# Popular Science Monthly/Volume 17/September 1880/Aesthetic Feeling in Birds

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* ÆSTHETIC FEELING IN BIRDS.
By Professor GRANT ALLEN.
THERE is no portion of Mr. Darwin's great superstructure which has been subjected to more searching criticism than his theory of sexual selection—the theory that beauty in animals is dependent, in part at least, upon the choice of brightly colored, ornamented, or musically endowed mates by one or other sex among all the more highly developed classes, such as insects, crustaceans, birds, and mammals. Not only have opponents argued strongly against the existence of such æsthetic tastes in the lower animals as would account for the supposed preference for beautiful partners, but even many of those who accept the evolutionist hypothesis as a whole have declared themselves unable to give in their adhesion to this particular speculation. Professor Mivart has brought forward strong objections to the great naturalist's view, and Mr. A. R. Wallace has raised a counter-theory on the subject of coloration at least, which has done much to convince many wavering biologists, and to insure their rejection of the suggested cause as adequate for the production of that beauty which all alike recognize in the animal world. It seems to me, however, that a little too much stress has been laid upon the notion that comparatively advanced intelligence is necessary for the appreciation of beauty in the opposite sex. It is true that our own highly complex æsthetic feelings are largely composed of elevated intellectual and emotional elements; but it may perhaps be shown that aesthetic tastes quite sufficient for the production of the known results do actually exist in many cases, and consist ​almost entirely of very simple sensuous factors. In order for a butterfly or a humming-bird to admire its gorgeously appareled mate, it is not necessary that it should be capable of taking delight, like ourselves, in a Claude or a Rubens; it is enough that it should possess a nervous organization pleasurably affected by certain forms and colors in the same way as it is pleasurably affected by sweet fruits or the nectar of flowers. Nothing more than this need be postulated in order to establish the facts for which Mr. Darwin has contended with such wealth of illustration in the second part of the "Descent of Man."
It may be worth while, then, to examine a single large class of animals, in which the æsthetic nature is highly developed, for the purpose of discovering whether they do really afford proof of a sensibility to form, color, or musical sound. It must be remembered that even in our own race the sense of beauty in children, savages, and the uncultured classes, hardly rises above this simple level. We must not, of course, expect to find an appreciation of musical harmony, of imitative pictorial skill, of elaborate ornamentation, among birds or insects. We must be content if we see evidence of a love for red, blue, and yellow, for sweet perfumes and pleasant flavors, for symmetrical forms and simple patterns, for ringing notes and trilled resonances. The class of birds probably shows external marks of such tastes in a higher degree than any other; and, though many of them have been set forth by various writers elsewhere, it will perhaps repay the trouble to collect them into a single paper in order to show their bearing upon the general æsthetic sensibility of the class, as well as upon the specific question of sexual selection. For this purpose I shall first take for granted the fact of such selection, and afterward endeavor to justify it by analogy from known human practice.
Beginning with the lowest of the special senses, taste, we find ample evidence that very many birds have a strong liking for sugar. In confinement, canaries and parrots eagerly devour it in the manufactured form. In the wild state humming-birds, sun-birds, honey suckers, lories, and many other species, feed off the nectar of flowers, more or less mixed with insects. Mr. Webber, an American naturalist, found that the ruby-throats of the United States were attracted by a cup of sirup, and numerous other birds display a strong liking for the same mixture. Fruits, which have been developed especially to suit the tastes of birds, almost always contain an abundance of sugary juices; while the kernels within their stones are generally bitter, so as to prevent their winged allies from devouring the actual seed. Hence we may infer that all the vast tribes of toucans, hornbills, macaws, plantain-eaters, birds-of-paradise, and fruit-pigeons, possess a taste for sugar sufficiently strong to have produced the separate evolution of these sweet seed-coverings in a hundred different families of plants throughout the whole world. Indeed, the strength of the evidence thus afforded can not be overrated, when we remember that ​in every case the covering is a dead loss to the plant, except in so far as it aids the dispersion of seeds; and that it must have been developed over and over again in a thousand different cases by the action of the most widely different birds. It is impossible to believe that such a coincidence can be due to accident, impossible to doubt that it results from a genuine taste for sweet flavors.
