INSECTS AND FLOWERS.
Now that we have examined a few amongst the various modes by which waves of æther, as a whole, may affect the sensory system of animals, let us turn at last to our more proper subject, and inquire by what steps the different kinds of æther-waves came to be differentially cognised in consciousness as red, green, yellow, or blue. We find ourselves now face to face with the ultimate problem which we have determined, if possible, to solve in the present volume. We must see what immediate advantage animals could gain from the possession of a nervous structure capable of that differential stimulation by the diverse varieties of light which we know subjectively as the colour-sense.
There are two great classes of animals amongst which the existence of a colour-sense is most certain, and its reactions upon the external world most conspicuous. These are the articulates and the vertebrates. The first class affords us the beautiful butterflies, beetles, and crustaceans; the second gives us the golden coats of fish and lizards, the exquisite plumage of tropical birds, and the striped or dappled skins of the fur-bearing mammals. To the first we owe the existence of flowers, to the second we must refer the colours of all bright-hued seeds and fruits. Accordingly, for every practical purpose, we may narrow down our inquiry to the consideration of these two great classes; and amongst the articulates, the division which most obviously calls for special notice is that of insects. So our first task must be to account for the existence of a colour-sense in the insect eye, and to discover what were the objects for the sake of which this mode of perception was developed?
Clearly the inorganic world does not offer us any chance of a satisfactory solution. The dull clay and grey-blue rocks have none of the brilliancy and purity which is needed as the groundwork for a first differential stimulation. Such complex wave-systems as they reflect would be too mixed, too confused, too indefinite, too variable to afford any means of clear recognition by an early half-developed sense. And even if it were otherwise, the insect does not need to trouble itself about the chemical or mineralogical character of the ground upon which it crawls or alights. A few rare inorganic bodies do indeed possess the requisite simplicity and richness for the supposed first stimulus, as we see in the ruby, the emerald, the sapphire, and the topaz; nay, some much commoner substances, such as red sandstone and blue granite, are endowed with a moderately bright and pure colour. All these bodies, however, lie open to the fatal objection that they do not in any way contribute to the welfare of the insect or animal which looks upon them. A sense highly developed by other means may (as we see in the savage or barbarous love for precious stones) be agreeably exercised upon such objects; but in order to give that sense its first start, some direct advantage must be secured by the new mode of discrimination, either in the pursuit of food, the search for mates, or the avoidance of enemies. No faculty can possibly be originally developed for the sake of mere useless exercise upon unessential acts; although, as we shall see when we come to examine the æsthetic value of colour, each faculty, when once fully established, admits of immense pleasurable extension by being directed towards such secondary ends.
The same line of argument applies to those occasional displays of colour which are due to transient effects of sunlight, through the medium of a refractive vapour. Long before the first insect vision learnt to discriminate between red, yellow, and blue, the various rays which we call by those names poured down unnoticed upon the primeval world. Then, as now, the rainbow scattered ten thousand colours upon the dull grey clouds, but no eye drank in the diverse stimulation from its gorgeous undertones of melting orange and exquisite green. Then, as now, the sunset crimsoned the west with dying glory, and bathed the horizon in floods of golden light, but no living thing beheld its loveliness or revelled in its changeful wealth. Such distant and exceptional displays could have little or no effect upon the life of tiny creatures that picked out in fear and trembling a precarious livelihood amid palæozoic forest shades. Even our own nearest mammalian relatives seem totally unconcerned with regard to the magnificent pictures which are spread nightly before their eyes in tropical plains. Indeed, the savage members of our own race, or even the stolid peasantry of European countries, appear to notice such useless phenomena with little curiosity or admiration. Part of our business in this work will be to trace out the slow steps by which the love of bright-coloured food led on to the choice of bright-coloured mates; and how this again brought about a liking for bright colours in general, which shows itself in the savage predilection for brilliant dyes and glistening pebbles; till at last the whole long series culminates in that intense and unselfish enjoyment of rich and pure tints which makes civilised man linger so lovingly over the hues of sunset and the myriad shades of autumn. But if even the lower types of humanity are little stirred by such glowing sights, how could we expect the humble insect to have developed a new sense for their perception? Here, as elsewhere, the disinterested affection can only be reached by many previous steps of utilitarian progress.
It is to the organic world, then, the insect’s practical world of food and prey, that we must look for the first development of the colour-sense. In the origin of flowers we shall find also the origin of the insect’s perception; and though the inquiry will seem at first to lead us rather far afield from our proper path, I think we cannot do better than steadily set to work at unravelling the tangled thread of their mutual influence. In order to do so effectively we must first glance at the condition of the world before flowers, fruit, or colour-sense had yet begun the first stages of their reciprocal existence.
Brongniart long ago pointed out that the periods of geological vegetation fall into three main divisions, which he called the reigns of acrogens, of gymnosperms, and of angiosperms. Acrogens are plants like ferns and mosses, which bear no fruits or flowers, but produce their young by means of spores. Gymnosperms are plants like pines and firs, which have their blossoms and seeds in dry scaly cones. Angiosperms are true flowering plants, often bearing bright bells or brilliant clusters of bloom, and always having their seeds enclosed in some more or less conspicuous form of enveloping fruit. These three kinds of plants succeeded each other through the geological series in the order here assigned to them — members of every class still surviving on our earth, but outnumbered and overlived, as a rule, by those of the newer and more successful classes.
For our present purpose, however, we might say more truly that the great epochs of vegetal life naturally fall under three similar heads — the reign of flowerless plants, the reign of wind-fertilised flowering plants, and the reign of insect-fertilised flowering plants. These three heads correspond in the main with Brongniart’s divisions, but they serve to bring more clearly into prominence the salient functional facts with which we are here especially concerned.
Flowerless plants, or cryptogams, are those which have no conspicuous organs for the production of seeds or fruits. Their chief varieties are known to all as fungi, sea-weeds, mosses, and ferns. Flowering plants, or phanerogams, are those which possess more or less conspicuous organs for the production of seeds and fruits. They may be divided, not structurally but functionally, into two great sub-classes — the anemophilous and the entomophilous. Anemophilous plants include all species in which the pollen of the male flower is wafted to the stigma of the female flower by means of the wind. Entomophilous plants include all species in which the pollen is carried to the stigma upon the head, legs, or bodies of insects. Each of these classes possesses numerous species in which various modifications have been produced to aid fertilisation by the appropriate means. Some of these modifications we shall examine as we proceed; for others, the inquirer must be referred to special works upon the subject.
In the great forests of the Carboniferous era few or no flowers diversified the unbroken green of the primeval world. Almost all the plants which raised their heads above the dark mould of those forgotten deltas were acrogens or other cryptogams. Like the ferns and mosses of our own epoch, they reproduced their kind, not by means of flowers and seeds, but by inconspicuous little spores, each of which rooted itself on the ground independently, and grew into a young plant. Many of them resembled the bristling horse-tails of modern waste lands, magnified a hundred-fold, so as to present the appearance of huge jointed trees, to which geologists have given the name of calamites. Others were rather the gigantic analogues of our creeping club-mosses, with monstrous thickened stalks, clad in a sort of plated armour, and known to science as sigillariæ, lepidodendra, and haloniæ. Yet others, again, grew like the tree-ferns of our latter-day tropics, with graceful waving fronds, whose delicate outline is still faithfully preserved on the flat surface of many a coal-seam. But amongst them all not a single bright blossom anywhere displayed its crimson petals or its golden bells; not a single fruit gleamed red or orange among the embowering foliage. Green, and green, and green once more; wherever the eye of an imaginary visitor could turn, it would have rested on one unbroken sea of glistening verdure.
A few phanerogams there were, it is true, among this mass of cryptogamic vegetation, but they belonged entirely to the pine and cycad families, which grow their seed in hard scaly cones, and would never be included amongst the flowering plants by any but a technical botanist. Moreover, as their blossoms are green when young and brown when ripe, they would form little or no exception to the prevailing tints of the palæozoic world. Even if a few primitive grasses of some archaic form intermingled, as is possible, with the mosses and liverworts which carpeted the ground beneath the conifers, the tree-ferns, and the titanic lycopodites, yet these themselves would bear their seed in green panicles on a waving stem, and would still add no new element of colour to the one monotonous field. Not a trace of any vegetal organism has yet been discovered in the primary rocks to which we can even conjecturally attribute a possible tinge of red or orange, blue or yellow, in the form of flowers or fruit.
Equally unvaried, no doubt, was the hue of the articulate creatures which fed amid those green jungles of tangled fern and club-moss. A few scorpion-like insects, an occasional cockroach, beetle, or other uncanny creeping thing, may still be detected in the débris of a forgotten world; but no trace of a bee, a moth, or a joyous butterfly can be discovered in these earliest ages of animal life. Scarlet berry and crimson blossom, gorgeous bird and painted insect, were all equally absent from the unvaried panorama of green overhead and brown beneath.
