by Grant Allen
These various proofs, though indirect, can leave us in little doubt with regard to the general existence of a colour-sense among birds.
When we come to the highest class of vertebrates — the mammalia — strangely enough the evidence of a colour-sense almost entirely fails us. The antipathy of male ruminants for scarlet, and the curiosity which certain monkeys display with regard to bright-coloured objects, are the only facts in point which come under ordinary observation. This result, so contrary to what we might have expected, appears really quite natural when we examine more closely the circumstances of the case. By far the larger part of the mammalia are either herbivorous like the ruminants and pachyderms, or carnivorous like the technical carnivores, insectivores, and whales. Only a small portion of the class subsists upon fruits, while none of them are very specially connected with flowers. Hence a large set of possible tests which we can employ in the case of insects and birds are wholly inapplicable to mammals. Moreover, the want of close relations with the coloured parts of plants has probably resulted in a want of any peculiar love for bright colour, such as we see reason to suspect in the butterflies, humming-birds, and parrots. This absence of a taste for brilliancy is probably marked by the absence of brilliant hues in the animals themselves, the result of sexual selection; for these hues, as we shall see hereafter, only appear among the mammalia in a few higher arboreal and frugivorous species, such as the mandrill and certain squirrels. For the most part, throughout the mammalian series sexual selection seems only to have exerted itself, if at all, in the production of elegant shapes, protuberances like horns and dewlaps, and marked contrasts of light and shade, as in the zebra, giraffe, and hyæna.
Nevertheless we can hardly doubt that mammals do possess a considerable colour-sense, though, owing to the circumstances of their practical environment, their taste for any special hue is probably far from strong.
Here once more I must remind the reader that the proofs of a colour-sense throughout the whole infra-human world are necessarily very derivative, and that they owe their chief strength to their cumulative character. The fragmentary evidence collected in this chapter will be much corroborated and supplemented by that which will be detailed in the sequel. Enough will have been done if we succeed in showing that the hypothesis of a general colour-sense is consonant with all the facts of nature, and helps us to understand those facts in a way which no other hypothesis can do. For the present it will be sufficient if we bear in mind the one great point hitherto settled — that wherever any part of a plant, be it flower or fruit, will derive any benefit from attracting the eye of an animal, be it insect, bird, or mammal, that part is almost invariably coloured with some pure and brilliant hue, be it blue, red, yellow, pink, orange, violet, or lilac, quite distinctive from the green of ordinary vegetation. This one fact is the great pivot upon which turns our whole knowledge of the animal colour-sense.
CHAPTER VIII.
THE COMMUNITY OF TASTE BETWEEN FLOWER-FEEDING AND FRUIT-EATING SPECIES.
Before we proceed to consider the secondary reactions of the colour-sense in insects and vertebrates upon their own external appearance, we must glance for a moment at one of the determining causes which give approximate uniformity to the general results of such reactions in the animals with which we are most specially concerned. In the next chapter we shall have to examine the production of bright hues in the wings of butterflies, the skins of lizards, the feathers of birds, and the fur of mammals, due to the selective action of sexual preferences. But, as a necessary preliminary to that inquiry, we must first set ourselves to determine the principles which govern the formation of tastes generally among the flower-feeding and fruit-eating animals. Before we can trace to its final effects the action of a sexual preference for bright colouring, we must previously find out with certainty the reasons why a taste for such colouring should exist at all in the animal consciousness.
People are generally too apt to accept as ultimate and obvious every fact with which they have always been familiar. Seeing that bright colours as a rule attract children and savages, dogs, birds, fish, and insects, they do not trouble themselves to seek a reason for this preference, but take it for granted as an inherent and natural property of the animal organism, or, more often and more absurdly still, of the colours themselves. If, however, we reflect upon the subject for a moment, we shall see that there is no primitive and self-sufficing reason in the nature of things why any one colour should be more beautiful to us than another. Dull and dingy hues might conceivably have been just as pleasant to our sense, under slightly different conditions of our development, as we know bright and pungent hues to be, under the actual circumstances of humanity. “We must get a little deeper into the groundwork of our likes and dislikes if we would really understand the origin of our native preference for brilliant tints over mixed or unstimulating colours.