There is even some reason to believe that birds care for and discriminate other tastes besides the fundamental distinctions of sweet and bitter. All the small birds in Jamaica are particularly fond of the little scarlet capsicums grown in gardens, and devour them so greedily, that the fruit has acquired the common name of bird-peppers. If we remember how very hard is often the almost horny covering of a bird's tongue, there is nothing remarkable in the fact that the pungency of the capsicum should be felt as an agreeable stimulant, probably having effects analogous to those of mustard, water-cress, or peppermint, with human beings. The oft-quoted liking of tropical pigeons for the nutmeg, with its aromatic coating of mace, points in the same direction. Parrots in captivity frequently display very decided preferences and antipathies in their food. Owls can not be induced to taste meat in the slightest degree tainted. Again, all birds have a most accurate notion of the difference between ripe fruits and the unripe sour ones, besides carefully choosing the sunny side of peaches, pears, and apricots. The very frequency of distinct sapid principles in fruits would seem to favor the same supposition, as they have probably been acquired for the special allurement of particular species. Indeed, the more we consider the origin and nature of succulent fruits, the more does it become clear that they have been developed to suit the tastes of animals having essentially identical sensations with our own.
The case of the nutmeg leads us naturally on to the consideration of smell. Here we may conclude with great probability that the large class of aromatic fruits has acquired its perfume for the sake of attracting birds, especially when we recollect that flowers have acquired exactly similar perfumes for the sake of attracting insects. And although the possession of scent as a means of sexual allurement is rare among birds, being probably confined to the musk-duck and a few other species, yet it occurs frequently among butterflies, and is represented among mammals by the musk-deer, beaver, and many other ruminants or rodents. Curiously enough, the similarity of taste thus testified extends to the vegetal world in the case of the musk-plant; while even certain carnivores, such as the cat tribe, are extremely fond of "valerian, lemon-thyme, camomile, lavender, and many plants rich in essential oils." On the other hand, a good observer notes that cats have their dislikes, and he has often seen a tabby "smell at a fig-tree, and turn away with the disgusted air of a connoisseur." We have no such strong facts in the case of birds, but the frequency of perfumes in those fruits which depend upon them for the dispersion of their seeds, coupled ​with their total absence among most nuts, would lead to the conclusion that their likes and dislikes in the matter of smell are fully as marked.
But it is when we arrive at the sense of hearing that we come to the point where proper æsthetic feelings begin. It is quite impossible to doubt that birds are fond of musical sounds. The song of our own nightingales and linnets, the deep notes of the South American bellbird, the incessant cooing of the dove, the noisy chattering of the parrots, the ringing cry of the whippoorwill, all lead to the same conclusion. Here, again, these sounds are of precisely the same nature as those employed by the crickets, katydids, cicadas, and other musical insects, as well as by man himself in his vocal and instrumental music. Something of the same taste is displayed among the quadrumana by the howlers and other monkeys. But it is a noteworthy fact that a large majority of these presumably sexual calls, in birds, insects, and other animals, are true musical sounds, not mere noises. I have pointed out elsewhere the probable reason for this preference of pure tones in the case of mankind; and the same argument will apply, mutatis mutandis, to all other animals. But there is certainly a singular analogy in this respect between sounds and colors, most animals preferring the relatively pure and simple musical tones to confused noises; and the relatively pure and simple analytic colors, red, blue, green, and yellow, to confused mixtures such as brown, gray, and mud-color. At any rate, a bird evidently pays far more attention to the musical class of auditory perceptions than to mere noise. A canary will take no notice of ordinary confused sounds in a room; but, if one begins to chirp or whistle to it, it immediately responds with another chirp in emulation. So, too, when a piano or other musical instrument is played in the neighborhood of a singing bird, it will often show its recognition of the musical character by pouring out its very fullest flood of song, as if to conquer its unconscious rival. Of course, the singing-matches between birds themselves are too familiarly known to call for separate mention. It may be worth while, however, to notice that this love of musical sound exists even among certain reptiles; for I have often seen the common house-lizard of Jamaica listening with evident interest and attention to the playing of a piano, turning his head from side to side, and scampering away when disturbed, only to return again to the fascinating sound after a minute or two of hesitation.