Such, we may suppose, was the general appearance of our earth’s surface before the colour-sense had given rise to all the diverse wealth of hues which gladden the woodland and the meadow for modern eyes. First to be developed among the bright-coloured objects of the newer era were the brilliant whorls of abortive leaves which ordinary people know as flowers. Their origin affords us the key to all the subsequent evolution both of coloured organisms and of the sense whereby they are perceived.
The transition from the wholly green and spore-bearing cryptogams to the bright hues of entomophilous blossoms was through the intermediate stage of anemophilous plants. Already, in the Carboniferous era of the palæozoic world, we have seen that these organisms had begun to exist. The causes which led to their development throw so much light upon the subsequent evolution of insect-fertilised species, that we must pause for a moment to examine the history of their first appearance.
Every individual cryptogamic plant produces spores or young individuals by its own unaided generative power. It needs no co-operation from a partner of a different sex to fertilise the embryo germs which it puts forth. True, a male and a female element may always be discovered within the plant itself; but their occurrence does not militate against the general statement that cryptogamic reproduction is essentially hermaphrodite or non-sexual in its character. For real sexual generation consists fundamentally in this, that two independent parents combine to produce a brood of young, partaking equally, on the average, of the idiosyncrasies of each. Now, Mr. Darwin has shown that whenever an organism is the result of interaction between two anterior organisms, it possesses a vigour, a plasticity, and a hardiness which enable it to thrive far more easily than any similar organism resulting from the generative action of a single parent. Our great teacher has proved that self-fertilised flowers produce relatively weak, puny, and unhealthy young; while cross-fertilised flowers produce relatively strong, hearty, and vigorous young. The general principle upon which this effect depends has been exposed, with his usual luminous insight, by Mr. Herbert Spencer; but unfortunately its explanation would involve too many wide questions of biological theory for reproduction here.
If, then, by any special combination of circumstances, it should happen at any time that certain plants acquired the habit of fertilising the female element in one individual by the male element of another, it would necessarily result that such plants would produce exceptionally healthy young, and so gain unusual advantages for their descendants in the struggle for life. And this is exactly the case with flowering as opposed to flowerless plants. While the latter still continue to fertilise themselves in every instance, the former possess a special set of male and female organs, often situated on different individuals, and almost always so disposed that the pollen of any particular flower is specially prevented from quickening the ovules of its own pistil. Indeed, the very effect which Mr. Darwin’s experiments show us on a small scale, nature herself here shows us on a large scale; for when once the flowering plants had been introduced into the world, their superior vitality gave them such an increased chance in the struggle for life that they have now overrun the whole earth, and almost lived down the very memory of their cryptogamic predecessors, whose huge forms diversified the landscape of a palæozoic wild. Step by step, throughout the secondary and tertiary periods, we find the acrogens decreasing in number of species, in frequency of individuals, in size and height; while step by step we find the flowering plants dispossessing them over the whole world, and growing into more and more varied forms, with ever-increasing numbers and ever-widening girth; until at last forest trees, and herbs, and grasses cover the face of hill, and plain, and valley; while only in a few tropical jungles, or a stray patch of neglected English warren, do we still discover the degenerate descendants of those giant tree-ferns and horse-tails which flourished without a rival over vast continents during the earlier ages of vegetal life.
Among these flowering plants, which thus substituted the sexual for the hermaphrodite method of reproduction, the anemophilous or wind-fertilised division was the first to appear. The ever-moving currents of air naturally offer the earliest and simplest agency for the dispersion and transference of pollen from the stamens of one blossom to the pistil of another. Accordingly, we find the pines and cycads, both of which bear their flowers in the form of cones or other unnoticeable bunches of floral organs, as the earliest representatives of flowering plants. After them, in geological order, come the monocotyledons, represented by grasses, rushes, and other spiky species, whose blossoms assume the shape of green panicles or waving plumes; while, last of all, come the dicotyledons, whose anemophilous varieties are usually distinguished by those pretty hanging clusters of stamens or pistils which we know as catkins. Now, in all these cases, the mature male organs, covered with the fertilising pollen, necessarily protrude from the scales, sheathes, or glumes which guard their younger stage, and offer large surfaces to the wind, whose aid they require in the dispersion of their stores. Similarly, the pistils or female organs must possess spreading and feathery stigmatic processes, wherewith to catch any stray grains of pollen which the unconscious wind may waft to their neighbourhood. Hence these blossoms consist usually of bundles or masses of male and female organs, hanging out in such a way as to secure the favour of every passing breeze; but they never possess those bright whorls of coloured leaves which make up the popular idea of a flower. The latter notion is mainly based upon the peculiar structure of entomophilous plants.
As wind-fertilised flowers can only hope that a small fraction of their pollen will reach the stigmatic surface of their brides, and there be drunk in to fertilise the embryo within, they must needs produce enormous quantities of useless material, to be dissipated by the storm in every direction. The amount of pollen thus wasted is often incredibly great. The floor of a pine forest during the flowering season frequently lies thickly covered with the rich yellow dust that cost so much useless energy to the parent plant. Occasionally even showers of pollen grains have been noted in the neighbourhood of great forests or of fields thickly sown with anemophilous species. At Mumbles, near Swansea, a yellow-coloured rain fell in 1850, and left large spots of ochre-like matter, which proved on close examination to consist of willow-pollen. Similar showers, produced by the Canadian conifers, have often been observed along the shore of the great lakes, and others have taken place in Zurich, in Brunswick, and in Inverness-shire. Of course, the loss of energy which this waste expenditure involves for the parent plant must necessarily be very great; and any change in its circumstances which produced a more economical mode of applying the pollen to the pistil would naturally result in a saving of material, and so give the plants in which it occurred a fresh advantage over their less fortunate compeers. Such a change we see in the utilisation of insect agency by the entomophilous plants.
Even as early as the Carboniferous period we find traces of terrestrial articulates which might have sought their food among the few coniferous blossoms of that mainly flowerless world. Most of the plants about them were hard, scaly, innutritious acrogens, whose stem and leaves contained large quantities of silica, as we still see to be the case in the horse-tail family, their nearest modern allies. But the stray flowering species which grew at rare intervals in the midst of the calamites and lepidodendra must have offered special attractions to insects (or their undifferentiated ancestors) in the shape of soft, edible, nutritious pollen. And as the insects travelled from one flower to another, carrying on their legs or heads the fertilising powder, they would supply the plant with a cheaper and more certain means of impregnation than that afforded by the wasteful wind. Accordingly, any plants which offered special advantages to insects, in the shape of pollen, sweet juices, or soft edible matter, would thus obtain an extra chance in the general competition for a share of the earth’s surface, and hand down the peculiarity to an ever-increasing brood of descendants. So, when once the entomophilous flora began to exist, it gained ground rapidly on the anemophilous division, as the anemophilous flora had previously gained ground on the flowerless plants, until in our own day the two divisions divide the world between them; while in the future, doubtless, the balance will be still further disturbed in favour of the younger and more vigorous races.
Of course, the change from fertilisation by wind to fertilisation by insects could not be accomplished without many structural modifications, whereby the flower became adapted to the new and more specialised agency thus afforded it. Some of these modifications were concerned with the food offered by the flower to the fertilising insect. At first this food doubtless consisted of pollen alone, but after a time there was added that sweet matter known as sugar or honey, which is contained more or less in most plants, and which is especially developed during the two processes of flowering and fruiting. Now sugar, by its crystalline condition, so rare amongst energy-yielding organic products, seems specially adapted for affording pleasurable stimulation to the gustatory nerves of animals; and it has therefore been stored up by plants in all those cases where the attraction of some animal ally is desirable for their protection or the continuance of their species. Certain plants lay it by in glands upon their stems to allure the harmless ants who protect them from the ravages of their leaf-cutting congeners. Others, again, as we shall see in a later chapter, collect it in the pulpy covering of their seed-vessels, and thus induce parrots, pigeons, or monkeys to devour and distribute the undigested kernel. Yet others distil it in the recesses of their blossoms, and so tempt the bee, the butterfly, or the humming-bird to rifle their labyrinthine storehouses, and unconsciously aid in the impregnation of their embryos. The honey thus elaborated by the flower has become at last the main ultimate attraction for all fertilising insects, whose most specialised forms we find in the common hive-bees of domestic economy.
