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

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._

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.

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.

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]

very conspicuous animals may be adapted to their peculiar haunts.

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.

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]

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._

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]

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._

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]

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