The cases of the starling, the piping bullfinch, and the mockingbird, which can be taught to whistle a tune, show the same power still more highly developed. These instances prove not merely susceptibility to musical sounds, but also a capacity for distinguishing the harmonic intervals. It is stated that some birds, even in the wild state, display considerable knowledge of the musical scale; and a San Francisco naturalist is at present engaged upon a work in which he hopes to show that the human ear possesses in this respect merely a ​more highly developed form of the common vertebrate sensibility. When we reflect upon the purely physical and physiological basis which, as Helmholtz has taught us, underlies the musical intervals and the distinctions of harmony and discord, there is certainly no reason why they should not be perceived by all the higher animals alike, in a greater or less degree.
Considering, therefore, the evident susceptibility of birds to the simpler pleasures of music, and the interest which they show in it even apart from their domestic relations, there is no a priori difficulty in accepting the belief that their powers of song may have been developed by mutual selection, provided no adverse argument can be shown against the probability of such selection ever proving a cause of specific variation. To this last question, the question so ably raised by Mr. Wallace, I shall return on a later page.
Passing on to sight, we have first to observe the effects of mere light or brilliancy upon birds, apart from special effects of color or form. Now, birds certainly share with insects and many other creatures the common fascination for bright lights. "Owls and night-jars have been known to flutter against the window of a lighted room in the small hours." In the tropics, where windows are more constantly left open, birds frequently fly into houses, attracted by a lamp or candle. The reflected light of a mirror is employed to draw down larks. Magpies delight in secreting diamonds, gold, silver, and other shiny objects. The bower-birds use shells, polished pebbles, and like brilliant odds and ends in the construction of their bowers. So, too, metallic iridescence occurs frequently in the feathers of beautiful species, notably in the humming-birds, sun-birds, peacocks, and other flower-feeding or fruit-eating classes. But even the far less brilliant crows, gulls, ducks, and doves show exquisitely burnished gloss or luster on their coats, often specialized upon particular portions of the plumage, and apparently betraying the action of sexual selection.
Of the love for color shown by birds, I have already treated so fully elsewhere, that it will suffice here briefly to recapitulate the main facts. The universality of bright hues in the fruits which depend upon birds for the dispersion of their seeds clearly shows that fruit eating species are attracted by red, blue, purple, and yellow; just as the analogous case of insect-fertilized flowers shows the preference of bees and butterflies for similar tints. Mr. Darwin has collected several instances of interest displayed by birds in colored objects-, and of the attractiveness which color evidently possesses in their eyes. Of these, the most remarkable cases are those of the bower-birds' and the humming-birds' nests. And the constant occurrence of very brilliant hues among flower-feeding species, such as humming-birds, sun-birds, lories, and barbets, or among fruit-eaters, such as toucans, fruit-pigeons, ​birds-of-paradise, and parrots, induces the belief that in these classes the exercise of the structures upon the search for food has led to the formation of a very strong taste for color, ultimately resulting in sexual modifications.