A second class of modifications is connected with the shape of the flower. Most entomophilous blossoms possess, in addition to the pistil and the stamens, two other whorls of floral leaves — the corolla and the calyx. In the simplest form of flower, these whorls consist of separate leaves (petals and sepals), as we see in the buttercup or the dog-rose. But in certain more specialised flowers the long-continued action of the insect-fertilisers has unconsciously selected those blossoms which most easily suited themselves to the form of their visitor, and has thus produced a united corolla, all whose petals are joined into a regular tube or cup, as we see in the Canterbury bell, the convolvulus, and the lily of the valley. A number of these tubular flowers united form the head of the daisy, the marigold, and the sunflower. In still more specialised cases the cup becomes irregularly lobed, so as to suit still more closely the shape of its insect friend — a change whose first steps we see in honeysuckle and foxglove, while its completed stage is shown in mint, dead-nettle, snapdragon, lobelia, and orchids. All these varieties of entomophilous flowers we shall have to examine at greater length hereafter.
But the third class of modifications, the important class for our present subject, is that which refers to the colouring of flowers. By far the most conspicuous difference between entomophilous and anemophilous blossoms is the difference of colour. While wind-fertilised plants have seldom more than a few brownish scales or tiny green sepals around their fructifying organs, insect-fertilised plants are almost always distinguished by the growth of large, brilliantly-coloured petals outside the essential whorls, which act as guides and allurements to the eyes of bees or butterflies. These wide, expanded, and costly structures have absolutely no other purpose in the vegetal economy than that of attracting the fertilising agents; and they afford by themselves a strong presumption of developed colour-sense in the creatures for whose guidance they have been slowly evolved. Let us see by what steps they can gradually have reached their present conspicuous dimensions.
We will suppose that some of the flowering plants in the early ages of the world showed some slight tendency to develop the various attractive structures which we now observe in their completed form. They offered to insects the soft and nourishing pollen, and perhaps, too, small quantities of stimulating saccharine matter. Such saccharine matter we know is always evolved during the opening of flowers, at least in small amounts, for the nurture of the blossom itself; and there is nothing extravagant in the supposition that occasional individuals might produce it in more than the average quantity, and so might attract more than the average share of insect attention. In like manner they may possibly have shown a tendency to develop bright-coloured leaves around their essential organs; and if the eyes of insects were capable of distinguishing these bright colours, in however imperfect a degree, it would naturally follow that the hues would go on deepening from generation to generation among the plants, while the perception would go on sharpening itself from generation to generation among the insects. For while the flowers which thus become more and more readily distinguishable by their fertilisers would thereby better secure the chance of descendants, the insects which most readily distinguished flowers would thereby secure for themselves the greatest amount of the available food-stores. So that, supposing such a tendency once set up on either side, we can see every reason why it should ultimately develop to its present noticeable extent.
This, however, is mere a priori hypothesis. The experimental philosopher will ask at once whether we have any grounds for believing that the tendency in point would ever be set up. I think we have such grounds in abundance, and although the question involves a little closer application and more technical considerations than any we have yet encountered, I shall ask the reader patiently to follow me through the exposition, because it really encloses the whole fundamental basis of the developed colour-sense in every terrestrial animal. The point which we have first to consider is this: Did flowers show an original tendency to the production of coloured adjuncts prior to the selective action of insects? And when we have answered that question we must proceed to the second one: Did insects possess any tendency vaguely to discriminate colours apart from the reactive influence of entomophilous flowers?
The solar radiations, falling upon the green portions of plants, are the sole ultimate source of all the energy existing in the animal or vegetal organism. Under their influence, the plant separates carbon and hydrogen from the oxygen with which they were originally combined, stores them up in some part of its own tissues, and turns the free oxygen adrift upon the atmosphere around. In this process, the kinetic or active energy of the solar undulations has assumed the potential or dormant form. The potential energy thus laid up is associated with the carbon and hydrogen of the plant on the one hand, and with the free oxygen of the atmosphere on the other. “Whenever they may recombine, the dormant energy will assume once more the active form, and be yielded up in the shape of mechanical motion, heat, or light.
This reconversion of contained energy into its mobile mode may be brought about in many ways. Sometimes the plant may be cut down and burnt, as we all see in wood-fires, and then the energy will be given out rapidly as heat and light, while part of it will also go off as motion of the surrounding air. Sometimes the plant may fall as it lies, be changed into peat or coal, and finally burnt, like the wood, in a human grate, with the same concomitant phenomena as in the first case. Sometimes, too, these same materials — wood, coal, peat — may be used to feed a steam-engine, and mainly converted into visible movements of the locomotive or its parts, which are finally dissipated by friction into the circumambient æther. In yet other cases, the plant may be eaten by an animal, and then its elements will recombine within his body with the free oxygen supplied by his lungs or gills, and will give off heat and motion, less conspicuously perhaps, but quite as truly as in the engine. There remains, however, another instance, fully as common as these, yet far less generally observed — the instance in which the elements recombine in the tissues of the living plant, and yield up their dormant energies in producing growth, development, and rearrangement of its parts. This metamorphosis of energy (known as Stoffwechsel or Metastasis) actually takes place in every active portion of a plant which does not itself assimilate nutritive material from the surrounding air. And all such parts of plants may be considered as carrying on essentially animal functions — that is to say, functions by which potential energy becomes kinetic, oxygen unites with carbon to form carbonic anhydride, heat is evolved, and motion is given out.
The most noticeable cases of such quasi-animal processes may be seen in the germination of seeds, the growth of bulbs and tubers, the unfolding of flowers, and the ripening of fruits. In fact, every growing and active part of a plant, unless it be itself assimilating kinetic energy from solar undulations, must necessarily be using up energies assimilated elsewhere. Otherwise, it would be manufacturing new energies for itself out of nothing, which we know to be impossible, and inconceivable as a direct contravention of all physical and mental laws.
Now, the active agent of deoxidation in ordinary plants is that peculiar compound substance known as chlorophyll, the pigment which gives a green colour to healthy leaves. Hence all the active organs of plants are usually green in hue, because the chlorophyll is seen through the transparent cell-walls of the epidermis. But there are reasons for believing that wherever the reverse process of metastasis is taking place, other bodies are frequently formed, which reflect the light in slightly different manners, and so give rise to tints of red, orange, yellow, pink, mauve, purple, or blue. We will examine the evidence in order, and see whether we can gather from it any inference as to the origin of coloured flowers.
In the first place, even in active leaves, the presence of green chlorophyll is often masked by the occurrence of other pigments, which give the foliage a tinge of brown, russet, scarlet, or golden yellow. Cases of this sort are commonly known in the copper beech, the red cabbage, and the various species of purple coleus, crimson-hearted caladium, pink dracæna, or pale mauve begonia. Here the colouring matter doubtless belongs to some one among the many by-products of vegetal physiology, which must necessarily occur from time to time in one part or another as the results of assimilative or metastatic changes. But in the more noticeable cases of coloured juices or pigments, other than green, we shall find that the special colouring matter is almost always more or less connected with those portions of the plant where energy is being liberated, and where accordingly oxidation is necessarily taking place.
The only class of plants in which green rarely — we might almost say never — occurs, is that of fungi. But fungi differ from all other plants (except a few parasites and saprophytes) and agree with animals in this, that they derive their energies not directly from solar undulations, but from organised matter already contained in the soil or matrix on which they grow. And there is something in the vivid orange, yellow, lilac, and crimson of their hues, as well as in the pasty whiteness of their common tissues, which strikingly recalls the possibly adventitious colouring of the lower animal forms, such as sea-anemones, star-fish, and medusæ. This analogy, as we shall note hereafter, is not without a deep significance for our present purpose.
When, however, we go on to those plants which have normally green leaves, we see a like result. In the first place, dying leaves, as we all know, assume the most brilliant tints of red, yellow, orange, and brown. Even in our own damp and uncertain climate, the Virginia creeper glows with the richest crimson, while the forest trees shade off into delicate tones of golden gloss and occasional flashes of deep scarlet light. But in American woodlands these displays assume grander dimensions and more glorious beauties, forming perhaps the most magnificent fields of gorgeous colour in the whole organic world. Now, Macaire-Princep has shown, that as leaves begin to turn yellow they give up the function of deoxidation, while a reverse process at once sets in. Mr. Sorby traces the gradual loss of vitality in the fading leaf from bright green through greenish-brown, red, scarlet, and orange-brown to the final dull and dingy hue of the dry leaf. That this change results from some degradation of energy, in whatever component of the leaf it may take place, is beyond all doubt.
Any injury to leaves causes similar-effects, whether due to disease, external impact, or the attacks of insects. Gall-nuts and rose-blights have generally an outer coating of small reddish excrescences, while feeble plants produce yellow-spotted or pink-speckled leaves. Here, too, oxidation, or some other de-energising action, is most probably the cause of the change observed.