Now, after this preamble, most readers will imagine that I mean to explain the liking of flower-feeders and fruit-eaters for bright hues by means of that grand but somewhat vaguely employed shibboleth, the Association Theory. I know that to the mass of loose thinkers association is a sort of psychological deus ex machina which satisfactorily accounts for every ill-defined mental problem, just as electricity is a sort of physical deus ex machina which similarly gets over every ill-comprehended material problem. Such persons say to themselves at once, “Oh, of course, birds and butterflies feed off bright-coloured objects; so bright colours set associations with their food, and are consequently pleasant to them.” Having thus satisfied their nascent critical doubts by the easy application of an accepted formula, they never pause to translate their vague speculation into thinkable terms, but leave it as they took it up, a mere algebraical expression, incapable of rational statement in a concrete form. For how can association with food make a colour or anything else pleasant in itself? This is the true crux of the Association Theory, a crux which, as I humbly believe, few of its adherents have ever perceived in its full significance. Until it has been solved, the theory remains a mere verbal explanation, adding nothing to our real knowledge of the subject, yet deluding us by its specious resemblance to an explanatory truth.
The mode of exposition here adopted will be a very different one, based upon the known psycho-physical law of pleasure and pain. According to that law, pleasure is the psychical aspect of an ultimate physiological fact, which in its physical aspect may be summed up as the unimpeded activity of a fully nurtured and not overworked nervous structure in unbroken connection with the cerebro-spinal or other central sentient system. Conversely, or nearly so, pain is the psychical aspect of an ultimate physiological fact, which in its physical aspect may be summed up as the disintegration, insufficient nutrition, or excessive activity of a nervous structure, similarly connected with the sentient organism. With the latter half of this important law we have here little or nothing to do; but the former half so intimately concerns our subject that I shall make no apology for endeavouring briefly to explain its meaning in simpler language than that of the above abstract formula.
A pleasure, then, is the feeling which results when any sentient nervous centre receives a stimulation not excessive in quantity, nor beyond the existing power of the structures concerned. Every centre undergoes at each stimulation a certain amount of disintegration; and if that disintegration pass beyond the easy repairing-point of the system, pain sets in. But, on the other hand, so long as the stimulation is moderate, by exercising the structures it promotes their general efficiency, and hence it is accompanied by a feeling of pleasure. Or to translate our law into still more concrete and ordinary language, we may say that whenever an organ which can feel at all is exercised not beyond the due amount, pleasure is the result. Hence the pleasurableness of any activity may be accepted as a rough gauge of its general desirability for the organism as a whole, while conversely its painfulness may be regarded as a certain proof of its general undesirability.
Now the more fully-nurtured an organ may be, the higher
is its functional efficiency, and the greater the pleasure to be derived from its exercise. We all know that the fresher our limbs, our muscles, our nerves, and our eyes, the greater the enjoyment we derive from a country walk or a game of cricket. After long fasting we eat our food with greater relish; after long confinement we use our legs and arms with redoubled delight. But we also know that in order to keep up a state of high efficiency in any organ, frequent exercise is necessary. Only by running, jumping, rowing, and gymnastics can we bring our muscles into a proper condition for hard athletic work. Only by constant practice can we retain any accomplishment which we have learnt by dint of serious effort. And just the same is true of nerves. Their existing structure has been acquired by continuous function in past generations, and continuous function is necessary still if we would prevent them from rusting into obsolescence.
