by Grant Allen
CHAPTER VII.
THE COLOUR-SENSE IN VERTEBRATES.
Although the perception of colour by birds and mammals has been taken for granted in the preceding chapter, and although it is practically all but incontestable that the higher vertebrates are quite as fully endowed in this respect as ourselves, yet the positive proofs which can be advanced in favour of the belief are very meagre and insufficient. The only real evidence is that supplied by our everyday observation, but no person familiarly acquainted with the habits of birds and mammals ever doubts for a moment their essential agreement with ourselves so far as concerns the visual faculties. Nevertheless, it may be well to point out the few positive facts which are forthcoming on the subject, connecting them at the same time with the probable genesis of the developed sense.
Apparently the perception of colour is inherited by the whole vertebrate series from some earlier common ancestor. At any rate, considerable traces of the colour-sense may be detected among many marine invertebrates. The best known instance is that of the chameleon shrimp (Mysis chamæleo), which has the power of altering its own colour in correspondence with the material among which it is found. When lying on a sandy bottom it appears grey; but when lurking among seaweed, it is green or reddish brown, according to the nature of the background. This change is produced by means of a reflex action connected with the eye, for when the animal is blinded it no longer occurs. In other words, we must suppose that when the optic nerve of the shrimp is affected by the green light from a piece of seaweed, certain muscles are set in action, voluntarily or automatically, which cause a corresponding change in the arrangement of the pigment cells, so that the animal appears green itself. It may be added, too, that this peculiarity affords an indirect proof of a colour-sense in the enemies of the chameleon shrimp (which cannot be more highly developed animals than fishes, and may perhaps by other crustaceans); because the creature can only possess this power for the sake of escaping the observation of its foes. If we believe it to be so provided for the purpose of deceiving its prey, then we must allow the existence of colour-perception even in more lowly forms on the average than fishes or Crustacea.
Indeed, the brilliant nature of many marine animals and plants affords an excellent opportunity for the development of a colour-sense. Instead of the uniform green of the forest, with the dingy black or brown of soil and rocks, we have here the exquisite colours of sea-anemones, star-fish, corals, serpulæ, jelly-fish, callianiridæ, aphroditidæ, ascidians, sea-slugs, and shell-covered mollusca, which browse amid groves of variegated algæ, whose hues are far more diversified than those of terrestrial vegetation. Amongst such surroundings it would almost be impossible that a colour-sense should not take its rise; and many indirect proofs conspire to show that in the class of fishes at least it exists in high perfection.
The most striking evidence is that afforded by certain flat-fish, which, like the chameleon shrimp, possess the power of changing their colour, so as to suit the bottom upon which they lie. Here again the peculiarity not only shows that the fish themselves are differentially affected by the various colours, but also that their enemies or prey are conscious of similar differences, or else the disguise would be useless.
Equally significant is the colouration of the common sole, brill, dab, and flounder. Any person who has seen these fishes lying on a natural bottom or in an aquarium must have been struck by the perfection of the imitation, which often baffles even human eyes, in spite of the actual knowledge that a fish is somewhere to be found upon the spot.
In like manner, the fishes, mollusca, and crustaceans which inhabit the sargasso weed, are all protectively coloured of exactly the same pale buff hue as the sargasso itself. One may often closely examine a piece of the weed, freshly brought up in a bucket of sea-water, and yet fail to detect any sign of life, until the attempt to raise the weed from the water reveals the fact that some small crabs or tiny fishes are lurking unseen among its waving branches. In all such cases, the existence of the imitative colouring is fair proof that it subserves the good of the species by protecting its members against enemies, or enabling them more readily to secure their prey.
Perhaps the only direct evidence, however, is that of the baits used by fishermen. Mackerel and other fish are often taken by means of red rags. A spoon painted bright scarlet forms a capital trolling bait. All anglers are agreed that trout can discriminate between the various imitation flies offered to their notice, and that the original colours must be carefully copied. Indeed, the facts rather tend to show, not only that fishes can discriminate colours, but also that they are attracted by metals or other brilliant objects, and by pure or intense hues. A taste for colour as well as a mere neutral perception seems to be implied by these observations.
Those who have given the greatest attention to the subject are inclined to credit fish with a very high degree of colour sensibility; and their opinion may be set down here as having some weight in so uncertain a subject. Mr. H. N. Mosely, the accomplished naturalist of the Challenger expedition, believes that almost all the colours of marine animals have been acquired for purposes of warning, protection, or attraction of prey, and that they have special reference to the eyes of fishes and higher crustaceans. The whole colouration of the lower aquatic organisms is exactly what we should expect it to be if the more highly evolved marine creatures were possessed of a colour-sense; and it is quite inexplicable and gratuitously complex if we suppose them to be destitute of such a faculty. Mr. Darwin is further of opinion that the colours of many fishes have been produced by the action of sexual selection; and though I do not mean to treat this part of our subject in detail till a later chapter, I think the conclusion of so careful and masterly an observer has considerable substantive value as corroborative of the positive facts.