Leaves which have given up their natural functions frequently assume bright hues. Thus the pitchers of the side-saddle flower (Sarracenia rubra) have purple tips. Those of the pitcher-plant (Nepenthe) are “tinted and mottled with red and purple.” The leaves of the curious insectivorous plants, with whose habits Mr. Darwin has made us so familiar, are apt to be speckled with similar hues, especially in the active portions, which show, by their movements and secretions, some approach to animal functions. The common English sundew (Drosera rotundifolia), which may be found in all boggy or peaty places, has bright red glands scattered over its leaves. The Venus fly-trap (Dionæa muscipula) is “thickly covered with minute glands of a reddish or purplish colour,” while the spikes which close upon the insect prey have small projections described as “reddish-brown or orange.” Like organs in Drosophyllum lusitanicum are bright pink, and in Pinguicula lusitanica purple. Our own common butterwort and saxifrage, which share to a less extent the same peculiarity, have also slightly reddish or yellowish foliage.
Parasites which live upon the energetic matter stored up by other plants fall obviously under the same class. Their whole existence consists in a continuous metastasis, that is to say, in the expenditure and liberation of previously accumulated nutriment, under the influence of oxidation. The common European broom-rapes (Orobanche) have no green leaves, but merely pink, purple, brown, yellow, blue, or rose-coloured scales and flowers. Cytinus hypocistis, which grows parasitically on the roots of the Cistus, has a bright orange stem and leaves. The common English dodder is noticeable for its pretty twining red filaments, while its Indian congeners display brilliant hanging masses of golden threads. In fact, almost all true and perfect parasites are remarkable for the absence of green and the presence of other bright hues. Of course, many plants usually included under that name, like the mistletoe family, have foliage of the ordinary colour; but these are in reality only half-parasitical, a kind of stepping-stone between the epiphytic plants (orchids and bromelias) and the thorough-going parasites, such as Rafflesia. To the very end, indeed, the degraded leaves or scales of flowering plants contain some traces of chlorophyll, which, however, like the leaves themselves, must be regarded as mere obsolescent relics of their earlier state. It should also be noticed in passing, that many parasites, like Rafflesia and Hydnora, have exceptionally large and brilliant flowers. The blossom of R. Arnoldi sometimes measures three feet in diameter.
Still more noticeable in hue are the plants known as saprophytes, which live like fungi on the decaying matter contained in dead foliage or other organic remains. These, too, have no real assimilative leaves, while their functions are purely animal, consisting in the absorption of oxygen and the expenditure of previously accumulated energies. The Indian-pipe plant of Canada (Monotropa uniflora) has a pure white scaly stem and flower, exactly resembling a fungus to the untrained eye; it grows under the shade of pine forests, amid the rich débris of their pollen and their fallen foliage. The beautiful Neottia speciosa has a scape and rudimentary leaves of bright scarlet. Corallorhiza and many other saprophytes are equally remarkable for their exquisitely coloured scales. It is true that several, if not all, of these plants contain small quantities of chlorophyll or xanthophyll; but here again, we must regard the pigment as a mere remnant of earlier ancestors; while the plant, as a whole, mainly consists of metastatic materials, or, in other words, of oxidation products.
The resemblance which both parasites and saprophytes bear to fungi is certainly remarkable when we remember their close community of nature and function. All alike live upon previously organised material, and all have the same flabby, succulent, pulpy appearance. The Indian-pipe plant is always described by Canadian farmers as “a kind of toadstool;” the Rafflesia is noted for its fungoid look and animal odour; the Cytinus exhales a meaty flavour; and the Cynomorium coccineum is known to druggists by the technical name of Fungus melitensis or Malta mushroom. Putting these facts by the side of their very similar colouration, we are not unnaturally led to expect some causal connection such as that of which we are now in search. Let us pass on to other coloured portions of ordinary plants, which may throw a little more light upon the question at issue.
Buds contain energetic material, stored up by the plant during the preceding season, and expended, presumably, by union with oxygen, during the spring. The sprouting buds of the hawthorn and of many other plants present exquisite tints of pink and mauve. The bulbils of the tiger-lily are covered by purple scales. The various devices by which plants lay by nutriment during one season for their growth in the next are known as bulbs, tubers, corms, or rhizomes. All of these are apt to produce young sprouts of dainty colouring and bright hue. The growing sprays of the potato, when kept carefully from the light, exhibit distinct tinges of pink, blue, violet, and yellow. Asparagus shoots and blanched sea-kale have scales or leaves of mauve, lilac, and greenish brown. Almost all bulbs, on first producing leaves, show very decided colours, which change to green under the action of light. Beet-root, permitted to sprout in the dark, sends up beautiful bunches of deep crimson foliage. Carrots, under like circumstances, put forth golden sprays, varying from light primrose to bright orange. Sprouting peonies are of a full dark red. Rhubarb has rosy stems and pink or yellow leaves. In many of these cases, the colour is most conspicuous in the thin laminated portion of the young leaves, which offer the best medium for the display of delicate pigments. In every case, exposure to the sunlight brings about reversion to the original assimilative function, and results in the final triumph of green chlorophyll.
The young shoots at the end of branches are in the same position, as regards energy, with the sprouts which arise from bulbs or tubers. They cannot yet feed themselves, but they are nourished by energetic materials from the older leaves, whose carbon combines with oxygen in their tissues to yield the energy whereby their growth is carried on. Now the bright tints of these young shoots are very noticeable (as may be especially observed in the fuchsia, the hawthorn, and the rose-apple), and they can be skilfully arranged in such combinations as to produce a visible effect not at all unlike that of flowers.
If we compare these various cases with those of bright-hued entomophilous blossoms and brilliant fruits, we shall find that they have all one quality in common — they occur in parts which are expenders, not accumulators, of energy. Hence we are led to suppose that those portions of plants which subsist upon previous accumulations are apt to assume bright hues of different sorts. To what can we attribute the tendency which we thus observe? Can we give any causal formula for the empirical generalisation at which we have now arrived? I think we can, and in the following manner: —
Chlorophyll, the active deoxidising principle, has a definite composition, which enables it to carry on its proper functions, and a definite mode of reflecting light, which we call green. How far its greenness is bound up with its other physical properties we cannot say. Perhaps, as has been objected, it might equally well perform its physiological purpose were it red or yellow. But more probably its special reaction upon light is intimately connected with its special reaction upon carbonic anhydride under the influence of light. However this may be, at least we know that active chlorophyll is always green; and the more active, the brighter its hue, as Mr. Sorby has abundantly shown. Hence, every part of a plant which performs deoxidising functions has necessarily a green pigment for its foundation. The greenness may indeed be masked by other dyes (perhaps themselves the products of oxidation), as in cell-sap or epidermis, but in the actual active principle itself, greenness is apparently always present as an essential and inherent property.
So leaves as a rule, where exposed to sunlight, are green, but the remaining portions of the plant do not seem to be bound by such a stringent law of colouration. There is no reason why other colours should not appear in them from time to time, and, if they prove useful, be perpetuated through the action of natural selection. How, then, do they arise?
Colour, we have seen already, is merely the mode in which various bodies react upon light, reflecting or absorbing its constituent elements in varying proportions of their several rays. But there is no property of different bodies more variable in its nature than this particular mode of reaction. The slightest change in the molecular constitution of a substance is apt to be accompanied by considerable changes in its hue. Materials which appear chemically almost identical pass through strange varieties of tint with the greatest readiness. And this is particularly the case with organic matter, which differs from all other matter in the striking effects produced upon its physical constitution by apparently trifling causes. Hence we might naturally expect that very small changes in the constituents or contents of plant tissues would be likely to produce great alterations in their colour. And we find accordingly in all non-active parts of a plant that by-products of various tints do actually occur with considerable frequency: take, for example, the bright hues of many stems, barks, and juices, the red under-side of the Victoria regia leaf, the amber nether foliage of the star-apple, and the beautiful scales of the gold and silver ferns.
