopus cinctus) sate motionless during the
heat of the day in numbers, on well-exposed branches; but it was with
the utmost difficulty that I or my sharp-eyed native servant could ever
detect them, even in trees where we knew they were sitting."[66] The
trees referred to are species of Eucalyptus which abound in Timor. They
have whitish or yellowish bark and very open foliage, and it is the
intense sunlight casting black curved shadows of one branch upon
another, with the white and yellow bark and deep blue sky seen through
openings of the foliage, that produces the peculiar combination of
colours and shadows to which the colours and markings of this bird have
become so closely assimilated.
Even such brilliant and gorgeously coloured birds as the sun-birds of
Africa are, according to an excellent observer, often protectively
coloured. Mrs. M.E. Barber remarks that "A casual observer would
scarcely imagine that the highly varnished and magnificently coloured
plumage of the various species of Noctarinea could be of service to
them, yet this is undoubtedly the case. The most unguarded moments of
the lives of these birds are those that are spent amongst the flowers,
and it is then that they are less wary than at any other time. The
different species of aloes, which blossom in succession, form the
principal sources of their winter supplies of food; and a legion of
other gay flowering plants in spring and summer, the aloe blossoms
especially, are all brilliantly coloured, and they harmonise admirably
with the gay plumage of the different species of sun-birds. Even the
keen eye of a hawk will fail to detect them, so closely do they resemble
the flowers they frequent. The sun-birds are fully aware of this fact,
for no sooner have they relinquished the flowers than they become
exceedingly wary and rapid in flight, darting arrow-like through the air
and seldom remaining in exposed situations. The black sun-bird
(Nectarinea amethystina) is never absent from that magnificent
forest-tree, the 'Kaffir Boom' (Erythrina caffra); all day long the
cheerful notes of these birds may be heard amongst its spreading
branches, yet the general aspect of the tree, which consists of a huge
mass of scarlet and purple-black blossoms without a single green leaf,
blends and harmonises with the colours of the black sun-bird to such an
extent that a dozen of them may be feeding amongst its blossoms without
being conspicuous, or even visible."[67]
Some other cases will still further illustrate how the colours of even
very conspicuous animals may be adapted to their peculiar haunts.
The late Mr. Swinhoe says of the Kerivoula picta, which he observed in
Formosa: "The body of this bat was of an orange colour, but the wings
were painted with orange-yellow and black. It was caught suspended, head
downwards, on a cluster of the fruit of the longan tree (Nephelium
longanum). Now this tree is an evergreen, and all the year round some
portion of its foliage is undergoing decay, the particular leaves being,
in such a stage, partially orange and black. This bat can, therefore, at
all seasons suspend from its branches and elude its enemies by its
resemblance to the leaves of the tree."[68]
Even more curious is the case of the sloths--defenceless animals which
feed upon leaves, and hang from the branches of trees with their back
downwards. Most of the species have a curious buff-coloured spot on the
back, rounded or oval in shape and often with a darker border, which
seems placed there on purpose to make them conspicuous; and this was a
great puzzle to naturalists, because the long coarse gray or greenish
hair was evidently like tree-moss and therefore protective. But an old
writer, Baron von Slack, in his _Voyage_ _to Surinam_ (1810), had
already explained the matter. He says: "The colour and even the shape of
the hair are much like withered moss, and serve to hide the animal in
the trees, but particularly when it has that orange-coloured spot
between the shoulders and lies close to the tree; it looks then exactly
like a piece of branch where the rest has been broken off, by which the
hunters are often deceived." Even such a huge animal as the giraffe is
said to be perfectly concealed by its colour and form when standing
among the dead and broken trees that so often occur on the outskirts of
the thickets where it feeds. The large blotch-like spots on the skin and
the strange shape of the head and horns, like broken branches, so tend
to its concealment that even the keen-eyed natives have been known to
mistake trees for giraffes or giraffes for trees.