As for the harmony of color usually observable in birds, it must be remembered that our feeling of harmony probably depends upon the due intermission and alternation of sense-stimulants, and therefore ought naturally to be shared by us more or less definitely with all other animals having a like constitution of the eye. Now, the mammalian and avian eye being derived from a common ancestor, who already possessed a highly developed power of vision, we might reasonably expect that our feelings of harmony would be essentially identical; and this expectation is fully borne out both by the coloration of fruits and of birds themselves, which seldom or never present what we should regard as discordant coloring. Furthermore, as the most beautiful classes of birds are those which live perpetually among tropical flowers and fruits, in the most beautiful forests or meadows, surrounded by exquisite insects and reptiles, and for ever exercising their vision upon the most diversely colored environment in the whole world, it would seem far from impossible that their chromatic sensibility is even more highly developed than that of average humanity, and therefore that harmony or discord of colour would bear a relatively greater importance in their eyes than in those of any human being except the most artistically endowed. This conclusion will doubtless sound strange and even grotesque to those who are always accustomed to postulate for man a kind of absolute supremacy in the scheme of Nature; but it appears to me almost as obvious and as simply accounted for as the superiority of scent in the dog and the deer, or of distant vision in the eagle and the vulture. Lastly, it may be noted that much of the beauty of birds, as of insects, fruits, and flowers, is due to the delicate gradation of tints which they display. But in all natural products such gradation is an almost necessary result of the mode by which they have been evolved. It is only in human manufactures, where pigment is laid on with a brush or stamp, that colors can be placed in crude juxtaposition to one another, giving rise to the worst form of chromatic discord. Doubtless our native feeling of dislike to such discords, based upon their immediately fatiguing effect upon the nerves employed, has been heightened intellectually by the knowledge that they differ so widely from the dainty gradations to be found in the handiwork of Nature. Besides being sensuously recognized as discordant, they are intellectually recognized as inartistic. Thus a large part of our art-progress has consisted in an advance from the harsh and monotonous fields of primary red and blue, divided by very hard and definite lines, which we find in Egyptian painting, to the faithful representation of graduated tints and shades which appears upon a modern canvas. But in the petal of a rose, the ray of ​a daisy, the wing of a butterfly, the tail-covert of a peacock, such gradual merging of tint in tint could hardly fail to occur spontaneously, as a product of evolution; while the comparatively definite marking off of special spots or lines, as in some orchids and other flowers, could only present itself as a result of very intense competition between species, carried on under highly complex conditions. The views set forth by Mr. Bates upon the progressive modification of patches or regions in a butterfly's wing, and by Mr. Darwin and Mr. Wallace on the feathers of the peacock and the Argus-pheasant, though widely differing as to the particular mode of their evolution, yet alike convince us that the inevitable result must be just such a graceful running together of contiguous colors as we actually find to obtain in every case.
Lastly, we arrive at the sensibility to form, symmetry, arrangement of patterns, and the like higher sensuous aesthetic feelings, which remains in the eyes of many the chief stumbling-block in the way of accepting the theory of sexual selection. The pleasure derived from sweet tastes and fragrant perfumes is so purely sensuous that nobody doubts its universal existence among all the higher animals. The pleasure derived from musical sounds and bright colors, though more intimately bound up in the human consciousness with intellectual and higher emotional elements, yet contains so large a factor of mere sensuous stimulation that we can easily conceive of it as appealing to the ears and eyes of insects and vertebrates. But the still higher pleasure derived from graceful curves, symmetrical ornamentation, and elaborate tracery is so largely made up of intellectual feelings, and so largely supplemented in our own case by associations of costliness, human handicraft, or imitative skill, that we find it hard at first sight to believe in the existence of similar feelings among pheasants of the Indian jungle, antelopes of the African plains, or monkeys of the Brazilian forest. Even here, however, a little consideration may convince us that the æsthetic appreciation of form and its connected varieties is not necessarily above the narrow intellectual faculties of the higher vertebrates and articulates at least.
In the first place, if we look at the human race itself we shall find that a comparatively high susceptibility to form occurs even among very low races. Indeed, most exquisite patterns are produced by savages whose taste in color is apparently far less developed than that of parrots, humming-birds, and fruit-pigeons. The tattooed tracery of the Polynesians and many other savage tribes presents beautiful designs of which even a European decorative artist need not be ashamed. The New Zealand canoes, the paddles and clubs of the Admiralty-Islanders, the shields of the Zooloos, are all most graceful in their shapes and most daintily wrought with interlacing patterns in carved work. Calabashes, cocoanuts, ostrich-eggs, and other early vessels are always cut in sections which exactly coincide with the demands of the most ​developed taste. The huts of savages are generally square, circular, or oval in shape, neatly wattled at symmetrical distances. The earliest architecture consists of regular stone rings, avenues, tumuli, and other definitely shaped monuments. Dr. Schweinfurth's "Heart of Africa" contains pictures of pottery as beautiful as anything ever produced in Greece or Etruria, stools, chairs, and other furniture as gracefully shaped as anything ever wrought by a Renaissance carver, and villages as prettily arranged after their simple fashion as the architects of the Parthenon or Cologne could have arranged them. If we look back in time, we find the stone hatchets and arrow-heads, not only of the neolithic but even of the palæolithic age, carefully symmetrical in shape, and that at a time when the extra labor of chipping the flints into comeliness must have entailed a considerable waste of human or half-human energy. At the same early date we find fossil shells, symmetrical bones, teeth, and other like objects, already drilled to serve as necklaces or other ornaments, which analogy with the similar ornaments now in use would lead us to believe were symmetrically strung together into definite patterns. Indeed, the more we look at the products of the very lowest savages and the very earliest men, the more shall we be convinced that they possessed in the germ all those aesthetic feelings which have finally developed our existing architecture and other decorative or semi-decorative arts.