Whether such colours are always due to oxidation, it would be difficult to say; but in a large number of instances it is quite clear that oxidation is going on in the tissues where the colours appear. Obviously, in all cases of metastasis, the recombination of oxygen with the accumulated hydrocarbons is the only source of the energy whereby growth is carried on. Sometimes as much as 40 or even 50 per cent. by weight of the organic matter contained in seeds which germinate in the dark is lost by conversion into carbonic anhydride and water; and somewhat the same change must take place in bulbs, tubers, corms, and rhizomes. Almost all the above-quoted cases fall apparently under a like generalisation. The red colouring matter of persistent winter leaves, as seen in Sempervivum, Mahonia, Vaccinium, and Sedum, is clue to a substance mainly consisting of tannin. Mr. Sorby does not absolutely say that the colours of autumn foliage are due to oxidation, but he refers them on the whole to decreased vitality, absorption of chlorophyll, and similar causes, which bring into prominence various minor principles otherwise unnoticed. Of the lichnoxanthine series he says expressly, “They are probably only products of the oxidisation of chlorophyll, from which they may be prepared artificially.” Of the erythrophyll series, on the other hand, he merely observes, “They are usually indicative of low constructive energy.” The chrysotannin group, again, “when oxidised, give rise to various brown substances, which are the cause of many of the characteristic tints of autumnal foliage.” But with regard to the pigments of entomophilous flowers his language is much more decided in tone. “The coloured substances in the petals are in many cases exactly the same as those in the foliage from which chlorophyll has disappeared; so that the petals are often exactly like leaves which have turned yellow or red in autumn, or the very yellow or red leaves of early spring. . . . The colour of many crimson, pink, and red flowers is due to the development of substances belonging to the erythrophyll group, and not unfrequently to exactly the same kind as that so often found in leaves . . . The facts seem to indicate that these various substances may be due to an alteration of the normal constituents of leaves, some being probably formed from chlorophyll, others from the xanthophylls, and perhaps some from other constituents. So far as I have been able to ascertain, their development seems as if related to extra oxidisation, modified by light and other varying conditions not yet understood.” In like manner Lory found that parasites, as exemplified by broom-rapes, absorbed oxygen and exhaled carbonic anhydride in all stages of growth, whether exposed to the sun or not. So, also, Morot showed that in etiolated plants the coloured portion of the tissues gave out carbonic anhydride, while the green portion gave out oxygen. In short, without going into the lengthy ultimate question — scarcely soluble at present — whether all bright vegetal pigments (except chlorophyll) are themselves actually oxidation products, we may at least affirm that they occur with exceptional frequency in those plants or parts of plants where oxidation is largely taking place. They may be always directly due to the absorption of oxygen, or they may be merely secondary results of that action; but they certainly show a great tendency to present themselves wherever energy is being expended; and that conclusion is quite sufficient for our immediate object.
Here, to guard against an obvious criticism, it should be added that only a tendency, not a universal law, in such a direction is believed to exist. For example, the leaves of the sensitive plant and the Desmodium, which exhibit movements far more marked than those of the insectivorous species already noticed, are perfectly green. But the whole conclusion here suggested amounts in brief to the following principle: Wherever considerable changes occur in the nature of the vegetal tissues or their contents, they are apt to be accompanied by similar changes in the reaction of the tissues upon the incident sunbeams.
Yet it is a noteworthy fact of great importance, as shedding light upon the origin of the colour-sense, that such brilliant tints are everywhere exceptionally common in the organic world. Besides the green of chlorophyll, the orange and scarlet of autumn leaves, or the varied hues of flowers and fruits, we find unusually bright colouring in many parts of animals, especially the very simplest, such as jelly-fish and sea-anemones. Although, as we shall observe hereafter, many of these are doubtless due to the selective action of sexual preference, acting through the colour-sense itself, yet in the lowest organisms there is some reason to believe that the purity and splendour of the prevailing hues are only due to the adventitious composition of their molecules. And when we further notice the brightness of mammalian blood, besides the numerous changeful hues of sundry viscera or their contents, we shall probably be willing to allow that organic bodies habitually display pure and gorgeous tints, which the mineral world only shows us in a few rare and exceptional jewels.
Before we proceed, however, to apply these general principles to the genesis of entomophilous flowers, it will be well to glance briefly at a distinction of considerable importance, already hinted at in the preceding paragraph. Colour, as such, cannot of itself subserve any special function except in connection with the animal eye. The hues of all inorganic and of most organic bodies depend entirely upon the fortuitous molecular constitution of the particular body. But when a colour so reflected happens to produce some specific effect upon the eyes of any animal, whose interference is either useful or noxious to the animal or plant reflecting it, then the principle of natural selection will come into play, and the colour, as such, may be said to subserve the special function of attraction or protection, as the case may be. Henceforward, in the present work, a colour which seems simply to depend upon molecular constitution, apart from function to be subserved, will be described as adventitious; while a colour which also subserves a special function will be described as purposive.
Now all the coloured objects with which we have so far dealt — green leaves, autumn foliage, young shoots, sprouting buds — are purely adventitious in their tints. Flowers, however, which we next approach, are purposive; but, like all other purposive adaptations, they must necessarily have taken their rise in some adventitious circumstance, afterwards increased and developed by selective action.
With such data before us, then, let us proceed to inquire what was the genesis of those bright entomophilous flowers, which present brilliant tints in specialised thin leaves or petals, admirably adapted alike for rapid oxidation, and for the ostentatious display of delicate pigments.
The flower is one of the purely expensive structures which we noticed above as seats of oxidation and liberated energy. The well-known experiments of Saussure, Dutrochet, Vrolik, and De Vriese, detailed in all handbooks of physiological botany, sufficiently prove that during the act of flowering oxygen is consumed, carbonic anhydride evolved, and heat liberated. These experiments have been generally conducted upon various species of Arum, which are insect-fertilised flowers; but similar phenomena have also been observed in the cones of cycads, whose blossoms are strictly anemophilous. Indeed, as the absorption of oxygen is chiefly concerned with the maturation of the pollen, and, to a less extent, of the pistil, it is clear that it can be but little influenced by the nature of the surrounding structures.
Hence we would naturally expect that all floral organs, wind-fertilised or insect-fertilised alike, would show a tendency to the production of bright colours, in accordance with the general principle here laid down. This a priori expectation is fully justified by the actual facts.
In the first place, even among flowerless plants, the purely expensive structures employed in the elaboration of young spores are almost always tinged with some other hue than that of the green pigment which distinguishes the active and assimilating leaves. In mosses the graceful little spore-cases, which rise like miniature fruits at the extremity of the tall spiky stems, are usually pink or reddish brown in colour. The beautiful Splachnum rubrum of the Canadian forests has a cup of brilliant scarlet, which has led the children who pick it to give it the pretty popular name of red-cap moss. Many lycopodiums produce bright golden fructifications, very conspicuous in the lovely exotic L. dendroideum. Ferns generally bear their spores on the under surface of the frond, where their brown or russet colour makes them very noticeable and pretty objects. So that, in spite of their ill-chosen name, the cryptogams themselves exhibit the universal tendency to varied colouration in the reproductive organs.
Next, when we examine the phanerogamous division of plants, we see at once that the actual floral structures themselves are always more or less marked by distinctive colours. The pollen is generally of a rich golden yellow, while the surrounding scales show tints of silvery grey or faint pink. Even among the wind-fertilised blossoms, not a few are thus rendered conspicuous when they hang thickly together in large close-set masses. Many catkins, several grasses, the larch and other conifers, the dock and its congeners, all display blossoms of considerable distinctness, quite uninfluenced by the selection of insects. The inner bracts of the unopened artichoke head are often a brilliant mauve, not less beautiful than that of many flowers. The glumes which surround the floral organs of grasses are ruddy purple. The female flowers of the common hazel are a fine red, as Mr. Darwin reminds me. Evidently we have here a groundwork of differential colouring upon which selection might set to work, and ultimately produce the striking results that we see to-day in every flower-garden.
These, then, are the ultimate elements of our problem. Flowers consist essentially of male and female organs, which really represent aborted leaves, greatly modified for their special function, as Wolff and Goethe long since pointed out. These reproductive organs are situated at the ends of axes, where growth is failing; and Mr. Herbert Spencer observes that such points are just the ones where coloured leaves, as noted above, frequently make their appearance. In anemophilous flowers, as a rule, we find only the two whorls of essential floral organs; but in entomophilous flowers, as a rule, we find two additional whorls, the petals and the sepals, one or both of which are brilliantly coloured, the colouration apparently subserving no other purpose than the attraction of insects who aid in fertilising the flowers. We can hardly resist the inference that the coloured whorls represent an intensification of the natural tint in growing shoots and floral organs, slowly modified by the selective action of the insect eye.
When we look more closely at the nature of showy entomophilous flowers, this conviction becomes greatly strengthened. If colouration depends wholly or in part upon oxidation of previously stored material, it will follow that very large and massive blossoms can only be produced by the aid of considerable prior accumulations in so depend for their support upon bulbs, corms, tubers, or other like bulky reservoirs of energetic material. It will be sufficient to mention the cases of the water-lilies, the lotus, the dahlias, the orchids, the iris, the crocus, the gladiolus, the narcissus, the snowdrop, the daffodil, the tulip, the various lilies, the tuberose, the hyacinth, and the meadow-saffron. In many of these plants the handsomest heads of bloom are secured by cutting off the flower-buds for several successive years, and so preventing the expenditure of material until enough has been accumulated for a gorgeous display of blossom. Certain other flowers, again, depend for support upon starch or other nutriment laid by in the fleshy receptacle from which they spring. This is the case with the artichoke, the dandelion, and many of their sister composites. A third class lives upon materials stored up in the woody branches, as in the almonds, flowering cherries, and other trees, which bloom in the spring before the fresh leaves make their appearance. Yet a fourth sort maintain themselves cheaply upon the manufactured juices of other plants, like the leafless parasite, Rafflesia, whose flower measures three feet in diameter, or the pretty little English dodder, whose suckers fasten themselves tightly upon the growing stems of gorse. A great number of the most beautiful exotics are saprophytes, which live entirely upon the decaying vegetable mould in which they are embedded. Indeed, whenever showy flowers, like poppies and convolvulus, grow without the aid of some such accumulated nutriment, it will generally be found that their petals are thin and papery, so that the total cubical content of the flower-bud is really quite inconsiderable. Such plants, in fact, have learnt to make a very great display at very little actual expense.