Innumerable examples of this kind of protective colouring occur among
insects; beetles mottled like the bark of trees or resembling the sand
or rock or moss on which they live, with green caterpillars of the exact
general tints of the foliage they feed on; but there are also many cases
of detailed imitation of particular objects by insects that must be
briefly described.[69]
_Protective Imitation of Particular Objects._
The insects which present this kind of imitation most perfectly are the
Phasmidae, or stick and leaf insects. The well-known leaf-insects of
Ceylon and of Java, species of Phyllium, are so wonderfully coloured and
veined, with leafy expansions on the legs and thorax, that not one
person in ten can see them when resting on the food-plant close beneath
their eyes. Others resemble pieces of stick with all the minutiae of
knots and branches, formed by the insects' legs, which are stuck out
rigidly and unsymmetrically. I have often been unable to distinguish
between one of these insects and a real piece of stick, till I satisfied
myself by touching it and found it to be alive. One species, which was
brought me in Borneo, was covered with delicate semitransparent green
foliations, exactly resembling the hepaticae which cover pieces of
rotten stick in the damp forests. Others resemble dead leaves in all
their varieties of colour and form; and to show how perfect is the
protection obtained and how important it is to the possessors of it, the
following incident, observed by Mr. Belt in Nicaragua, is most
instructive. Describing the armies of foraging ants in the forest which
devour every insect they can catch, he says: "I was much surprised with
the behaviour of a green leaf-like locust. This insect stood immovably
among a host of ants, many of which ran over its legs without ever
discovering there was food within their reach. So fixed was its
instinctive knowledge that its safety depended on its immovability, that
it allowed me to pick it up and replace it among the ants without making
a single effort to escape. This species closely resembles a green
leaf."[70]
Caterpillars also exhibit a considerable amount of detailed resemblance
to the plants on which they live. Grass-feeders are striped
longitudinally, while those on ordinary leaves are always striped
obliquely. Some very beautiful protective resemblances are shown among
the caterpillars figured in Smith and Abbott's _Lepidopterous Insects of
Georgia_, a work published in the early part of the century, before any
theories of protection were started. The plates in this work are most
beautifully executed from drawings made by Mr. Abbott, representing the
insects, in every case, on the plants which they frequented, and no
reference is made in the descriptions to the remarkable protective
details which appear upon the plates. We have, first, the larva of
Sphinx fuciformis feeding on a plant with linear grass-like leaves and
small blue flowers; and we find the insect of the same green as the
leaves, striped longitudinally in accordance with the linear leaves, and
with the head blue corresponding both in size and colour with the
flowers. Another species (Sphinx tersa) is represented feeding on a
plant with small red flowers situated in the axils of the leaves; and
the larva has a row of seven red spots, unequal in size, and
corresponding very closely with the colour and size of the flowers. Two
other figures of sphinx larvae are very curious. That of Sphinx
pampinatrix feeds on a wild vine (Vitis indivisa), having green
tendrils, and in this species the curved horn on the tail is green, and
closely imitates in its curve the tip of the tendril. But in another
species (Sphinx cranta), which feeds on the fox-grape (Vitis vulpina),
the horn is very long and red, corresponding with the long red-tipped
tendrils of the plant. Both these larvae are green with oblique stripes,
to harmonise with the veined leaves of the vines; but a figure is also
given of the last-named species after it has done feeding, when it is of
a decided brown colour and has entirely lost its horn. This is because
it then descends to the ground to bury itself, and the green colour and
red horn would be conspicuous and dangerous; it therefore loses both at
the last moult. Such a change of colour occurs in many species of
caterpillars. Sometimes the change is seasonal; and, in those which
hibernate with us, the colour of some species, which is brownish in
autumn in adaptation to the fading foliage, becomes green in spring to
harmonise with the newly-opened leaves at that season.[71]
Some of the most curious examples of minute imitation are afforded by
the caterpillars of the geometer moths, which are always brown or
reddish, and resemble in form little twigs of the plant on which they
feed. They have the habit, when at rest, of standing out obliquely from
the branch, to which they hold on by their hind pair of prolegs or
claspers, and remain motionless for hours. Speaking of these protective
resemblances Mr. Jenner Weir says: "After being thirty years an
entomologist I was deceived myself, and took out my pruning scissors to
cut from a plum tree a spur which I thought I had overlooked. This
turned out to be the larva of a geometer two inches long. I showed it
to several members of my family, and defined a space of four inches in
which it was to be seen, but none of them could perceive that it was a
caterpillar."[72]
One more example of a protected caterpillar must be given. Mr. A.
Everett, writing from Sarawak, Borneo, says: "I had a caterpillar
brought me, which, being mixed by my boy with some other things, I took
to be a bit of moss with two exquisite pinky-white seed-capsules; but I
soon saw that it moved, and examining it more closely found out its real
character: it is covered with hair, with two little pink spots on the
upper surface, the general hue being more green. Its motions are very
slow, and when eating the head is withdrawn beneath a fleshy mobile
hood, so that the action of feeding does not produce any movement
externally. It was found in the limestone hills at Busan, the situation
of all others where mosses are most plentiful and delicate, and where
they partially clothe most of the protruding masses of rock."