Again, we can not fail to be struck by the fact that man has always employed for ornamental purposes exactly those very appendages of animals which, if the theory of sexual selection be correct, have been produced by the animals themselves as ornamental adjuncts. The feathers of peacocks, the plumes of the ostrich and the bird-of-paradise, the antlers of deers, the horns of antelopes, the tusks of elephants, mammoths, and musk-deer, the striped, spotted, or dappled skins of mammals, all these have been used from the earliest periods as materials for decoration by mankind. Exactly the same curls, twists, and patterns which seem to please the eyes of animals are known to please the eyes of man, even in his lowest developments. If these ornaments were not produced because the creatures themselves found them beautiful, at least they are the same as those which would have been produced had the taste of such creatures coincided in the main with that which runs throughout the whole of humanity, from the most degraded savage to the highest artist.
Moreover, part at least of the pleasure of form probably has a purely sensuous origin. The superiority of curved lines to straight, of the waving or sinuous contour to the angular, is apparently connected with the muscular process in the act of vision. Hence there is no reason why it might not be felt by intelligent animals, just as we know that it is felt, and acutely felt, by hardly more intelligent men.
Similar conclusions are forced upon us if we look at the nature of the supposed ornaments themselves. They are almost always, like the ​horns of several ruminants, the tail-coverts of the peacock, and the lappets or crests of many birds, apparently devoid of any functional use whatsoever, unless that use be the attraction of the opposite sex. They are also marked by the extreme definiteness of their shape, color, or sculpture—a definiteness which never occurs in similar structures among the lower animals. For though some echinodermata, as for example the sea-urchins, are very beautifully and regularly marked, yet their markings are purely dependent upon the structural arrangements of the animal, and can not generally be detected till after death. So, too, the shells of many mollusca, such as scalaria and the murices, are very beautifully sculptured; but this sculpture is structurally necessary for the animal, and apparently depends entirely upon the shape and markings of the mantle. Among birds, however, as among the ruminants, all the structures ascribed by Mr. Darwin to sexual selection are marked by a kind of definiteness, quite unconnected with ordinary functions, which it is difficult to describe in words, but which can immediately be felt if we compare the coloration of a peacock with that of a sea-anemone or a medusa. The former is perfectly definite without being obviously connected with structure; the latter is very indefinite, and yet bears a clear relation to the general shape of the animal. This combination of great specific distinctness with little apparent functional value appears to me the genuine hall-mark of organs due to sexual selection.
There is even some little external evidence in favor of a love for symmetry among birds. The nests of weaver-birds and many other species, as well as the bowers of the bower-birds, display a considerable taste for orderly arrangement. For one must remember that the building of such nests, though doubtless instinctive and inherited, is not a mere organic process, like the secretion of a molluscan shell; it is as much an art as the building of a honeycomb or of a savage hut. The flight of birds in play, the antics of many humming-birds, the strange eddyings and aërial evolutions of several other species, all approach very nearly to our own idea of dancing. I am almost afraid to hazard the observation, yet on the other hand I can not avoid risking it, that the attitudes taken up by the turkey-buzzards or John crows of the West Indies upon the tops of houses frequently seemed to me intentionally symmetrical. I have observed them sitting in every variety of position—one at each end of a long roof; one at each of the two points half-way between ends and middle; three arranged in either of these forms, with one in the middle; five arranged in the order C, B, A, B, C, etc. If any other observer can supplement this experience, which I record with great diffidence, I shall be very glad.