Furthermore, flowers often exhibit different colours according to the state of oxygenation which their juices have reached, and these differences, as I shall endeavour to show hereafter, bear a definite relation to the various periods of maturity, and the particular insect whose assistance is required. Almost all blossoms in their early stages contain green pigments and perform foliar functions; but as they mature, they gradually assume their proper hues of yellow, blue, or red. “The endochrome of the rudimentary petals,” says Mr. Sorby, “approximates in character to that of the leaves; and, during their development, their leaf-like character is gradually lost, and often new colouring matters are formed.” The series of changes may be easily followed in a hyacinth, a tulip, or a daffodil; but perhaps the garden hydrangea (H. hortensis) offers the best opportunity for watching this interesting phenomenon, because the structures in which the mauve or pink pigment finally appears are exposed to view during the whole process of maturation. Other changes also frequently take place after the flower is fully developed. “Cheiranthus chamæleo has at first a whitish flower, then a citron-yellow, then red or slightly violet; the petals of Stylidium fruticosum are pale yellow at first, then lightish rose-coloured; the flowers of Œnothera tetraptera are first whitish, then rose-coloured or nearly red; the corolla of Cobæa scandens is greenish-white the first day, and violet the day following; the flowers of Hibiscus mutabilis appear in the morning of a white colour, towards midday they become flesh-coloured, and at night they are red.” F. Müller has observed a Lantana at Sta. Catherina in Brazil, the flowers of which last three days, “being yellow on the first, orange on the second, purple on the third day;” and his interesting explanation of this peculiarity will find further mention when we come to treat of the parallel adaptation whereby insects have accommodated themselves to the colours of flowers. Indeed, Delpino believes that all such changes of hue are specially intended to inform the fertilising insects of the proper moment for effecting impregnation.
We conclude, then, with much probability, that the bright pigments of entomophilous plants are due originally to the natural oxidation taking place in all purely expensive structures, aided by the selective action of insects. It is noteworthy, as proving the functional origin of these pigments, that both great divisions of flowering plants, the monocotyledons and the dicotyledons, have independently hit upon the very same device of coloured leaves for attracting their insect allies. But this could hardly have happened had not some original groundwork existed in the mere fact of oxidation, upon which selective action might be successfully exerted. Still more clear does this argument become when we recollect that in almost every family under these two great divisions, anemophilous and entomophilous genera may be found side by side, thus proving that the device of colour has been independently adopted by different plants, not twice alone, but a thousand times over. Whenever brilliant leaves showed any tendency to appear in the neighbourhood of the floral organs, no matter what the species, genus, family, or class, it would seem that the plant thereby derived such an advantage as to perpetuate the habit in future, under the constant stimulus of over-population and natural selection, resulting in survival of the fittest.
When we pass on to examine the various parts of the flower which may thus become devoted to the attractive function, we find still plainer evidence to the same effect. The essential floral organs themselves, already so conspicuous in the various catkins, may be specially modified for the sake of displaying brilliant pigments. The common meadow rue depends almost entirely for attraction upon these organs. In the family of Mesembryanthemums, the outer stamens become flattened and petaloid, so as to resemble the corolla of ordinary flowers. In the water-lilies, the tendency towards a similar change is always noticeable. Indeed, if one may hazard a guess in so uncertain a question, analogy would rather lead us to suppose that all petals are modified stamens than that the transition has taken place in the opposite direction. However this may be, the corolla, or petaline whorl, forms in most flowers the main attractive organ. Roses, buttercups, violets, blue-bells, and primroses may stand as sufficient examples. Next in order comes the calyx, or sepaline whorl, usually a protective organ, but often so modified as to aid in the function of alluring the insect guests. In the fuchsia, the bright sepals make the most striking part of the whole blossom; while in the tulip, crocus, and other brilliant monocotyledonous plants, both sepals and petals are coloured alike, so as to be usually lumped together under the common name of perianth pieces. In the marsh marigold, the marvel of Peru, the purple clematis, and the crimson Aristolochia cordata, the petals are wholly wanting, and the calyx alone performs the task of ostentatious chromatic display.
Nor does the process of colouration stop short at the regular floral whorls. The bracts and other secondary adjuncts often aid in the attractive effect. Several euphorbias have separately inconspicuous flowers, enclosed in a common involucre of the most beautiful scarlet hue. Poinsettia pulcherrima bears tiny yellow blossoms, which would doubtless fail by themselves to catch even the microscopic eye of a tropical butterfly; but they are surrounded by a thick mass of gorgeous crimson bracts, so strikingly lovely as to ensure for the plant a place in all our great conservatories. The various arums bear their minute flowers on a yellow spadix, about which grows a huge white or purply-green sheath, known as a spathe, whose large size and bright colour makes up for the relative inconspicuousness of the essential organs. In short, whatever part happened to display a tendency towards bright colouration, and thereby attracted the attention of insects, would naturally grow more and more prominent from generation to generation, till it reached the furthest limit of useful expenditure.
That the colour of the flower is a mere intensification of that prevailing in the stem has long since been recognised by painters. In some cases, as in Peperomia, the hue of the stem becomes itself very noticeable. In others, as in Echeveria, the stalk and bracts are pinkish, gradually growing deeper till we reach the calyx, while the petals themselves appear simply as an intensified form of the surrounding tint. In Epiphyllums, the end of the leaf-like peduncle is often bright red like the blossom itself. Amongst English plants, Echium, Sedum, Chrysosplenium, Rumex, and many other genera, show like phenomena. And when, as in the parasites and saprophytes, the stem and scales have no special reason for greenness, we find such brilliant examples as Lastræa, Monotropa, Neottia, and Corallorhiza, whose rudimentary leaves are quite as beautifully coloured as the flowers themselves.
From whatever point of view we regard the question, then, it seems equally probable that even before insect selection had come into play certain flowers would show a considerable tendency to the production of adventitious colours. Wherever such patches of red or blue shone out among the prevailing green of primitive forests, we may be sure they would act as beacons to the rudimentary eyes of unspecialised insects. At first their colours would doubtless be arranged in very indefinite patches; but as they were gradually selected by their insect visitors, the effects of cross-fertilisation, by weeding out individual peculiarities, would make their shape and hue more and more definite with each new generation. For such definiteness, as we shall observe abundantly hereafter, is a mark of contradistinction between adventitious and purposive colouration. Wherever we find a plant, like the common West Indian Bromelia pinguin, in which the spathes are coloured brightly but irregularly, the crimson fading off into white or green, we may fairly conclude that the selective process has not yet proceeded very far. But when we get a definite bunch of crimson bracts, as in Poinsettia pulcherrima, standing apart as a regular mass from the green foliage below, we may be sure that the selective process has continued for a considerable period of time; while in the three constant coloured leaves which surround the little blossoms of the Bougainvillea, we see a still further progress in numerical definiteness. So, too, if we compare the English cuckoo-pint with the Æthiopian lily (Richardia africana), or the apple with the orange, we shall see reason to believe that the former cases represent a relatively incomplete, and the latter cases a relatively complete, stage of the differentiating action. And we shall observe hereafter, when we come to examine the origin of bright-coloured fruits, that these structures, which have been developed to suit the eyes of birds and mammals, and are therefore comparatively late in geological time, possess on the whole much less definite colours than entomophilous flowers, which have been developed to suit the eyes of insects, and date far back in geological time.
The first step towards definiteness in colouration is gained by that dwarfing of the internodes which gives the floral whorls their circular appearance. The earliest entomophilous flowers probably belonged to the dicotyledonous group, which now exhibits the highest differentiation of any; but they consisted of separate petals, like the common dog-rose, instead of being tubular or bell-shaped, like the honeysuckle or the campanula. Gradually, however, the various petals in certain cases became adnate, that is to say, developed together, so as to form a single indented corolla. The former class of flowers are known as polypetalous, the other as gamopetalous. At a still later date came the irregular flowers, like the labiates and orchids, which are specially adapted to the shapes of insects; while the differentiating process is doubtless still going on under our very eyes whenever a bee visits a blossom in the meadows around us.
Side by side with this differentiation of various flowers went the differentiation of flower-haunting insects. Even in the Carboniferous world some vagrant species of that great class already lived in the hard siliceous underbrush; but Sir John Lubbock believes that Hymenoptera, Hemiptera, and Diptera first came into being during the Cretaceous era; while Lepidoptera, or butterflies, did not appear until the Tertiary times. Beetles first exhibit evident marks of flower-feeding during the Miocene epoch. As for honey-bees, they probably represent the very latest and most highly differentiated members of the whole class, and they could hardly have reached their present form till a very late period. In short, if we look at the correlation of the flowers and the insects, we shall see reason to believe, what is already suspected on purely palæontological grounds, that gamopetalous flowers could not be developed before the rise of specialised insects having a proper proboscis fitted for fertilising their bloom.