_How these Imitations have been Produced._
To many persons it will seem impossible that such beautiful and detailed
resemblances as those now described--and these are only samples of
thousands that occur in all parts of the world--can have been brought
about by the preservation of accidental useful variations. But this will
not seem so surprising if we keep in mind the facts set forth in our
earlier chapters--the rapid multiplication, the severe struggle for
existence, and the constant variability of these and all other
organisms. And, further, we must remember that these delicate
adjustments are the result of a process which has been going on for
millions of years, and that we now see the small percentage of successes
among the myriads of failures. From the very first appearance of insects
and their various kinds of enemies the need of protection arose, and was
usually most easily met by modifications of colour. Hence, we may be
sure that the earliest leaf-eating insects acquired a green colour as
one of the necessities of their existence; and, as the species became
modified and specialised, those feeding on particular species of plants
would rapidly acquire the peculiar tints and markings best adapted to
conceal them upon those plants. Then, every little variation that, once
in a hundred years perhaps, led to the preservation of some larva which
was thereby rather better concealed than its fellows, would form the
starting-point of a further development, leading ultimately to that
perfection of imitation in details which now astonishes us. The
researches of Dr. Weismann illustrate this progressive adaptation. The
very young larvae of several species are green or yellowish without any
markings; they then, in subsequent moults, obtain certain markings, some
of which are often lost again before the larva is fully grown. The early
stages of those species which, like elephant hawk-moths (Chaerocampa),
have the anterior segments elongated and retractile, with large eye-like
spots to imitate the head of a vertebrate, are at first like those of
non-retractile species, the anterior segments being as large as the
rest. After the first moult they become smaller, comparatively; but it
is only after the second moult that the ocelli begin to appear, and
these are not fully defined till after the third moult. This progressive
development of the individual--the ontogeny--gives us a clue to the
ancestral development of the whole race--the phylogeny; and we are
enabled to picture to ourselves the very slow and gradual steps by which
the existing perfect adaptation has been brought about. In many larvae
great variability still exists, and in some there are two or more
distinctly-coloured forms--usually a dark and a light or a brown and a
green form. The larva of the humming-bird hawk-moth (Macroglossa
stellatarum) varies in this manner, and Dr. Weismann raised five
varieties from a batch of eggs from one moth. It feeds on species of
bedstraw (Galium verum and G. mollugo), and as the green forms are less
abundant than the brown, it has probably undergone some recent change of
food-plant or of habits which renders brown the more protective colour.
_Special Protective Colouring of Butterflies._
We will now consider a few cases of special protective colouring in the
perfect butterfly or moth. Mr. Mansel Weale states that in South Africa
there is a great prevalence of white and silvery foliage or bark,
sometimes of dazzling brilliancy, and that many insects and their larvae
have brilliant silvery tints which are protective, among them being
three species of butterflies whose undersides are silvery, and which are
thus effectually protected when at rest.[73] A common African butterfly
(Aterica meleagris) always settles on the ground with closed wings,
which so closely resemble the soil of the district that it can with
difficulty be seen, and the colour varies with the soil in different
localities. Thus specimens from Senegambia were dull brown, the soil
being reddish sand and iron-clay; those from Calabar and Cameroons were
light brown with numerous small white spots, the soil of those countries
being light brown clay with small quartz pebbles; while in other
localities where the colours of the soil were more varied the colours of
the butterfly varied also. Here we have variation in a single species
which has become specialised in certain areas to harmonise with the
colour of the soil.[74]
Many butterflies, in all parts of the world, resemble dead leaves on
their under side, but those in which this form of protection is carried
to the greatest perfection are the species of the Eastern genus Kallima.
In India K. inachis, and in the larger Malay islands K. paralekta, are
very common. They are rather large and showy butterflies, orange and
bluish on the upper side, with a very rapid flight, and frequenting dry
forests. Their habit is to settle always where there is some dead or
decaying foliage, and the shape and colour of the wings (on the under
surface), together with the attitude of the insect, is such as to
produce an absolutely perfect imitation of a dead leaf. This is effected
by the butterfly always settling on a twig, with the short tail of the
hind wings just touching it and forming the leaf-stalk. From this a dark
curved line runs across to the elongated tip of the upper wings,
imitating the midrib, on both sides of which are oblique lines, formed
partly by the nervures and partly by markings, which give the effect of
the usual veining of a leaf. The head and antennae fit exactly between
the closed upper wings so as not to interfere with the outline, which
has just that amount of irregular curvature that is seen in dry and
withered leaves. The colour is very remarkable for its extreme amount of
variability, from deep reddish-brown to olive or pale yellow, hardly two
specimens being exactly alike, but all coming within the range of colour
of leaves in various stages of decay. Still more curious is the fact
that the paler wings, which imitate leaves most decayed, are usually
covered with small black dots, often gathered into circular groups, and
so exactly resembling the minute fungi on decaying leaves that it is
hard at first to believe that the insects themselves are not attacked by
some such fungus. The concealment produced by this wonderful imitation
is most complete, and in Sumatra I have often seen one enter a bush and
then disappear like magic. Once I was so fortunate as to see the exact
spot on which the insect settled; but even then I lost sight of it for
some time, and only after a persistent search discovered that it was
close before my eyes.[75] Here we have a kind of imitation, which is
very common in a less developed form, carried to extreme perfection,
with the result that the species is very abundant over a considerable
area of country.
