The Living Age 1743, 10.11.1877
From Macmillan's Magazine.*
*In the first part of this paper I used the term "voluntary sexual selection" to indicate the theory that many of the ornaments of male animals have been produced by the choice of the females, and to distinguish it from that form of sexual selection which explains the acquisition of weapons peculiar to male animals as due to the selective influence of their combats and struggles for the possession of the females. I find that Mr. Darwin thinks the terra "voluntary" not strictly applicable, and I therefore propose to alter it to "conscious" or "perceptive," which seem free from any ambiguity and make not the least difference to my argument.The coloring of plants is neither so varied nor so complex as that of animals, and its explanation accordingly offers fewer difficulties. The colors of foliage are, comparatively, little varied, and can be traced in almost all cases to a special pigment termed chlorophyll, to which is due the general green color of leaves; but the recent investigations of Mr. Sorby and others have shown that chlorophyll is not a simple green pigment, but that it really consists of at least seven distinct substances, varying in color from blue to yellow and orange. These differ in their proportions in the chlorophyll of different plants; they have different chemical reactions; they are differently affected by light; and they give distinct spectra. Mr. Sorby further states that scores of different coloring matters are found in the leaves and flowers of plants, to some of which appropriate names have been given, as erythrophyll which is red, and phaiophyll which is brown; and many of these differ greatly from each other in their chemical composition. These inquiries are at present in their infancy, but as the original term chlorophyll seems scarcely applicable under the present aspect of the subject, it would perhaps be better to introduce the analogous word chromophyll as a general term for the coloring matters of the vegetable kingdom.
Light has a much more decided action on plants than on animals. The green color of leaves is almost wholly dependent on it; and although some flowers will become fully colored in the dark, others are decidedly affected by the absence of light, even when the foliage is fully exposed to it. Looking therefore at the numerous colored substances which are developed in the tissue of plants; the sensitiveness of these pigments to light; the changes they undergo during growth and development; and the facility with which new chemical combinations are effected by the physiological processes of plants as shown by the endless variety in the chemical constitution of vegetable products, we have no difficulty in comprehending the general causes which aid in producing the colors of the vegetable world, or the extreme variability of those colors. We may therefore here confine ourselves to an inquiry into the various uses of color in the economy of plants; and this will generally enable us to understand how it has become fixed and specialized in the several genera and species of the vegetable kingdom.
In animals, as we have seen, color is greatly influenced by the need of protection from or of warning to their numerous enemies, and to the necessity for identification and easy recognition. Plants rarely need to be concealed, and obtain protection either by their spines, their hardness, their hairy covering, or their poisonous secretions. A very few cases of what seem to be true protective coloring do, however, exist, the most remarkable being that of the "stone mesembryanthemum," of the Cape of Good Hope, which in form and color closely resembles the stones among which it grows; and Dr. Burchell, who first discovered it, believes that the juicy little plant thus generally escapes the notice of cattle and wild herbivorous animals. Mr. J. P. Mansel Weale also noticed that many plants growing in the stony Karoo have their tuberous roots above the soil, and these so perfectly resemble the stones among which they grow that, when not in leaf, it is almost impossible to distinguish them (Nature, vol. iii. p. 507). A few cases of what seem to be protective mimicry have also been noted, the most curious being that of three very rare British fungi, found by Mr. Worthington Smith, each in company with common species, which they so closely resembled that only a minute examination could detect the difference. One of the common species is stated in botanical works to be "bitter and nauseous," so that it is not improbable that the rare kind may escape being eaten by being mistaken for an uneatable species, though itself palatable. Mr. Mansel Weale also mentions a labiate plant, the Ajuca ophrydis, of South Africa, as strikingly resembling an orchid. This may be a means of attracting insects to fertilize the flower in the absence of sufficient nectar or other attraction in the flower itself; and the supposition is rendered more probable by this being the only species of the genus Ajuga in South Africa. Many other cases of resemblinces between very distinct plants have been noticed — as that of some euphorbias to cacti; but these very rarely inhabit the same country or locality, an it has not been proved that there is in any of these cases the amount of interrelation between the species which is the essential feature of the protective "mimicry" that occurs in the animal world.