Accordingly, whenever we find that any activity is productive of immediate pleasure in ourselves, we may be sure that the activity in question is one which has long been practised by our human or ante-human ancestors. The greater the pleasure, as I have elsewhere endeavoured to show, the greater the intimacy of connection between the activity and the life of the species. Let us, for example, take the case of colour itself, with which we are here so fully engaged. If in any species the need for distinguishing different colours ever arose, and if by its side there also arose a nascent structure for so distinguishing them, then those individuals in which that structure was most fully developed would survive from generation to generation, in virtue of their superior adaptation to the needs of their environment above their less highly endowed compeers. But with each such increment in the structure there would go increased pleasure in the function. Conversely, the more fully the function was indulged the more would the structure increase and strengthen by exercise. So from generation to generation, as the power of distinguishing colours became more and more developed, the pleasure arising from their perception would grow more and more acute. Such pleasure forms the first groundwork for that differential preference in individuals or species which we know as taste.
But every colour would not probably prove equally pleasurable. Some, like the ordinary greys, greens, and browns, occur too often in the surrounding world to allow of any marked gratification, derivable only from the intermittent stimulation of little-worked nerves. Moreover, these common colours would have no special reference to the life of the race, and so would have few structural connections with other portions of the central nervous system. But in the case of fruit-eating and flower-feeding species, we may well suppose that the special nerves devoted to the perception of red, yellow, orange, and purple would naturally be much strengthened by constant hereditary use; while the comparatively intermittent nature of the stimulation would render the accompanying feeling far more pleasurable than in the more familiar instances of green or brown. Furthermore, the close relation of these colours with the food of the species would doubtless give rise to numerous nervous connections in the central system, whereby the sight of such coloured objects might set up the necessary movements for obtaining the booty. In this manner the central organs of special colour-perception for the brilliant hues of fruits and flowers would in all probability assume unusually large dimensions, and would certainly possess large numbers of concurrent fibres along which waves of discharge might readily travel, thus giving free vent for a considerable volume of pleasure-yielding energy. Such species might fairly be said to possess a taste for red, yellow, and other like pungent hues; and we might accordingly give a rough definition of taste as a special preference, in an individual or a race, for one or more out of several similar objective stimuli, depending ultimately upon special variations of nervous development.
Of course this hasty definition leaves out of consideration the other half of the subject, which we might perhaps sum up as distaste, — the special repugnance to one or more among such stimuli, ultimately due to like diversities of individual or generic organisation. In this case, however, we must distinguish between two widely different forms of feeling, which are apt at first to be mentally confused, — mere neutral indifference, which results from a stimulation too languid or too common to produce pleasure, and positive dislike or disgust, which arises from some actually painful and disintegrative action. Thus, in sight, an ugly colour, like that of mud, is simply neutral and unstimulating; but in taste, a bitter or acrid substance probably sets up material disintegration, and so of course produces a positively painful sensation. This distinction will become more obvious and more important as we proceed.
In this way only, then, as it seems to me, can association have anything to do with the intrinsic pleasurableness of any sensation, namely, by affording outlets for the overflowing nervous energy. But the main pleasure of the sensation itself, as I understand the question, must be due to inherited calibre of the nerves and nervous centres employed, that calibre being due itself to ancestral function throughout many previous generations. To put once more the concrete case, fruit-eaters and flower-feeders derive pleasure from brilliant colours (postulating the fact for the time being, argumenti gratia), not because those colours have mental associations with their food, but because the structures which perceive them have been continually exercised and strengthened by hereditary use. The connection with food has given numerous outlets for the nervous energy, and has been the ultimate cause for the extra development of colour-perceiving structures, but it has had no direct effect, as I believe, upon the immediate pleasure of sensation.
It is true that in highly evolved animals, with whom the emotions have attained an immense and preponderant influence, associations do largely enter into all pleasurable feelings. But even here the ultimate explanation is equally simple and straightforward. These emotions have their own proper nervous seats, as Ferrier’s experiments sufficiently show; and we must suppose that the actual sensation, being located in a centre which has connections with the seats of such emotions, rouses action in the emotional centres, more or less conspicuously, and so adds a more or less distinct factor to the total resulting consciousness. But it would be very foolish to transfer similar ideas into the simple nervous organisation of birds, and still more into that of bees and butterflies. Taste in these animals must be almost entirely a matter of direct sensation, dependent upon the calibre of the nerves employed, and little influenced by the few possible associated feelings.