_Protective Resemblance among Marine Animals._
Among marine animals this form of protection is very common. Professor
Moseley tells us that all the inhabitants of the Gulf-weed are most
remarkably coloured, for purposes of protection and concealment, exactly
like the weed itself. "The shrimps and crabs which swarm in the weed are
of exactly the same shade of yellow as the weed, and have white markings
upon their bodies to represent the patches of Membranipora. The small
fish, Antennarius, is in the same way weed-colour with white spots. Even
a Planarian worm, which lives in the weed, is similarly yellow-coloured,
and also a mollusc, Scyllaea pelagica." The same writer tells us that "a
number of little crabs found clinging to the floats of the blue-shelled
mollusc, Ianthina, were all coloured of a corresponding blue for
concealment."[76]
Professor E.S. Morse of Salem, Mass., found that most of the New
England marine mollusca were protectively coloured; instancing among
others a little red chiton on rocks clothed with red calcareous algae,
and Crepidula plana, living within the apertures of the shells of larger
species of Gasteropods and of a pure white colour corresponding to its
habitat, while allied species living on seaweed or on the outside of
dark shells were dark brown.[77] A still more interesting case has been
recorded by Mr. George Brady. He says: "Amongst the Nullipore which
matted together the laminaria roots in the Firth of Clyde were living
numerous small starfishes (Ophiocoma bellis) which, except when their
writhing movements betrayed them, were quite undistinguishable from the
calcareous branches of the alga; their rigid angularly twisted rays had
all the appearance of the coralline, and exactly assimilated to its dark
purple colour, so that though I held in my hand a root in which were
half a dozen of the starfishes, I was really unable to detect them until
revealed by their movements."[78]
These few examples are sufficient to show that the principle of
protective coloration extends to the ocean as well as over the earth;
and if we consider how completely ignorant we are of the habits and
surroundings of most marine animals, it may well happen that many of the
colours of tropical fishes, which seem to us so strange and so
conspicuous, are really protective, owing to the number of equally
strange and brilliant forms of corals, sea-anemones, sponges, and
seaweeds among which they live.
_Protection by Terrifying Enemies._
A considerable number of quite defenceless insects obtain protection
from some of their enemies by having acquired a resemblance to dangerous
animals, or by some threatening or unusual appearance. This is obtained
either by a modification of shape, of habits, of colour, or of all
combined. The simplest form of this protection is the aggressive
attitude of the caterpillars of the Sphingidae, the forepart of the body
being erected so as to produce a rude resemblance to the figure of a
sphinx, hence the name of the family. The protection is carried further
by those species which retract the first three segments and have large
ocelli on each side of the fourth segment, thus giving to the
caterpillar, when the forepart of its body is elevated, the appearance
of a snake in a threatening attitude.
The blood-red forked tentacle, thrown out of the neck of the larvae of
the genus Papilio when alarmed, is, no doubt, a protection against the
attacks of ichneumons, and may, perhaps, also frighten small birds; and
the habit of turning up the tail possessed by the harmless rove-beetles
(Staphylinidae), giving the idea that they can sting, has, probably, a
similar use. Even an unusual angular form, like a crooked twig or
inorganic substance, may be protective; as Mr. Poulton thinks is the
case with the curious caterpillar of Notodonta ziczac, which, by means
of a few slight protuberances on its body, is able to assume an angular
and very unorganic-looking appearance. But perhaps the most perfect
example of this kind of protection is exhibited by the large caterpillar
of the Royal Persimmon moth (Bombyx regia), a native of the southern
states of North America, and known there as the "Hickory-horned devil."
It is a large green caterpillar, often six inches long, ornamented with
an immense crown of orange-red tubercles, which, if disturbed, it erects
and shakes from side to side in a very alarming manner. In its native
country the negroes believe it to be as deadly as a rattlesnake, whereas
it is perfectly innocuous. The green colour of the body suggests that
its ancestors were once protectively coloured; but, growing too large to
be effectually concealed, it acquired the habit of shaking its head
about in order to frighten away its enemies, and ultimately developed
the crown of tentacles as an addition to its terrifying powers. This
species is beautifully figured in Abbott and Smith's _Lepidopterous
Insects of Georgia_.
_Alluring Coloration._
Besides those numerous insects which obtain protection through their
resemblance to the natural objects among which they live, there are some
whose disguise is not used for concealment, but as a direct means of
securing their prey by attracting them within the enemy's reach. Only a
few cases of this kind of coloration have yet been observed, chiefly
among spiders and mantidae; but, no doubt, if attention were given to
the subject in tropical countries, many more would be discovered. Mr.