When we proceed to examine the amphibia, much better evidence is available. The two colours green and blue are the least markedly different of all hues; and if they can be discriminated from one another by any species, we may be sure that that species possesses a very perfect form of colour-sense. Kühne of Heidelberg, in the course of certain researches on the nature and functions of retinal purple, discovered that if a number of frogs, Rana esculenta and R. temporaria, are confined in a shallow dish, one half of which is covered with green glass and the other half with blue, they will shortly all collect under the green portion. Great care was taken to eliminate all disturbing elements, such as unequal transparency to heat (diathermancy), or unequal intensity of illumination, and it was conclusively ascertained that an enormous majority of the frogs exhibited a distinct preference for the colour green over the colour blue. Blind frogs introduced into the same vessel showed no preference for one part over another. In this case, again, it is interesting to note that a special emotional taste, as well as an intellectual discrimination, is proved by the facts; and this taste becomes particularly interesting from the point of view of sexual selection, when we remember that green forms a very common colour amongst the Ranidæ.
Frogs likewise possess the power of changing their colour in correspondence with the environment, in the same manner as already noted in Mysis.
Reptiles also show some distinct marks of colour-perception. The most familiar instance is that of the chameleon, whose natural hue is a muddy white, changing with the nature of the background to yellow, brown, green, or bluish grey. The mechanism by which this change is effected has met with full treatment at the hands of Yon Wittich. Two layers of pigment cells are deeply seated under the skin, consisting of blue and yellow colouring matter respectively; and by forcing up one or other of these layers through muscular pressure, the animal assumes a bluish or yellowish tint, while the green is produced by simultaneous pressure upon both layers. In this manner the chameleon is able to simulate the appearance of the branches or leaves on which it stands, and so, perhaps, both to escape enemies and deceive prey. As before, the power of changing colour implies impressibility both in the animal itself and in certain other species for w
hose deception the habit has been acquired. The action is undoubtedly reflex, and ceases if the eyes be covered.
Many of the insects which mimic leaves or other like objects in the environment have probably gained this means of protection to escape the notice of lizards and other reptiles. But as the mimetic resemblance is oftener useful for deceiving birds, we may more fitly consider these cases when we pass on to examine the colour-sense in the higher vertebrates. There are one or two instances of protective colouring, however, which evidently have reference to the reptilian eye alone. Mr. Bates mentions a South American snake (Dryophis fulgida), whose pale green body exactly resembles the stem of a liana, and even imposed upon the keen-eyed naturalist himself at first sight. The prey for which this living branch lies in wait consists of tree-frogs and lizards. These, themselves, in turn, may perhaps escape it by their own prevailing greenness, which makes them so difficult of detection amongst the foliage on which they rest. Again, Sir Joseph Hooker found three ticks on an Indian lizard, each of which was coloured in imitation of that part of its host’s body on which it preyed. One from the yellow belly was yellow; one from the brown head was brown; and one from the parti-coloured scales was parti-coloured, “the hues corresponding with the individual scales which they covered.” Here we can hardly suppose that the imitation could be of any use except as a protection against the lizard himself and the other members of his family.
It may be worth while to mention in passing that many lizards besides the chameleons possess the power of changing their colour by inflating their lungs, which compresses or spreads the layers of pigment cells. The sexual colours of the beautiful Draco and other reptiles have been fully described by Mr. Darwin. “The shining appendages of the throat,” says Dr. Günther, “are merely folds of the skin, ornamental and sexual. Such appendages always betray an excitable temper,” or, in other words, co-exist with strong sexual jealousies. The significance of these facts will become more apparent when we pass on to the general question of selective preference for decorated mates.
Among birds, the perception of colour is shown by a large number of facts, collected by Mr. Darwin. A tame partridge described by Mr. Hussey “seemed fond ‘of gay colours, and no new gown or cap could be put on without catching his attention.’” Lord Lilford notices that the ruff “will dart down to a bright-coloured handkerchief, regardless of repeated shots.” The well-known bower-bird “collects gaily coloured articles, such as the blue tail-feathers of parrakeets, bleached bones, and shells.” Mr. Gould “found in one bower a neatly worked stone tomahawk and a slip of blue cotton.” “The regent-bird, as described by Mr. Ramsay, ornaments its short bower with bleached land-shells belonging to five or six species, and with berries of various colours, blue, red, and black, which give it when fresh a very pretty appearance. Besides these, there were several newly picked leaves and young shoots of a pinkish colour, the whole showing a decided taste for the beautiful.”
To these facts, which bear evidence to taste as well as perception, we may add the antipathy of the turkey-cock to scarlet, which is probably an effect of sexual jealousy, as the red would be ancestrally associated in his mind with the wattles of a rival. “The recognition of colour by small birds generally,” says a late writer, “is indisputable. Every one must have observed with varied feelings the discrimination with which they select the ‘sunny side’ of a pear, a plum, or a peach. It is also an established fact that they will attack the red currant in preference to the white variety, though the latter is much the sweeter of the two. Many observers during the last few years have pointed out how the yellow crocus is torn to pieces by sparrows and other birds, while the white and other varieties are unmolested.” I have myself often noticed in Jamaica the unerring certainty with which chickens darted from blossom to blossom of a yellow potentilla, for which they have a particular fancy, and which they always snapped up as though they supposed it to be alive.