Having premised these few considerations as to the nature of taste in general, let us now go on to examine the special tastes of fruit-eating and flower-feeding animals. We shall find on investigation that these appear to be approximately identical throughout the whole animal series; while they are more or less strongly marked off from the opposite tastes of carnivores and carrion-eaters. The very same sweet and sugary substances, the very same etherial and delicate perfumes, the very same bright and dainty colours, seem pleasing in the very same way to butterflies, and humming-birds, and parrots, and apes, and men. The similarity of nervous impressibility which we thus perceive to hold throughout the whole heterogeneous collection of fruit and flower haunters casts much light upon the nature of sexual selection, and upon the identity of taste between man and so many lower animals. It enables us to see why the flowers which the bee developed for his own delight and guidance should be the joy of children and the envy of artists; why the hues of the orange and the mango should be as beautiful to man as to the toucans and macaws which gave them origin; why the wing of the butterfly, the tail of the peacock, and the burnished throat of the sun-bird should be exquisite to our eyes as they were to those of their fastidious mates; and why human beings should dye their bodies with the woad of Britain and the ochre of Papua, or tinge their garments with the purple of Tyre and the thousand hues of Lyon, to vie with the gorgeous tints of bird and insect in the very self-same profusion of refulgent colours.
First, then, let us begin with the sense of taste. It is a most noteworthy fact that wherever any part of a plant can gain any advantage by attracting
the notice of animals, it always effects its purpose by the secretion of sugar, or, as we oftener though more incorrectly call it in this connection, honey. Now sugar, as I have already pointed out, has a special power of acting upon the gustatory nerves of animals, through the great solubility, diffusibility, and crystalline texture of its particles. Accordingly, we find that almost all classes of fruit-eaters and flower-feeders show a decided partiality for this pleasant stimulant — a partiality due, doubtless, to the long habits of their ancestors, which have developed correspondingly differentiated structures for the perception of the particular body in question. In flowers, sugar is secreted to attract bees and other insects; while in fruits, it acts as an allurement to birds and mammals. Furthermore, certain plants possess organs for the secretion of sugar on their stems or at the base of their leaf-stalks, of which Sir John Lubbock gives the following account:— “Belt and Delpino have, I think, suggested the true function of these extra-floral nectaries. The former of these excellent observers describes a South American species of Acacia, which, if unprotected, is apt to be stripped of its leaves by a leaf-cutting ant, which uses the leaves not directly for food, but, according to Mr. Belt, to grow mushrooms on. The Acacia, however, bears hollow thorns, and each leaflet produces honey in a centre-formed gland at the base, and a small sweet pear-shaped body at the tip. In consequence, it is inhabited by myriads of a small ant which nests in the hollow thorns, and thus finds meat, drink, and lodging all provided for it. These ants are continually roaming over the plant, and constitute a most efficient body-guard, not only driving off the leaf-cutting ants, but even, in Mr. Belt’s opinion, rendering the leaves less liable to be eaten by herbivorous mammalia.” Indeed, so universal is the taste for sugar among insects, that certain small animal creatures, like the Aphides and Cocci, have themselves acquired the habit of developing nectaries, and so gaining the protection of ants, which may be seen “assiduously running up the stems of plants to milk these curious little cattle.” And if we want further proof of the general love for sweet food-stuffs, we need only bethink us how the insects flock about a barrel of treacle in our streets, how the birds congregate in fruit-gardens, and how our own children gather around the windows of the confectioner’s shop to stare at the tempting wares within — rendered all the more enticing, be it observed, through the very same addition of red, blue, and yellow, which had already been invented by the fruit and the flower.