H.O. Forbes has described a most interesting example of this kind of
simulation in Java. While pursuing a large butterfly through the jungle,
he was stopped by a dense bush, on a leaf of which he observed one of
the skipper butterflies sitting on a bird's dropping. "I had often," he
says, "observed small Blues at rest on similar spots on the ground, and
have wondered what such a refined and beautiful family as the Lycaenidae
could find to enjoy, in food apparently so incongruous for a butterfly.
I approached with gentle steps, but ready net, to see if possible how
the present species was engaged. It permitted me to get quite close, and
even to seize it between my fingers; to my surprise, however, part of
the body remained behind, adhering as I thought to the excreta. I looked
closely, and finally touched with my finger the excreta to find if it
were glutinous. To my delighted astonishment I found that my eyes had
been most perfectly deceived, and that what seemed to be the excreta was
a most artfully coloured spider, lying on its back with its feet crossed
over and closely adpressed to the body." Mr. Forbes then goes on to
describe the exact appearance of such excreta, and how the various parts
of the spider are coloured to produce the imitation, even to the liquid
portion which usually runs a little down the leaf. This is exactly
imitated by a portion of the thin web which the spider first spins to
secure himself firmly to the leaf; thus producing, as Mr. Forbes
remarks, a living bait for butterflies and other insects so artfully
contrived as to deceive a pair of human eyes, even when intently
examining it.[79]
A native species of spider (Thomisus citreus) exhibits a somewhat
similar alluring protection by its close resemblance to buds of the
wayfaring tree, Viburnum lantana. It is pure creamy-white, the abdomen
exactly resembling in shape and colour the unopened buds of the flowers
among which it takes its station; and it has been seen to capture flies
which came to the flowers.
But the most curious and beautiful case of alluring protection is that
of a wingless Mantis in India, which is so formed and coloured as to
resemble a pink orchis or some other fantastic flower. The whole insect
is of a bright pink colour, the large and oval abdomen looking like the
labellum of an orchid. On each side, the two posterior legs have
immensely dilated and flattened thighs which represent the petals of a
flower, while the neck and forelegs imitate the upper sepal and column
of an orchid. The insect rests motionless, in this symmetrical attitude,
among bright green foliage, being of course very conspicuous, but so
exactly resembling a flower that butterflies and other insects settle
upon it and are instantly captured. It is a living trap, baited in the
most alluring manner to catch the unwary flower-haunting insects.[80]
_The Coloration of Birds' Eggs._
The colours of birds' eggs have long been a difficulty on the theory of
adaptive coloration, because, in so many cases it has not been easy to
see what can be the use of the particular colours, which are often so
bright and conspicuous that they seem intended to attract attention
rather than to be concealed. A more careful consideration of the subject
in all its bearings shows, however, that here too, in a great number of
cases, we have examples of protective coloration. When, therefore, we
cannot see the meaning of the colour, we may suppose that it has been
protective in some ancestral form, and, not being hurtful, has persisted
under changed conditions which rendered the protection needless.
We may divide all eggs, for our present purpose, into two great
divisions; those which are white or nearly so, and those which are
distinctly coloured or spotted. Egg-shells being composed mainly of
carbonate of lime, we may assume that the primitive colour of birds'
eggs was white, a colour that prevails now among the other egg-bearing
vertebrates--lizards, crocodiles, turtles, and snakes; and we might,
therefore, expect that this colour would continue where its presence had
no disadvantages. Now, as a matter of fact, we find that in all the
groups of birds which lay their eggs in concealed places, whether in
holes of trees or in the ground, or in domed or covered nests, the eggs
are either pure white or of very pale uniform coloration. Such is the
case with kingfishers, bee-eaters, penguins, and puffins, which nest in
holes in the ground; with the great parrot family, the woodpeckers, the
rollers, hoopoes, trogons, owls, and some others, which build in holes
in trees or other concealed places; while martins, wrens,
willow-warblers, and Australian finches, build domed or covered nests,
and usually have white eggs.
There are, however, many other birds which lay their white eggs in open
nests; and these afford some very interesting examples of the varied
modes by which concealment may be obtained. All the duck tribe, the
grebes, and the pheasants belong to this class; but these birds all have
the habit of covering their eggs with dead leaves or other material
whenever they leave the nest, so as effectually to conceal them. Other
birds, as the short-eared owl, the goatsucker, the partridge, and some
of the Australian ground pigeons, lay their white or pale eggs on the
bare soil; but in these cases the birds themselves are protectively
coloured, so that, when sitting, they are almost invisible; and they
have the habit of sitting close and almost continuously, thus
effectually concealing their eggs.