These instances lead us on to those of the fruits, whose development we examined in our last chapter. “Red,” says Mr. Wallace, “being a very common colour of ripe fruits which attract birds to devour them and thus distribute their seeds, we may be sure that the contrast of red and green is to them very marked.” But this seems to me a somewhat inadequate expression of the real evidence on the point. We have seen that almost all those seeds or fruits which would be injured by the interference of birds are protectively coloured green or brown, while almost all those seeds or fruits which would be aided by the interference of birds are attractively coloured red, pink, orange, yellow, purple, blue, lilac, or black. I think these facts fully justify us in concluding that birds are able to distinguish every one of these colours from green, and most likely from one another. Otherwise there would be no reason why succulent fruits should differ in colour from nuts. The single case of the almond and the plum will bring the question at issue into strong relief. As in the case of entomophilous flowers, so in the case of succulent fruits, unless we believe that the seemingly attractive organs were developed for the purpose of enticing animals, we must believe that they are a positive waste of energy to the parent plant.
The evolution of bright flowers themselves shows that birds as well as insects are attracted by their beautiful petals. Mr. Darwin has collected many instances in which blossoms are fertilised by birds; and Fritz Müller notes several species of Abutilon in Brazil, which he believes depend entirely on humming-birds for the dispersion of their pollen. Mr. Wallace observes that brilliant flowers with handsome corollas exist in many Oceanic islands, such as Juan Fernandez, where flying insects are almost unknown; but their place is supplied by humming-birds, which Mr. Mosely mentions as being “extraordinarily abundant.” Mr. Belt believes that a climbing plant of Central America, Marcgravia nepenthoides, has been specially adapted to the same birds; while Mr. Wallace thinks that many Australian and Malayan flowers have been similarly specialised for the visits of honey-eaters, lories, and sun-birds. “Only large flowers,” says Mr. J. E. Taylor, “can be visited by these birds, or those whose polypetalous corollas allow of the head being thrust into the centre. Hence we have, in some measure, a reason afforded us for the larger size of the flowers in regions where such birds are abundant. The large bushes and trees of such countries usually bear very fine showy flowers in order to attract the birds; and it is found that the brush-tongued parrakeets are particularly fond of the flowers which grow at a height above the ground.” Any one who has watched a humming-bird darting with lightning speed from blossom to blossom could hardly have a doubt of his acute colour-perception.
The proofs afforded by imitation and mimicry are stronger in the case of birds than of any other class. One may say generally that almost all insects which display protective or imitative colouring do so for the sake of escaping birds or lizards. A few cases must suffice to show the general tendency of the evidence. The leaf and stick insects (Phylliidæ and Phasmidæ) closely imitate the colours and shapes of leaves and sticks. One in particular, the Ceroxylus laceratus, is apparently overgrown by moss or jungermannia. Sir Emerson Tennent describes the leaf-insects as possessing “all varieties of hue, from the pale yellow of an opening bud to the rich green of the full-blown leaf and the withered tint of decay.” The Kallima paralekta, a leaf-like butterfly of the Malay Archipelago, always rests among dead or dry leaves, which it resembles in all their varying hues, even appearing to be spotted with small fungi. Canon Tristram has noted that almost every insect, bird, or reptile inhabiting the desert of Sahara is coloured exactly like sand, and Lord George Campbell mentions a butterfly similarly imitative of its background which frequented the sea-shore at Amboyna. A South American Leptalis so closely resembles an uneatable Ithomia “in every shade and stripe of colour,” that Mr. Bates could hardly distinguish them, even with the aid of his minute entomological knowledge. “One of the Hemiptera (Spiniger luteicornis),” says Mr. Belt, “had every part coloured like the hornet (Priocnemis) that it resembled. In its vibrating coloured wing-cases it departed
greatly from the normal character of the Hemiptera and assumed that of the hornets.” The same careful observer gives many similar instances of mimetic resemblance in the Coleoptera, and Lepidoptera. But perhaps the most astonishing of these imitative forms is that of a moss-like insect, the larva of a Phasma, which is prolonged into curious green filaments, to mimic the moss in which it lives. Of course these creatures could derive no advantage from their minute reproduction of spots, lines, and hues, unless the enemies against which they required protection were capable of distinguishing their colours.
Mimicry or imitative devices of this sort are not confined to insects. Many lizards, such as the geckos, have colours like those of the walls on which they creep; while the protective green hue of the tree-frogs has already been noticed. Even birds occasionally mimic one another in the same manner. For example, two species of Mimeta (a sort of oriole), in Bouru and Ceram, imitate two Tropidorhynchi (honeysuckers) in minute details of colour, thus escaping small birds of prey, as the Tropidorhynchi are strong and pugnacious creatures. For other cases the reader must be referred to Mr. Wallace’s admirable essay on “Mimicry and other Protective Resemblances among Animals.”
