The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

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]

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 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.

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.

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.

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.

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.

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.

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