The different colors exhibited by the foliage of plants, and the changes it undergoes during growth and decay, appear to be due to the general laws already sketched out, and to have little if any relation to the special requirements of each species. But flowers and fruits exhibit definite and well-pronounced tints, often varying from species to species, and more or less clearly related to the habits and functions of the plant. With the few exceptions already pointed out, these may be generally classed as attractive colors. The seeds of plants require to be dispersed so as to reach places favorable for germination and growth. Some are very minute, and are carried abroad by the wind, or they are violently expelled and scattered by the bursting of the containing capsules. Others are downy or winged, and are carried long distances by, the gentlest breeze. But there is a large class of seeds which cannot be dispersed in either of these ways, and are mostly contained in eatable fruits. These fruits are devoured by birds or beasts, and the hard seeds pass through their stomachs undigested, and, owing probably to the gentle heat and moisture to which they have been subjected, in a condition highly favorable for germination. The dry fruits or capsules containing the first two classes of seeds are rarely, if ever, conspicuously colored, whereas the eatable fruits almost invariably acquire a bright color as they ripen, while at the same time they become soft and often full of agreeable juices. Our red haws and nips, our black elderberries, our blue sloes and whortleberries, our white mistletoe and snow berry, and our orange sea-buckthorn, are examples of the colorsign of edibility; and in every part of the world the same phenomenon is found. The fruits of large forest trees, such as pines, oaks, and beeches, are not colored, perhaps because their size and abundance render them sufficiently conspicuous, and also because they provide such a quantity of food to such a number of different animals that there is no danger of their being unnoticed.
The colors of flowers serve to render them visible and recognizable by insects which are attracted by secretions of nectar or pollen. During their visits for the purpose of obtaining these products, insects involuntarily carry the pollen of one flower to the stigma of another, and thus effect cross-fertilization, which, as Mr. Darwin was the first to demonstrate, immensely increases the vigor and fertility of the next generation of plants. This discovery has led to the careful examination of great numbers of flowers, and the result has been that the most wonderful and complex arrangements have been found to exist, alt having for their object to secure that flowers shall not be selffertilized perpetually, but that pollen shall be carried, either constantly or occasionally, from the flowers of one plant to those of another. Mr. Darwin himself first worked out the details in orchids, primulas, and some other groups; and hardly less curious phenomena have since been found to occur, even among some of the most regularly-formed flowers. The arrangement, length, and position of all the parts of the flower is now found to have a purpose, and not the least remarkable portion of the phenomenon is the great variety of ways in which the same result is obtained. After the discoveries with regard to orchids, it was to be expected that the irregular, tubular, and spurred flowers should present various curious adaptations for fertilization by insect agency. But even among the open, cupshaped, and quite regular flowers, in which it seemed inevitable that the pollen must fall on the stigma, and produce constant selffertilization, it has been found that this is often prevented by a physiological variation — the anthers constantly emitting their pollen either a little earlier or a little later than the stigma of the same flower, or of other flowers on the same plant, were in the best state to receive it; and as individual plants in different stations, soils, and aspects, differ somewhat in the time of flowering, the pollen of one plant would often be conveyed by insects to the stigmas of some other plant in a condition to be fertilized by it. This mode of securing cross-fertilization seems so simple and easy. that we can hardly help wondering why it did not always come into action, and so obviate the necessity for those elaborate, varied, and highly complex contrivances found in perhaps the majority of colored flowers. The answer to this of course is, that variation sometimes occurred most freely in one part of a plant's organization, and sometimes in another, and that the benefit of cross-fertilization was so great that any variation that favored it was preserved, and then formed the startingpoint of a whole series of further variations, resulting in those marvellous adaptations for insect fertilization, which have given much of their variety, elegance, and beauty to the floral world. For details of these adaptations we must refer the reader to the works of Darwin, Lubbock, Herman Müller, and others. We have here only to deal with the part played by color, and by those floral structures in which color is most displayed.
