by Grant Allen
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.
