by Grant Allen
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.
CHAPTER V.
THE COLOUR-SENSE IN INSECTS.
Throughout the whole of the preceding chapter we have taken for granted the existence of a developed colour-sense in some at least amongst the insects of modern times. We have tried to show what were the circumstances which gave it origin, and what the steps by which it reached its present supposed perfection. But now a deeper question arises, a question due to a destructive criticism which might seek to overthrow our whole superstructure by denying that modern insects do, as a matter of fact, possess any colour-sense whatsoever. In the face of such a possible criticism we must review all the various proofs of colour-perception in articulate animals which experiment or observation may reveal to our patient search.
This course is rendered the more necessary because within the last few years the existence of a faculty for the discrimination of different hues, even in primitive man himself, has been gravely called in question, both in this country and in Germany, by competent authorities in various walks of science or criticism. Mr. Gladstone first suggested that the Homeric poems contained no evidence of a colour-sense amongst the Akhaians of that early date. Many years later, Dr. Lazar Geiger noticed that the colour-words employed in the Bible, the Yedas, the Zend-Avesta, and other early works, were very vague and indeterminate. Dr. Magnus, a distinguished German oculist, next followed up the hint thus thrown out, and endeavoured to prove, in two learned pamphlets, that the colour-perception of civilised man was a faculty of quite recent development, and that so lately as some 3000 years ago mankind was utterly incapable of distinguishing between violet, green, blue, and yellow. These views were further popularised by Mr. Gladstone in a later paper, and have been partially adopted by several scientific authorities, including even that staunch evolutionist, Mr. A. E. Wallace, the joint-discoverer of natural selection. Indeed, although the allegations of Dr. Magnus and his friends have not gone entirely unanswered, yet it would seem as though the scientific world generally, in Germany at least, was prepared to accept them as representing the approximate truth.
To the evolutionist, however, this crude and ill-digested theory can hardly seriously recommend itself. The supposition that any mode of perception so distinct and so varied as our colour-sense could be developed in the short space of time intervening between the Homeric Akhaians and our own epoch seems little short of incredible. The few centuries which have rolled past during that interval form but a single pulse of the pendulum whose seconds make up the epochs of geological evolution. To me, it appears rather that the colour-sense of man is derived, through his mammalian ancestry, from a long line of anterior generations, and that its origin must be sought for in ages before a solitary quadrumanous animal had appeared upon the face of the earth. Holding this view, it becomes incumbent on me to propose a counter-theory to that of Dr. Magnus and Mr. Gladstone; a theory which will trace back our colour-sense to its ultimate sources in the bright hues of vegetal products like fruits and flowers. And it becomes necessary also to seek for every possible fact which goes to prove the existence of a similar faculty throughout the whole animal world. For if even man himself, “the head and crown of things,” did not possess any such power until a few hundred years since, how can we suppose that the lower animals, including the humble little insects, have been for ages in enjoyment of this highest sensuous gift?
It may be asked, however, “Why take the trouble to search for recondite proofs on such a plain and straightforward subject? Why not try at once a few simple and direct experiments upon the colour-perception of various insects, beasts, and birds?” The suggestion is a natural one, and yet it is not so easy to act upon as would at first appear to be the case. Experiments of the sort are difficult to devise, and still more difficult to carry out successfully to any definite result. We cannot ask the animals to detail their sensations, and we find it hard to invent decisive or crucial tests of an objective character. A few lucky exceptions will be described in the following pages, but they are mere oases amongst a desert of lamentable failures. As a rule, animals refuse in the most provoking manner to take any notice of the psychological traps which you have carefully baited as tests of their sensations. For the most part, we must rely upon the less satisfactory method of observation, and upon various indirect conclusions, each of which has separately very little weight. In short, the evidence in favour of a colour-sense amongst the lower animals is purely cumulative. Each link in the argument is but a slender support; yet I hope to show as we proceed that the whole strand, formed of variously twisted chains, is collectively strong enough and sure enough to support the burden of a weighty conclusion.
Happily, as regards the higher insects, we can start fair with a set of decisive experiments tried by Sir John Lubbock. That patient and minute observer saw grounds for believing that bees were attracted by the hues of flowers. However, to make assurance doubly sure, he placed slips of glass smeared with honey on paper of various colours, black, white, yellow, orange, blue, and red. The general results may be given in the original words. “A bee which was placed on the orange returned twenty times to that slip of glass, only once or twice visiting the others, though I moved the position and also the honey. The next morning again two or three bees paid twenty-one visits to the orange and yellow, and only four to all the other slips of glass. I then moved the glass, after which, out of thirty-two visits, twenty-two were to the orange
and yellow.” However, this preference did not depend upon an inability to discern the blue, for on another occasion, says the author, “I had ranged my colours in a line, with the blue at one end. It was a cold morning, and only one bee came. She had been several times the preceding day, generally to the honey which was on the blue paper. This day also she came to the blue. I moved the blue gradually along the line one stage every half hour, during which time she paid fifteen visits to the honey, in every case going to that which was on the blue paper.” Sir John Lubbock, however, never relies upon single or few experiments. Accordingly, he tried once more at a later date with greater variation in the circumstances. “On the 12th of July,” he says, “I brought a bee to some honey which I placed on blue paper, and about three feet off I placed a similar quantity of honey on orange paper. After she had returned twice, I transposed the papers; but she returned to the honey on the blue paper. After she had made three more visits, always to the blue paper, I transposed them again, and she again followed the colour, though the honey was left in the same place.” A series of careful observations followed, which are detailed in a tabular form; but my readers will probably be satisfied with a general summary, to the effect that thirty consecutive visits were all made to the same colour, in spite of four separate transpositions. On one of these occasions, says Sir John, “At 8.5 she returned to the old place, and was just going to alight; but observing the change of colour, without a moment’s hesitation darted off to the blue. No one who saw her at that moment could have entertained any further doubt about her perceiving the difference between the two colours.”