* Trees and Shrubs for English Plantations, by Augustus Moogredien. Murray, 1870.The sweet odors of flowers, like their colors, seem often to have been developed as an attraction or guide to insect fertilizers, and the two phenomena are often complementary to each other. Thus, many inconspicuous flowers, like the mignonette and the sweet-violet, can be distinguished by their odors before they attract the eye, and this may often prevent their being passed unnoticed; while very showy flowers, and especially those with variegated or spotted petals, are seldom sweet. White, or very pale flowers, on the other hand, are often excessively sweet, as exemplified by the jasmine and clematis; and many of these are only scented at night, as is strikingly the case with the nightsmelling stock, our butterfly orchis (Habenaria chlorantha), the greenish-yellow Daphne pontica, and many others. These white flowers are mostly fertilized by night-flying moths, and those which reserve their odors for the evening probably escape the visits of diurnal insects which would consume their nectar without effecting fertilization. The absence of odor in showy flowers and its preponderance among those that are white, may be shown to be a fact by an examination of the lists in Mr. Mongredien's work on hardy trees and shrubs.* He gives a list of about one hundred and sixty species with showy flowers, and another list of sixty species with fragrant flowers; but only twenty of these latter are included among the showy species, and these are almost all white-flowered. Of the sixty species with fragrant flowers, more than forty are white, and a number of others have greenish, yellowish, or dusky and inconspicuous flowers. The relation of white flowers to nocturnal insects is also well shown by those which, like the evening primroses, only open their large white blossoms after sunset. The red Martagon lily has been observed by Mr. Herman Müller to be fertilized by the hummingbird hawk moth, which flies in the morning and afternoon when the colors of this flower, exposed to the nearly horizontal rays of the sun, glow with brilliancy, and when it also becomes very sweet-scented.
To the same need of conspicuousness the combination of so many individually small flowers into heads and bunches is probably due, producing such broad masses as those of the elder, the gueldre-rose, and most of the umbelliferæ, or such elegant bunches as those of the lilac, laburnum, horse-chestnut, and wistaria. In other cases minute flowers are gathered into dense heads, as with Globularia, Jasione, clover, and all the compositæ and among the latter the outer flowers are often developed into a ray, as in the sunflowers, the daisies, and the asters, forming a starlike compound flower, which is itself often produced in immense profusion.
*Nature, vol. xi., pp. 32. 110The beauty of alpine flowers is almost proverbial. It consists either in the increased size of the individual flowers as compared with the whole plant, in increased intensity, of color, or in the massing of small flowrs into dense cushions of bright color; and it is only in the higher Alps, above the limit of forests and upwards towards the perpetual snow-line that these characteristics are fully exhibited. This effort at conspicuousness under adverse circumstances may be traced to the comparative scarcity of winged insects in the higher regions, and to the necessity for attracting them from a distance. Amid the vast slopes of debris and the huge masses of rock so prevalent in higher mountain regions, patches of intense color can alone make themselves visible and serve to attract the wandering butterfly from the valleys. Mr. Herman Müller's careful observations have shown, that in the higher Alps bees and most other groups of winged insects are almost wanting, while butterflies are tolerably abundant; and he has discovered, that in a number of cases where a lowland flower is adapted to be fertilized by bees, its alpine ally has had its structure so modified as to be adapted for fertilization only by butterflies.* But bees are always (in the temperate zone) far more abundant than butterflies, and this will be another reason why flowers specially adapted to be fertilized by the latter should be rendered unusually conspicuous. We find, accordingly, the yellow primrose of the plains replaced by pink and magenta-colored alpine species the straggling wild pinks of the lowlands by the masses of large flowers in such mountain species as Dianthus alpinus and D. glacialis; the saxifrages of the high Alps with bunches of flowers a foot long, as in Saxifraxa longifolia and S. cotyledon, or forming spreading masses of flowers, as in S. oppositifolia; while the soapworts, silenes, and louseworts are equally superior to the allied species of the plains.