Pigeons and doves offer a very curious case of the protection of exposed
eggs. They usually build very slight and loose nests of sticks and
twigs, so open that light can be seen through them from below, while
they are generally well concealed by foliage above. Their eggs are white
and shining; yet it is a difficult matter to discover, from beneath,
whether there are eggs in the nest or not, while they are well hidden by
the thick foliage above. The Australian podargihuge goatsuckers--build
very similar nests, and their white eggs are protected in the same
manner. Some large and powerful birds, as the swans, herons, pelicans,
cormorants, and storks, lay white eggs in open nests; but they keep
careful watch over them, and are able to drive away intruders. On the
whole, then, we see that, while white eggs are conspicuous, and
therefore especially liable to attack by egg-eating animals, they are
concealed from observation in many and various ways. We may, therefore,
assume that, in cases where there seems to be no such concealment, we
are too ignorant of the whole of the conditions to form a correct
judgment.
We now come to the large class of coloured or richly spotted eggs, and
here we have a more difficult task, though many of them decidedly
exhibit protective tints or markings. There are two birds which nest on
sandy shores--the lesser tern and the ringed plover,--and both lay
sand-coloured eggs, the former spotted so as to harmonise with coarse
shingle, the latter minutely speckled like fine sand, which are the
kinds of ground the two birds choose respectively for their nests. "The
common sandpipers' eggs assimilate so closely with the tints around them
as to make their discovery a matter of no small difficulty, as every
oologist can testify who has searched for them. The pewits' eggs, dark
in ground colour and boldly marked, are in strict harmony with the sober
tints of moor and fallow, and on this circumstance alone their
concealment and safety depend. The divers' eggs furnish another example
of protective colour; they are generally laid close to the water's edge,
amongst drift and shingle, where their dark tints and black spots
conceal them by harmonising closely with surrounding objects. The snipes
and the great army of sandpipers furnish innumerable instances of
protectively coloured eggs. In all the instances given the sitting-bird
invariably leaves the eggs uncovered when it quits them, and
consequently their safety depends solely on the colours which adorn
them."[81] The wonderful range of colour and marking in the eggs of the
guillemot may be imputed to the inaccessible rocks on which it breeds,
giving it complete protection from enemies. Thus the pale or bluish
ground colour of the eggs of its allies, the auks and puffins, has
become intensified and blotched and spotted in the most marvellous
variety of patterns, owing to there being no selective agency to prevent
individual variation having full sway.
The common black coot (Fulica atra) has eggs which are coloured in a
specially protective manner. Dr. William Marshall writes, that it only
breeds in certain localities where a large water reed (Phragmites
arundinacea) abounds. The eggs of the coot are stained and spotted with
black on a yellowish-gray ground, and the dead leaves of the reed are of
the same colour, and are stained black by small parasitic fungi of the
Uredo family; and these leaves form the bed on which the eggs are laid.
The eggs and the leaves agree so closely in colour and markings that it
is a difficult thing to distinguish the eggs at any distance. It is to
be noted that the coot never covers up its eggs, as its ally the
moor-hen usually does.
The beautiful blue or greenish eggs of the hedge-sparrow, the
song-thrush, and sometimes those of the blackbird, seem at first sight
especially calculated to attract attention, but it is very doubtful
whether they are really so conspicuous when seen at a little distance
among their usual surroundings. For the nests of these birds are either
in evergreens, as holly or ivy, or surrounded by the delicate green
tints of our early spring vegetation, and may thus harmonise very well
with the colours around them. The great majority of the eggs of our
smaller birds are so spotted or streaked with brown or black on
variously tinted grounds that, when lying in the shadow of the nest and
surrounded by the many colours and tints of bark and moss, of purple
buds and tender green or yellow foliage, with all the complex glittering
lights and mottled shades produced among these by the spring sunshine
and by sparkling raindrops, they must have a quite different aspect from
that which they possess when we observe them torn from their natural
surroundings. We have here, probably, a similar case of general
protective harmony to that of the green caterpillars with beautiful
white or purple bands and spots, which, though gaudily conspicuous when
seen alone, become practically invisible among the complex lights and
shadows of the foliage they feed upon.
In the case of the cuckoo, which lays its eggs in the nests of a variety
of other birds, the eggs themselves are subject to considerable
variations of colour, the most common type, however, resembling those of
the pipits, wagtails, or warblers, in whose nests they are most
frequently laid. It also often lays in the nest of the hedge-sparrow,
whose bright blue eggs are usually not at all nearly matched, although
they are sometimes said to be so on the Continent. It is the opinion of
many ornithologists that each female cuckoo lays the same coloured eggs,
and that it usually chooses a nest the owners of which lay somewhat
similar eggs, though this is by no means universally the case. Although
birds which have cuckoos' eggs imposed upon them do not seem to neglect
them on account of any difference of colour, yet they probably do so
occasionally; and if, as seems probable, each bird's eggs are to some
extent protected by their harmony of colour with their surroundings, the
presence of a larger and very differently coloured egg in the nest might
be dangerous, and lead to the destruction of the whole set. Those
cuckoos, therefore, which most frequently placed their eggs among the
kinds which they resembled, would in the long run leave most progeny,
and thus the very frequent accord in colour might have been brought
about.