Again, the entomophilous monocotyledons are probably far more modern in date than the bright-coloured dicotyledons, and they are also on the whole far more leaf-like and less definite. Most of them consist of six perianth pieces, shaped very much like the ordinary leaves, and seldom having any specialised features. Yet, as they found the field already occupied by bright-hued dicotyledons, it was necessary, if they would secure the attention of insects, to bid for their favour by very large and showy blossoms. Accordingly, these newest comers amongst the insect-fertilised plants form a large proportion of our choicest garden species. It will suffice merely to enumerate the iris, crocus, narcissus, daffodil, snowdrop, amaryllis, aloe, tulip, tiger-lily, fritillary, crown-imperial, tuberose, hyacinth, star of Bethlehem, meadow-saffron, hellebore, arum, and Æthiopian lily, to show how many of the most brilliant flowers belong to this class. Even here, however, a large number of species have advanced to a high degree of differentiation, due to the agency of insects. While many lilies have six separate perianth pieces, as we see in the tulip and the fritillary, others, like the lily of the valley, have become quite gamopetalous, or, to speak more correctly, the petaline and sepaline whorls have coalesced into a bell-shaped cup. But the orchid family display the most curious adaptations of all, being modified in an infinite variety of ways to suit the insects of their several countries, and presenting the most marvellous tricks of mimicry, mechanical device, and sportive cunning, which at first sight almost compel us to imagine an inherent consciousness guiding the blind course of their strange developments.
It has been remarked, too, that, as a rule, flowers whose forms are highly modified, so as to admit of fertilisation with considerable certainty by a single insect visitor, do not need the same large display of showy corollas as those which trust almost to chance for the conveyance of their pollen to the proper receptacle. Thus Sprengel contrasts the great size and numerous petals of the water lily, whose shape has no special reference to the organs of the fertilising insect, with the little labiates, whose form ensures the due application of the pollen at every visit. So, too, we may compare the common orchid with the fritillary, the lily of the valley with the tulip, and the composites with the rose family. Of course many interfering causes must be understood as putting a limitation upon the truth of this roughly generalised statement. For example, the great tropical butterflies, the larger bees, and the humming-birds, form fertilising agents who naturally demand large masses of colour as an attraction; or, again, the presence of scent, honey, or other special allurements, may make up in particular cases for the lack of bright corollas. Yet, on the whole, it may be said that, other things equal, high modification in form is accompanied by a decreased expenditure on coloured adjuncts.
Nor is it only in the shape and colour of individual flowers that plants vie with one another for the favours of their insect guests. Like varieties are also to be found in the mode of massing the blossoms so as to attract from a great distance the eyes of passing bees or butterflies. We must remember that the facets of the articulate visual organ are not adapted for perceiving small objects except at a comparatively close range. Hence those plants which can group their several blossoms into large and conspicuous bunches may derive special advantages from the extra attractiveness thus attained. Such species as the peony or the tulip bear a single terminal blossom at the end of their stalk. Others, like the pimpernel or the veronica, have a few tiny flowers half hidden at the axes of the leaves. But the hyacinth, the laburnum, and the lilac, group their bloom into large upright or hanging masses; while the cowslip, the carrot, and the calceolaria produce flattened heads which strike the eye from a considerable distance. The dog-rose, with its scattered flowers, does not catch our passing glance so readily as the apple-tree or the may; and the great tropical flowering forest trees may often be discerned by human sight at almost incredible distances for the stay-at-home European.
But the composite plants offer by far the most instructive example of the effect produced after many generations of unconscious selection by the visits of insects. The first approach toward their mode of aggregation may be seen in the head of clover, where a number of separate little pea-blossoms are collected into a compact assemblage by the shortening of their several stalks. In the scabious we find the like tendency carried still further by the addition of a broad receptacle and a bunch of surrounding leaves, known as an involucre, which fulfils the protective functions of a calyx for the compound group. The real calyx, however, on each single blossom, still retains its original form, and doubtless assists in the performance of its proper office. But in the true composites, like daisies or dandelions, the separate flowers have almost merged their distinct individualities in that of the complex whole. The calyx has become degraded into a mere bundle of hairs (known as a pappus), which serves as a float for the mature seed, and forms the “clock,” blown away by village children from the withered dandelion head, as well as the gossamer-like wings that carry the thistle seeds among the farmer’s corn. The involucre here usurps the whole protective function: and the head of flowers is mistaken by the ordinary human observer for a single blossom. But if we look close into the daisy, we see that its centre comprises a whole mass of little yellow bells, each of which consists of corolla, stamens, and pistil. The insect who alights on the head can take his fill in a leisurely way without moving from his standing-place; and meanwhile he is proving himself a good ally to the plant by fertilising one after another of its numerous ovaries. Each tiny bell by itself would prove too inconspicuous to attract much attention from the passing bee; but union is strength for the daisy as for the state, and the little composites have found their co-operative system answer so well, that late as was their appearance upon the earth, they are generally considered at the present day to be the most numerous family both in species and individuals of all flowering plants.
Nor has the process of differentiation stopped even here. Amongst the composites themselves great variety may be observed in the means adopted for the attraction of insects. The simplest form of composite head, which we see in the thistle and the artichoke, consists of uniform flowers, none differing in shape or colour from their neighbours. The common English centaury shows an intermediate stage, in which the outer florets are longer and larger than those in the centre of the head. The sunflower and the ragwort advance a step farther in the same direction, their outer florets having become ray-shaped or ligulate, but still preserving the yellow hue of the central mass. The ray florets, in these cases, practically fulfil the functions of petals, while the inner blossoms continue to act as true floral organs. Finally, in the daisy and in many chrysanthemums, the outer florets, besides being prolonged into petal-like rays, are coloured white, pink, mauve, or blue, while the central mass retains its original colouration. Here we find the external row of flowers quite diverted from its true purpose, and devoted almost exclusively to the attractive function.
Even now we have not yet arrived at the last stages of the differentiating process. The complex heads of flowers thus formed again unite into still more complex masses. The daisy and the sunflower bear only one composite head on each stalk, but the common thistle produces a whole mass of heads in a kind of umbel, and the ragwort has bunches of such umbels growing together side by side. In the groundsel, each head of flowers looks like a single blossom; in milfoil, the umbellate form is almost exactly reproduced in still wilder profusion; while the pretty waving golden-rod caps the climax by collecting compound bundles of heads into a many-branched and multitudinous plume. Flowers too small to succeed individually thus succeed in serried masses; and masses, again, too small for success in single complexity, achieve attention in their turn by reuniting into yet more complex groups.
As to the special colouring matter employed in each case, but little can be said as yet about its determining causes. In a few cases, indeed, we can conclude with some probability that the existing hue has been developed because it subserved, as such, some special function. Thus night-flowering plants are usually pure white or pale yellow, the very colours best adapted for scattering the scanty moonbeams or the dying twilight, and so attracting the eyes of moths and other crepuscular insects. Again, Rafflesia, Hydnora, Stapclia, and many other fetid flowers, which obtain fertilisation by deceiving flies through their resemblance to putrid meat, imitate the lurid appearance as well as the noisome smell of carrion. Many orchids are believed to be coloured in mimicry of insects, either for the sake of attraction or of protection from hurtful creatures. Other flowers appear to cater specially for the peculiar tastes of certain insects, which exhibit a preference for red, blue, yellow, or orange, as the case may be, and these will receive more extended treatment in the succeeding chapter. Sir John Lubbock thinks that the lines or spots on many flowers act as guides for the bees, pointing out the exact spot where the honey may be found; and Fritz Müller suggests that their changing hues serve as timepieces to show the right moment for effecting fertilisation. But in the majority of cases we cannot point to any such special determining cause for the particular hue which we find in nature. It is known that the colouring matters of flowers may be divided into two classes, the xanthic and the cyanic, whose types are respectively yellow and blue; and these two classes do not readily pass into one another. Thus, we cannot have a blue rose or a blue dahlia, though we may vary the hues of either blossom by proper treatment almost indefinitely within the prescribed limit. Hence, it might appear that each flower produced as a rule those colours which most readily result from the chemical properties of its constituents, varying the tint, so far as possible, under the influence of insect selection, in accordance with the nature of the percipient eye, of the surrounding foliage, and of other adventitious circumstances in the environment. It might well happen, however, in the majority of cases, that any bright colour would equally answer the attractive purpose, supposing only it contrasted sufficiently with the green leaves or other objects in the natural background. Such, at least, we know to be the fact with the eye of man, who is struck indifferently by the golden orange, the ruddy strawberry, the rosy-cheeked mango, or the purple grape.