*Nature, vol. ic., p.164.Again, Dr. Müller has discovered that when there are showy and inconspicuous species in the same genus of plants, there is often a corresponding difference of structure, those with large and showy flowers being quite incapable of self-fertilization, and thus depending for their very existence on the visits of insects; while the others are able to fertilize themselves should insects fail to visit them. We have examples of this difference in Malva sylvestris, Epilobium angustifolium, Polyganum historta, and Geranium pratense,— which have all large or showy flowers and must be fertilized by insects, — as compared with Malva rotundifolia, Epilobium parviflorum, Polygonum aviculare, and Geranium pusilium, which have small or inconspicuous flowers, and are so constructed that if insects should not visit them they are able to fertilize themselves.*
As supplementing these curious facts showing the relation of color in flowers to the need of the visits of insects to fertilize them, we have the remarkable, and on any other theory utterly inexplicable circumstance, that in all the numerous cases in which flints are fertilized by the agency of the wind they never have specially colored floral envelopes. Such are our pines, oaks, poplars, willows, beeches, and hazel; our nettles, grasses, sedges, and many others. In some of these the male flowers are, it is true, conspicuous, as in the catkins of the willows and the hazel, but this arises incidentally from the masses of pollen necessary to secure fertilization, as shown by the entire absence of a corolla or of those colored bracts which so often add to the beauty and conspicuousness of true flowers.
*See Nature, September 6th, 1876.The adaptation of flowers to be fertilized by insects — often to such an extent that the very existence of the species depends upon it — has had widespread influence on the distribution of plants and the general aspects of vegetation. The seeds of a particular species may be carried to another country, may find there a suitable soil and climate, may grow and produce flowers, but it the insect which alone can fertilize it should not inhabit that country, the plant cannot maintain itself, however frequently it may be introduced or however vigorously it may grow. Thus may probably be explainea the poverty in flowering plants and the great preponderance of ferns that distinguishes many oceanic islands, as well as the deficiency of gailycolored flowers in others. This branch of the subject is discussed at some length in my address to the Biological Section of the British Association,* but I may here just allude to two of the most striking cases. New Zealand is, in proportion to its total number of flowering plants, exceedingly poor in handsome flowers, and it is correspondingly poor in insects, especially in bees and butterflies, the two groups which so greatly aid in fertilization. In both these aspects it contrasts strongly with southern Australia and Tasmania in the same latitudes, where there is a profusion of gaily-colored flowers and an exceedingly rich insect fauna. The other case is presented by the Galapagos Islands, which, though situated on the equator off the west coast of South America, and with a tolerably luxuriant vegetation in the damp mountain zone, yet produce hardly a single conspicuously-colored flower; and this is correlated with, and no doubt dependent on, an extreme poverty of insect life, not one bee and only a single butterfly having been found there.
Again, there is reason to believe that some portion of the large size and corresponding showiness of tropical flowers is due to their being fertilized by very large insects and even by birds. Tropical sphinx moths often have their probosces nine or ten inches long, and we find flowers whose tubes or spurs reach about the same length; while the giant bees, and the numerous flowersucking birds, aid in the fertilization of flowers whose corollas or stamens are proportionately large.
I have now concluded this sketch of the general phenomena of color in the organic world. I have shown reasons for believing that its presence, in some of its infinitely varied hues, is more probable than its absence, and that variation of color is an almost necessary concomitant of variation of structure, of development, and of growth. It has also been shown how color has been appropriated and modified both in the animal and vegetable world, for the advantage of the species in a great variety of ways, and that there is no need to call in the aid of any other laws than those of organic development and "natural selection" to explain its countless modifications. From the point of view here taken it seems at once improbable and unnecessary that the lower animals should have the same delicate appreciation of the intl. nite variety and beauty of the delicate contrasts and subtle harmonies of color — which are possessed by the more intellectual races of mankind, since even the lower human races do not possess it. All that seems required in the case of animals, is a perception of distinctness or contrast of colors; and the dislike of so many creatures to scarlet may perhaps be due to the rarity of that color in nature, and to the glaring contrast it offers to the sober greens and browns which form the general clothing of the earth's surface.