Taking for granted, then, this appreciation of form and symmetry, we shall find that it has produced many notable effects in the world of birds. To it, apparently, we owe the crests of cockatoos, pigeons, herons, and a hundred other species; the wattles, combs, hackles, and ​lappets of the gallinaceous birds; the beaks of toucans, hornbills, and cassowaries; the wonderful marking of the peacock and the Argus pheasant. Any one who wishes really to understand the immense variety of ornamentation which has thus resulted should pay a visit to the ornithological rooms in the British Museum, and observe the innumerable devices for attracting attention which exist in almost every order of birds. Perhaps the familiar lyre-bird offers the very finest example of all, so far as beauty of form and symmetry of arrangement are concerned. It is specially noticeable, however, that in almost every case the decorations are lavished on the very same parts on which they would have been bestowed by human taste.
If, then, we put together all the scattered indications thus afforded us, if we consider the taste for sweet food and delicate perfumes, the song of the nightingale and the graceful movements of the swan, the metallic colors of the flower-feeders, the exquisite hues of the fruit eaters, the varied plumage of the birds-of-paradise, the beautiful nests and bowers, the habit of abstracting brilliant objects, the universal loveliness of shape or tint throughout the whole class—we can hardly doubt that birds, as a whole, possess aesthetic endowments of a very high order. Let us proceed to consider the general bearings of these views upon the question of sexual selection.
Mr. Herbert Spencer, in a very remarkable essay upon personal beauty, has shown that in the human race we regard as beautiful, on the whole, just those personal peculiarities which are, roughly speaking, the external marks of fitness for the conditions of human life. More especially do we admire those points which bespeak a physique adapted for the duties of paternity and maternity. We dislike excessive leanness or excessive fat; a sallow or a bloated complexion; deformity or extreme departure from the normal type. On the other hand, we like in man robust and muscular limbs, an erect carriage, an open chest, a virile development of beard and whiskers, with all the other outward signs of health and strength. We like in woman a womanly and tender face, a fine and well-developed figure, and all the other outward signs of health, and more especially of healthy maternal capacities. We like in both sexes an abundant crop of hair, clear and bright eyes, white and well-set teeth, red lips, and cheeks which show a good and sound circulation; we like an expression which betokens good humor, moral qualities, and refinement; lastly, we like a face which indicates intellectual power and ability to succeed in the highly complex struggle for life in the midst of which our lot is cast. One or other of these points we may occasionally waive in consideration of other special claims; but if anybody asks in the abstract whether we prefer a stunted physique to well-grown limbs and muscles; a flat-chested woman to one with a finely-proportioned bust—unhealthy and sallow skin to a clear complexion; a sour-looking, mean, or brutal face to a bright, joyous, open, and honest countenance; silly ​or idiotic features to an expression full of liveliness and intelligence—there can be but one answer possible. Leaving out of consideration for the present all other elements of the involved and complex problem, we may conclude that beauty, from one point of view at least, consists for each species in the outward signs of specific adaptation to specific necessities.
On the other hand, beauty also consists from a different point of view of stimulation by a certain relatively fixed number of external stimulants—musical sound, brilliant light, analytic colors, curved shapes, symmetrical arrangements of form, etc.—which appear to act directly upon the nervous system. This is clearly the view which Mr. Darwin implicitly accepts, especially with regard to tone and color. The facts at which we have briefly glanced above respecting the aesthetic feelings in birds, and the beauty of the birds themselves, take for granted some such theory of the aesthetic faculty. How are we to find a reconciliation between this view and that of Mr. Herbert Spencer?
I believe the true clew has been given us by Mr. A. R. Wallace, in the able essays on "Color in Plants and Animals" which originally appeared in "Macmillan's Magazine," and were afterward reprinted in his work on "Tropical Nature." It is true that Mr. Wallace utterly rejects sexual selection as a vera causa, and substitutes for it several separate minor modifications of natural selection; yet it seems to me that a compromise between his view and the two other views of Mr. Darwin and Mr. Herbert Spencer would more really represent the actual state of the case in nature. Or, to put it more correctly, the three ideas are not in reality contradictory or even opposite, but are rather different and complementary aspects of one and the same fundamental truth.
