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

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]

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).

senegalensis (below).]

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).

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._

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.

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._

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.

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.

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.

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.

_Transactions of the Royal Society_, vol. clxxviii. B. pp. 311-441.]

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_.]

edition, p. 130), with figure.]

1871.]
[Footnote 78: _Nature_, 1870, p. 376.]

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.]

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.]

first stated in my article on "The Colours of Animals and Plants" in