The general view of the subject now given must convince us that, so far from color being — as it has sometimes been thought to be — unimportant, it is intimately connected with the very existence of a large proportion of the species of the animal and vegetable worlds. The gay colors of the butterfly and of the alpine flower which it unconsciously fertilizes while seeking for its secreted honey, are each beneficial to its possessor, and have been shown to be dependent on the same class of general laws as those which have determined the form, the structure, and the habits of every living thing. The complex laws and unexpected relations which we have seen to be involved in the production of the special colors of flower, bird, and insect, must give them an additional interest for every thoughtful mind; while the knowledge that, in all probability, each style of coloration, and sometimes the smallest details, have a meaning and a use, must add a new charm to the study of nature.
Throughout the preceding discussion we have accepted the subjective phenomena of color — that is, our perception of varied hues, and the mental emotions excited by them — as ultimate facts needing no explanation. Yet they present certain features well worthy of attention, a brief consideration of which will form a fitting sequel to the present essay.
The perception of color seems, to the present writer, the most wonderful and the most mysterious of our sensations. Its extreme diversities and exquisite beauties seem out of proportion to the causes that are supposed to have produced them, or the physical needs to which they minister. If we look at pure tints of red, green, blue, and yellow, they appear so absolutely contrasted and unlike each other, that it is almost impossible to believe (what we nevertheless know to be the fact) that the rays of light producing these very distinct sensations differ only in wavelength and rate of vibration; and that there is from one to the other a continuous series and gradation of such vibrating waves. The positive diversity we see in them must then depend upon special adaptations in ourselves; and the question arises — for what purpose have our visual organs and mental perceptions become so highly specialized in this respect ? When the sense of sight was first developed in the animal kingdom, we can hardly doubt that what was perceived was light only, and its more or less complete withdrawal. As the sense became perfected, more delicate gradations of light and shade would be perceived; and there seems no reason why a visual capacity might not have been developed as perfect as our own, or even more so, in respect of light and shade, hut entirely insensible to differences of color, except in so far as these implied a difference in the quantity of light. The world would in that case appear somewhat as we see it in good stereoscopic photographs; and we all know how exquisitely beautiful such pictures are, and how completely they give us all requisite information as to form, surface-texture, solidity, and distance, and even to some extent as to color; for almost all colors are distinguishable in a photograph by some differences of tint, and it is quite conceivable that visual organs might exist which would differentiate what we term color by delicate gradations of some one characteristic neutral tint. Now such a capacity of vision would be simple as compared with that which we actually pos. secs; which, besides distinguishing infinite gradations of the quantity of light, distinguishes also, by a totally distinct set of sensations, gradations of quality, as determined by differences of wave-lengths or rate of vibration. At what grade in animal development this new and more complex sense first began to appear we have no means of determining. The fact that the higher vertebrates, and even some insects, distinguish what are to us diversities of color, by no means proves that their sensations of color bear any resemblance whatever to ours. An insect's capacity to distinguish red from blue or yellow may be (and probably is) due to perceptions of a totally distinct nature, and quite unaccompanied by any of that sense of enjoyment or even of radical distinctness which pure colors excite in us. Mammalia and birds, whose structure and emotions are so similar to our own, do probably receive somewhat similar impressions of color; but we have no evidence to show that they experience pleasurable emotions from color itself when not associated with the satisfaction of their wants or the gratification of their passions.