Some writers have suggested that the varied colours of birds' eggs are
primarily due to the effect of surrounding coloured objects on the
female bird during the period preceding incubation; and have expended
much ingenuity in suggesting the objects that may have caused the eggs
of one bird to be blue, another brown, and another pink.[82] But no
evidence has been presented to prove that any effects whatever are
produced by this cause, while there seems no difficulty in accounting
for the facts by individual variability and the action of natural
selection. The changes that occur in the conditions of existence of
birds must sometimes render the concealment less perfect than it may
once have been; and when any danger arises from this cause, it may be
met either by some change in the colour of the eggs, or in the
structure or position of the nest, or by the increased care which the
parents bestow upon the eggs. In this way the various divergences which
now so often puzzle us may have arisen.
_Colour as a Means of Recognition._
If we consider the habits and life-histories of those animals which are
more or less gregarious, comprising a large proportion of the herbivora,
some carnivora, and a considerable number of all orders of birds, we
shall see that a means of ready recognition of its own kind, at a
distance or during rapid motion, in the dusk of twilight or in partial
cover, must be of the greatest advantage and often lead to the
preservation of life. Animals of this kind will not usually receive a
stranger into their midst. While they keep together they are generally
safe from attack, but a solitary straggler becomes an easy prey to the
enemy; it is, therefore, of the highest importance that, in such a case,
the wanderer should have every facility for discovering its companions
with certainty at any distance within the range of vision.
Some means of easy recognition must be of vital importance to the young
and inexperienced of each flock, and it also enables the sexes to
recognise their kind and thus avoid the evils of infertile crosses; and
I am inclined to believe that its necessity has had a more widespread
influence in determining the diversities of animal coloration than any
other cause whatever. To it may probably be imputed the singular fact
that, whereas bilateral symmetry of coloration is very frequently lost
among domesticated animals, it almost universally prevails in a state of
nature; for if the two sides of an animal were unlike, and the diversity
of coloration among domestic animals occurred in a wild state, easy
recognition would be impossible among numerous closely allied forms.[83]
The wonderful diversity of colour and of marking that prevails,
especially in birds and insects, may be due to the fact that one of the
first needs of a new species would be, to keep separate from its nearest
allies, and this could be most readily done by some easily seen external
mark of difference. A few illustrations will serve to show how this
principle acts in nature.
My attention was first called to the subject by a remark of Mr. Darwin's
that, though, "the hare on her form is a familiar instance of
concealment through colour, yet the principle partly fails in a closely
allied species, the rabbit; for when running to its burrow it is made
conspicuous to the sportsman, and no doubt to all beasts of prey, by its
upturned white tail."[84] But a little consideration of the habits of
the animal will show that the white upturned tail is of the greatest
value, and is really, as it has been termed by a writer in _The Field_,
a "signal flag of danger." For the rabbit is usually a crepuscular
animal, feeding soon after sunset or on moonlight nights. When disturbed
or alarmed it makes for its burrow, and the white upturned tails of
those in front serve as guides and signals to those more remote from
home, to the young and the feeble; and thus each following the one or
two before it, all are able with the least possible delay to regain a
place of comparative safety. The apparent danger, therefore, becomes a
most important means of security.
The same general principle enables us to understand the singular, and
often conspicuous, markings on so many gregarious herbivora which are
yet, on the whole, protectively coloured. Thus, the American prong-buck
has a white patch behind and a black muzzle. The Tartarian antelope, the
Ovis poli of High Asia, the Java wild ox, several species of deer, and a
large number of antelopes have a similar conspicuous white patch behind,
which, in contrast to the dusky body, must enable them to be seen and
followed from a distance by their fellows. Where there are many species
of nearly the same general size and form inhabiting the same region--as
with the antelopes of Africa--we find many distinctive markings of a
similar kind. The gazelles have variously striped and banded faces,
besides white patches behind and on the flanks, as shown in the woodcut.
The spring-bok has a white patch on the face and one on the sides, with
a curiously distinctive white stripe above the tail, which is nearly
concealed when the animal is at rest by a fold of skin but comes into
full view when it is in motion, being thus quite analogous to the
upturned white tail of the rabbit. In the pallah the white rump-mark is
bordered with black, and the peculiar shape of the horns distinguishes
it when seen from the front. The sable-antelope, the gems-bok, the oryx,
the hart-beest, the bonte-bok, and the addax have each peculiar white
markings; and they are besides characterised by horns so remarkably
different in each species and so conspicuous, that it seems probable
that the peculiarities in length, twist, and curvature have been
differentiated for the purpose of recognition, rather than for any
speciality of defence in species whose general habits are so similar.