With regard to the infinite variety of tints which we find in various flowers, it is sufficient to remember that very slight alterations in the physical conditions or in the particular stock suffice artificially to produce such varieties among cultivated plants. Any one who looks at the multitudinous shades of garden hyacinths, dahlias, fuchsias, chrysanthemums, tulips, and pansies, need not wonder at the great profusion of colour in wild plants. Almost any shade seems easily procurable from another, provided only it does not overstep the natural limitation set down above. In all probability, the ordinary colouring matters of flowers differ from one another only in the minutest particulars of chemical composition.
So far we have been engaged in answering, to the best of our knowledge, the first question proposed above: Did flowers show an original tendency to the production of coloured adjuncts even prior to the selective action of insects? We have settled to our own satisfaction — I hope also to the satisfaction of the critical reader — that such an original and adventitious tendency did really exist; and we have traced it up through its various stages, as it became, from generation to generation, more and more purposive, until at last we have seen it culminate in the gorgeous peonies, tulips, lilies, and rhododendrons of our modern flower-gardens. But all this time we have been putting off the consideration of our second question: Did insects possess any tendency vaguely to discriminate the various colours apart from the reactive influence of entomophilous flowers? To this further inquiry we must now address ourselves for a few short minutes.
The answer must be a somewhat dubious one — in a certain sense negative, in another sense affirmative. There is no reason to think that insects could be definitely affected by various colours before the rise of bright-hued flowers had developed their colour-sense. But we must remember that while colours differ qualitatively for us, they also differ quantitatively in an absolute manner. Now, “to be affected more or less,” as Professor Bain well puts it, “is a consequence of being affected at all;” and therefore every animal which has any organ for the perception of light must be capable of quantitatively differential stimulation by its greater or less intensity. Herein we have a slight original groundwork through which white might be distinguished by the primitive eye from green, brown, or black. But the growth of a distinctive mode of consciousness, or, to put it objectively, of distinct nerve-organs for the various waves of æther, must needs have been the result of long ages, during which those insects who best discriminated colour lived down on the average their less gifted compeers. How this result was brought about we cannot even guess, for here we find ourselves on the threshold of an ultimate metaphysical problem, unfathomable as yet — perhaps unfathomable for ever! Why the sensations of the auditory central organs should differ from those of the optical central organs; why the stimulation of a certain fibre and its connected ganglia should yield the feeling of blue, while the stimulation of its neighbour yields that of red — these final questions we cannot even pretend to guess. How the differentiation began, how it continued, how it acts to-day, we do not know, and very probably we may never know. But we do know this, that in a developed sensorium a differential sensation is attached to the differential stimulation of each among several very like nervous bodies; and that if it were not so, consciousness itself would be impossible.
Passing over this ultimate problem, however, it is not difficult to see how a substance so unstable and so modifiable as nerve-matter might easily present various modifications which answered to the various varieties of æther-wave falling upon it. If once such differentiated nerve-terminals began to exist, all experience and analogy show us that they would be followed by the differentiation of their connected nerve-centres, to each of which, in this mysterious way, a differentiated mode of consciousness would come to be attached. And this is what we mean by colour-sense.
Vague and symbolical as such a sketch confessedly must be, it would be foolish and premature to fill in any further conjectural details in the present state of our knowledge. We must accept it as a bare skeleton of the possible truth which fuller acquaintance with the nature of nerve-substance may some day flesh out for us in all its minor aspects. But we are not wholly without analogies which allow us faintly to foreshadow in our minds some indefinite hypothesis of its evolution. We know that a single material, such as glass, may be so moulded into globes that each globe will not only yield, when struck, a single constant note, but will also answer sympathetically to that note alone when sounded on another instrument. Now if we suppose that the nerve-terminals of the insect eye were similarly tuned at first, but so badly as to vibrate sympathetically with the whole gamut of separate æther-waves, we shall have a symbolical picture of an eye without a colour-sense. But if we further suppose that, under the influence of sundry incident causes unknown, certain among these nerve-terminals became restricted in the range of their sympathies, so as only to vibrate in unison with æther-waves having a limited range of frequency, then we shall have a symbolical picture of an eye with a rudimentary colour-sense. And if natural selection, picking out, as we know it would, from the whole number of variations in either direction those which varied most on the side of a still more limited range, at last produced terminals which were affected only by waves lying within an extremely small compass, we should then have the symbolical picture of an eye with a highly developed colour-sense. Rude as this representation of the possible course of evolution must obviously be, it may still answer the purpose of enabling the reader diagrammatically to grasp the idea which would otherwise float vaguely through his mind and elude every attempt to fix and crystallise it into thought. More than this humble service our rough and materialistic metaphor cannot pretend to perform.
And now, to recapitulate the chief points of this lengthy chapter, let us look back in imagination over the whole complex process here so imperfectly sketched out, and state our hypothetical conclusions, for clearness’ sake, in the language of established fact. Amid the earliest forests of our earth, green cryptogamic vegetation formed the whole flora. But as time went on, the advantages of cross-fertilisation produced, through some unknown combination of circumstances, the earliest flowering plants. These, strengthened by the constant infusion of fresh blood (to use the familiar phrase), lived down the consanguineous offspring of the great ferns and horse-tails amongst which they grew. But such primeval flowering species were all fertilised by the aid of the wind, and possessed no bright corollas or other coloured adjuncts. The aspect of a palæozoic forest presented an almost unbroken sheet of monotonous verdure. Even then, however, a tendency towards the production of red or yellow juices and other colouring matters might have been noticed in certain portions of the different plants. The tendency was especially displayed in those parts of the organism where energies were being used up in the performance of physiological functions; this effect being due, perhaps, to the process of oxidation. Such phenomena might be noticed both in the dying leaves and in the youngest shoots; but they were also to be found in the floral organs and their neighbourhood. As yet, however, no eye could distinguish them as colours: they had only an objective existence as æther-waves of unusual simplicity and purity. But among these flowers a few undeveloped and unspecialised insects sought their food. Some of the blossoms thus obtained fertilisation more easily than before; and those among them which offered special attractions to the insects were able to effect a great economy of pollen, besides being impregnated with immensely greater certainty than their anemophilous competitors. Thus certain plants became permanently and regularly entomophilous. Thenceforward those entomophilous plants which produced the greatest quantities of insect food, as honey or pollen, were most often visited, and so most regularly fertilised. Again, out of this number, whatever individuals most conspicuously displayed the original tendency toward bright and distinctive colouration were most likely to strike the eyes of insects. Conversely, whatever insects most readily discriminated the nascent patches of colour were best able (other things equal) to secure their food. So the production of coloured floral whorls, and the perfectioning of the insect colour-sense, went on progressing side by side. The various flowers entered into unconscious competition with one another for the visits of their fertilisers; and those which could specially lay themselves out for the attention of a single species thereby procured impregnation with greater ease and certainty. Thus arose the quaintly-shaped bells, labiates, snapdragons, orchids, and other irregular flowers, whose forms are definitely correlated to those of their insect allies. Similarly, an insect with a specially long proboscis, and with certain hairy appendages on his legs or forehead, might at once abstract honey from flowers which no other insect could reach, and fertilise deeply-seated organs which no other insect would affect. Thus arose the specialised flower-feeders like bees and butterflies. Again, other flowers which separately failed to attract the proper insects might prove very alluring when massed in large bunches. The result is seen in the development of compound blooms like clover, lilac, horse-chestnut, and the various composites, which last undergo still further selections, ultimately producing yet more compound forms. At length the colour-sense of insects, thus aroused, strengthened, and fully developed, is employed for other purposes, of defence, protection, the chase after prey, the search for mates, or similar life-serving actions; and these activities once more react on the growing sense, so as to increase its definiteness and its worth. Last of all, the colour-sense is employed by the insects themselves, as we shall see in a future chapter, as an æsthetic instrument in the choice of mates, and so indirectly produces, through sexual selection, the brilliant hues of butterflies, beetles, and all the other exquisite winged or creeping articulates which fill the gorgeous cabinets of our museums.
In this list of what the colour-sense owes to the hues of blossoms, we might further include many facts with regard to humming-birds, sun-birds, and other flower-feeding vertebrates. But these belong properly to a later stage in our inquiry; and enough has already been said or hinted, I believe, to show how fundamental a fact in the history of the colour-sense and its reactions is the primitive tendency towards the display of bright hues around the floral reproductive organs. Already we have here, indeed, the origin of many among those brilliant objects which we noted as wanting in the Carboniferous world — the world without a colour-sense. We must hereafter go on to inquire what was the development of the remainder; and we shall find, when we search the records of evolution, that no small proportion of these, too, may be ultimately traced back, through some remote and indirect pedigree, to the lovely and varied tints of tropical or woodland flowers.