* There is reason to believe that our capacity of distinguishing colors has increased even in historical times. The subject has attracted the attention of German philologists, and I have been furnished by a friend with some notes from a work of the late Lazarus Geiger entitled. "Zur Entwickelungsgeschichte der Menschheit" (Stuttgart, 1871). According to this writer it appears that the color of grass and foliage is never alluded to as a beauty in the Vedas or the Zendavesta, though these productions are continually extolled for other properties. Blue is described by terms denoting sometimes green, sometimes black, showing that it was hardly recognized as a distinct color. The color of the sky is never mentioned in the Bible, the Vedas, the Homeric poems, or even in the Koran. The first distinct allusion to it known to Geiger is in an Arabic work of the ninth century. "Hyacinthine locks" are black locks, and Homer calls iron "violet-colored." Yellow was often confounded with green, but, along with red, it was one of the earliest colors to receive a distinct name. Aristotle names three colors in the rainbow - red, yellow, and green. Two centuries earlier Xenophanes had described the rainbow as purple, reddish, and yellow. The Pythagoreans admitted four primary colors — white, black, red, and yellow; the Chinese the same, with the addition of green. If these statements fairly represent the early condition of color-sensation they well accord with the view here maintained, that green and blue were first alone perceived, and that the other colors were successively separated from them. These latter would be the first to receive names; hence we find purple, reddish, and yellow first noticed in the rainbow as the tints to be separated from the widespread blue and green of the visible world which required no distinctive color-appellation. If the capacity of distinguishing colors has increased in historic times, we may perhaps look upon color-blindness as a survival of a condition once almost universal; while the fact that it is still so prevalent is in harmony with the view that our present high perception and appriciation of color is a comparatively recent acquisition, and may be correlated with a general advance in mental activity.The primary necessity which led to the development of the sense of color, was probably the need of distinguishing objects much alike in form and size, but differing in important properties; such as ripe and unripe, or eatable and poisonous fruits; flowers with honey or without; the sexes of the same or of closely allied species. In most cases the strongest contrast would be the most useful, especially as the colors of the objects to be distinguished would form but minute spots or points when compared with the broad masses of tint of sky, earth, or foliage against which they would be set. Throughout the long epochs in which the sense of sight was being gradually developed in the higher animals, their visual organs would be mainly subjected to two groups of rays — the green from vegetation, and the blue from the sky. The immense preponderance of these over all other groups of rays would naturally lead the eye to become specially adapted for their perception; and it is quite possible that at first these were the only kinds of lightvibrations which could be perceived at all. When the need for differentiation of color arose, rays of greater and of smaller wavelengths would necessarily be made use of to excite the new sensations required; and we can thus understand why green and blue form the central portion of the visible spectrum, and are the colors which are most agreeable to us in large surfaces; while at its two extremities we find yellow, red, and violet, colors which we best appreciate in smaller masses, and when contrasted with the other two or with light neutral tints. We have here probably the foundations of a natural theory of harmonious coloring, derived from the order in which our color-sensations have arisen, and the nature of the emotions with which the several tints have been always associated.* The agreeable and soothing influence of green light may be in part due to the green rays having little heating power; but this can hardly be the chief cause, for the blue and violet, though they contain less heat, are not generally felt to be so cool and sedative. But when we consider how dependent are all the higher animals on vegetation, and that man himself has been developed in the closest relation to it, we shall find, probably, a sufficient explanation. The green mantle with which the earth is overspread caused this one color to predominate over all others that meet our sight, and to be almost always associated with the satisfaction of human wants. Whcrc the grass is greenest, and vegetation most abundant and varied, there has man always found his most suitable dwellingplace. In such spots hunger and thirst are unknown, and the choicest productions of nature gratify the appetite and please the eye. In the greatest heats of summer, coolness, shade, and moisture are found in the green forest glades; and we can thus understand how our visual apparatus has became especially adapted to receive pleasurable and soothing sensations from this class of rays.
The preceding considerations enable us to comprehend, both why a perception of difference of color has become developed in the higher animals, and also why colors require to be presented or combined in varying proportions in order to be agreeable to us. But they hardly seem to afford a sufficient explanation, either of the wonderful contrasts and total unlikeness of the sensations produced in us by the chief primary colors, or of the exquisite charm and pleasure we derive from color itself, asdistinguished from variouslycolored objects, in the case of which association of ideas comes into play. It is hardly conceivable that the material uses of color to animals and to ourselves required such very distinct and powerfully contrasted sensations; and it is still less conceivable that a sense of delight in color per se should have been necessary for our utilization of it.
The emotions excited by color and by music, alike, seem to rise above the level-of a world developed on purely utilitarian principles.
- Alfred R. Wallace.
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