[Illustration: FIG. 18.--Gazella soemmerringi.]
It is interesting to note that these markings for recognition are very
slightly developed in the antelopes of the woods and marshes. Thus, the
grys-bok is nearly uniform in colour, except the long black-tipped ears;
and it frequents the wooded mountains. The duyker-bok and the rhoode-bok
are wary bush-haunters, and have no marks but the small white patch
behind. The wood-haunting bosch-bok goes in pairs, and has hardly any
distinctive marks on its dusky chestnut coat, but the male alone is
horned. The large and handsome koodoo frequents brushwood, and its
vertical white stripes are no doubt protective, while its magnificent
spiral horns afford easy recognition. The eland, which is an inhabitant
of the open country, is uniformly coloured, being sufficiently
recognisable by its large size and distinctive form; but the Derbyan
eland is a forest animal, and has a protectively striped coat. In like
manner, the fine Speke's antelope, which lives entirely in the swamps
and among reeds, has pale vertical stripes on the sides (protective),
with white markings on face and breast for recognition. An inspection of
the figures of antelopes and other animals in Wood's _Natural History_,
or in other illustrated works, will give a better idea of the
peculiarities of recognition markings than any amount of description.
Other examples of such coloration are to be seen in the dusky tints of
the musk-sheep and the reindeer, to whom recognition at a distance on
the snowy plains is of more importance than concealment from their few
enemies. The conspicuous stripes and bands of the zebra and the quagga
are probably due to the same cause, as may be the singular crests and
face-marks of several of the monkeys and lemurs.[85]
[Illustration: FIG. 19--Recognition marks of three African plovers.]
Among birds, these recognition marks are especially numerous and
suggestive. Species which inhabit open districts are usually
protectively coloured; but they generally possess some distinctive
markings for the purpose of being easily recognised by their kind, both
when at rest and during flight. Such are, the white bands or patches on
the breast or belly of many birds, but more especially the head and neck
markings in the form of white or black caps, collars, eye-marks or
frontal patches, examples of which are seen in the three species of
African plovers figured on page 221.
Recognition marks during flight are very important for all birds which
congregate in flocks or which migrate together; and it is essential
that, while being as conspicuous as possible, the marks shall not
interfere with the general protective tints of the species when at rest.
Hence they usually consist of well-contrasted markings on the wings and
tail, which are concealed during repose but become fully visible when
the bird takes flight. Such markings are well seen in our four British
species of shrikes, each having quite different white marks on the
expanded wings and on the tail feathers; and the same is the case with
our three species of Saxicola--the stone-chat, whin-chat, and
wheat-ear--which are thus easily recognisable on the wing, especially
when seen from above, as they would be by stragglers looking out for
their companions. The figures opposite, of the wings of two African
species of stone-curlew which are sometimes found in the same districts,
well illustrates these specific recognition marks. Though not very
greatly different to our eyes, they are no doubt amply so to the sharp
vision of the birds themselves.
Besides the white patches on the primaries here shown, the secondary
feathers are, in some cases, so coloured as to afford very distinctive
markings during flight, as seen in the central secondary quills of two
African coursers (Fig. 21).
[Illustration: FIG. 20.--Oedicnemus vermiculatus (above). Oe.
senegalensis (below).]
Most characteristic of all, however, are the varied markings of the
outer tail-feathers, whose purpose is so well shown by their being
almost always covered during repose by the two middle feathers, which
are themselves quite unmarked and protectively tinted like the rest of
the upper surface of the body. The figures of the expanded tails of two
species of East Asiatic snipe, whose geographical ranges overlap each
other, will serve to illustrate this difference; which is frequently
much greater and modified in an endless variety of ways (Fig. 22).
Numbers of species of pigeons, hawks, finches, warblers, ducks, and
innumerable other birds possess this class of markings; and they
correspond so exactly in general character with those of the mammalia,
already described, that we cannot doubt they serve a similar
purpose.[86]
[Illustration: FIG. 21.--Secondary quills.]
[Illustration: FIG. 22.--Scolopax megala (upper). S. stenura (lower).]
Those birds which are inhabitants of tropical forests, and which need
recognition marks that shall be at all times visible among the dense
foliage, and not solely or chiefly during flight, have usually small but
brilliant patches of colour on the head or neck, often not interfering
with the generally protective character of their plumage. Such are the
bright patches of blue, red, or yellow, by which the usually green
Eastern barbets are distinguished; and similar bright patches of colour
characterise the separate species of small green fruit-doves. To this
necessity for specialisation in colour, by which each bird may easily
recognise its kind, is probably due that marvellous variety in the
peculiar beauties of some groups of birds. The Duke of Argyll, speaking
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.]