Beauty in the abstract and for all species, as it seems to me, consists of pleasurable stimulation of the higher sense-organs. Such pleasurable stimulation must, on the average of cases, be given rather by brilliancy than by dullness; rather by analytic colors than by confused hues; rather by curved or flowing forms than by angularity; rather by musical sounds than by mere noises. But beauty relatively to the particular species, and especially as regards the sexual relation, must be largely due to special inherited tastes, doubtless ingrained and physically registered in the nervous system, leading the animal to derive pleasure from the typically healthy and normal form of the opposite sex. For, if any individual possesses divergent tastes, they must either be for relatively unhealthy and typically defective forms, in which case they will tend to be promptly suppressed by natural selection; or for neutral or improved forms, in which case they will help to give rise to new varieties, ultimately culminating in separate species. Such divergent tastes seem to be shown in all large dominant families, such as the humming-birds, where specific variation and ornamentation ​have been carried out to a very great extent. But all such divergent fancies must themselves tend to become distinctly fixed for purposes of specific identification; and we find as a matter of fact that each species does readily recognize its mates, even when the differences between closely allied species are only very slight.
Now, this special hereditary liking for a particular form and type will not interfere with the general love for color, brilliancy, sweet tones, and perfumes. Accordingly, wherever the circumstances which give rise to a taste for these sense-stimulants exist, it would naturally follow that the taste would help to determine the choice of mates. But, again, as Mr. Wallace has fully shown, the most vigorous individuals would usually possess the most highly developed ornaments, the brightest colors, the largest scent-glands, and the loudest or most musical voices. Hence the very animals most likely to be sexually selected are also, on the average, those most likely to be naturally selected. Yet sexual selection really differs from natural selection, in that it gives a special direction to the ornamentation. For example, one can hardly believe that mere masculine vigor will account for the gorgeous and positively inconvenient plumage of the bird-of-paradise, nor for the exquisite coloring of the peacock, nor for the extremely ungainly air-bladders of many insects. It is quite easy to conceive that the general vigor implied by the possession of these extended ornamental adjuncts may have helped their possessors in the general struggle for life; but it is hardly possible to believe that they could have reached their present definite development without the aid of sexual selection. In short, where an ornament, or what seemed to any particular individual an ornament, proved hurtful to the race, it would be eliminated by natural selection; but where it proved neutral it would be spared, and if it coincided with advantageous qualities it would be further developed. Yet, even if only neutral, sexual selection alone would give it an extra chance, and, as it would doubtless be correlated on the one hand with certain special tastes and habits, and on the other hand with certain slight modifications of structure, it would doubtless succeed on an average of cases in producing a new species.
The familiar facts of human beauty will probably serve to make this reconciliation of the conflicting views a little clearer. Man of course admires in the abstract bright colors, brilliancy, musical notes, graceful curves, and symmetrical form. But, as applied to the human face and figure, he admires these in certain special and typical arrangements. Thus, while our general love for color leads us to prize golden hair, we do not like a sallow complexion; while it leads us to see beauty in rosy cheeks and red lips, which are signs of a healthy circulation, we do not admire the same redness in the nose, where it is usually a result of dyspepsia or dissipated habits, either of which is bad for the race at large. Again, though we admire pearly teeth, clear eyes, and a white skin, all of which are obviously the external ​marks of useful properties, we do not admire white cheeks, which are the external mark of weakness or anæmia. Similarly, our idea of beauty demands that the figure should neither be too fat nor too thin, but should possess that graceful development of all the muscles which is the outward symbol of ability to move and act with ease and effect. If any large number of persons were ever actuated by opposite tastes, if they preferred pale cheeks and lips to rosy ones, thin and haggard faces to full and rounded ones, weak and angular limbs to strong and graceful ones, a flat and undeveloped chest to a fine and healthy bust, then they and their taste must rapidly die out through the inferior physique they would hand on to their descendants. And as every individual is himself the product of countless thousands of prior individuals, all of whom have been in the main successful in the struggle for life and the search for mates, it must follow that he will have inherited from them, on the average, a healthy taste for that particular arrangement of limbs and features which best suits the essential conditions of the species. Not, of course, that he will consciously recognize this fact in most cases; but the mere presentation of such a typical combination will instinctively rouse in him, through the organized correlation of nervous centers, the hereditary feeling of beauty. Hence this feeling will probably be most strongly aroused in each species by the sight of the sex which in that species has undergone the greatest differentiation through sexual selection: just as we know that the feeling is most strongly aroused in mankind by the beauty of woman. On the other hand, we are still able to perceive, when we look at a peacock or a humming-bird, that, thought his specific hereditary feeling is absent, yet the strength of the purely abstract elements—color, brilliancy, symmetry, form, and minute workmanship—is so unusually great that we have no hesitation in pronouncing them also beautiful after their kind.
If, then, we admit the reality and potency of sexual selection, in however modified a form, it must follow that birds, being on the whole the most ornamental of all classes in the animal world, are also the most æsthetic, with the exception of man. It might, at first sight, seem that consistency would demand the sacrifice even of this exception; but a moment's reflection will disclose an important difference between the two cases. Man possesses the active power of direct artistic creation; the birds only possess the passive power of selection from among the forms produced for them by Nature. The ordinary workman who selects his wife partly or wholly on the ground of beauty, thereby does something toward perpetuating and improving the beauty of the race; he stamps the impress of his taste upon future generations; but such mere passive choice differs widely from the ability to depict or create on canvas such a beautiful woman. In this way, the actual loveliness of birds may lead us somewhat to over-estimate their aesthetic sensibility; for, though within their own species they may be ​capable of distinguishing between comparatively minute shades and degrees of beauty, just as we can distinguish between such minute points in human faces as would doubtless absolutely escape the notice of any other animal, it is yet improbable that they would be equally discriminative outside the limits of their own species. Again the principle of "gradation of characters" necessitates certain artistic effects in their plumage which they themselves may be only half able to admire. So, too, the necessarily symmetrical arrangement of the two sides of the body and the mode of growth of feathers may often have helped, unintentionally, as it were, in producing the total effect. In other words, it may well be that the birds, while selecting their partners on the ground of bright color, exceptionally long plumes, and other ornamental characters which they could understand and admire, may have succeeded in producing harmonies of tone, delicate gradations of tint, and other similar effects which they could not understand or admire, or at least could only admire very partially.
Yet, after making all allowances for possible reading in of human feelings, it may probably be asserted with safety that the actual appearance of birds entitles them to rank, on the whole, higher in the aesthetic scale than any other animals except man. Whether we look at their graceful shapes, in the swan and the heron; their beautiful plumes, in the ostrich and the bird-of-paradise; their exquisite color, in the sunbird and the lory; their ornamental crests and lappets, in the humming-birds, the pigeons, and the parrots; or their song in the linnet, the mocking-bird, and the nightingale—we must confess that they give extraordinary evidence of a taste for all that man considers lovely or artistic. And this is just what we might expect from their free mode of life, their rapid motion, their highly developed senses, their comparative freedom from enemies, their long and almost uninterrupted rivalry between themselves for the possession of their mates. Especially should we expect this splendid outburst of aesthetic sensibility exactly where we find it in its greatest glory, among the flower haunting and fruit-eating species of the Brazilian forests, the Indian jungles, and the Malay Archipelago. Surrounded for generations and generations by gorgeous orchids and trumpet-creepers, from which they sucked the stored-up nectar, by gleaming purple or golden fruits, by burnished beetles, metallic butterflies, bronze-scaled lizards, and coral snakes, their prey or their enemies, exercising their eyes perpetually in the search for food among the exquisite objects of their environment, and safe from almost all foes except those of their own class, tropical birds have naturally developed the most gorgeous and the most perfect forms and colors in the whole animal creation. And, above all, they have stamped the mark of their peculiarly high aesthetic feelings upon their own shapes by the wonderful definiteness of their patterns and their ornamental adjuncts, nowhere equaled, save in the most perfect decorative handicraft of man himself.