by Grant Allen
If this be so, then it must follow that flower-feeding and fruit-eating insects or vertebrates will be specially distinguished from other animals by the exceptional brilliancy of their colouration. In the next chapter we will submit our general conclusion to the test thus suggested to us, and if we find that bright hues are, as a matter of fact, unusually common amongst those species in which we have inferred a priori that a taste for brilliancy would have been evolved by the circumstances of their life, then we shall have added another item to our cumulative proof of the existence and influence of a colour-sense among the lower animals. Just as we saw that the taste for sweets, formed upon flowers and fruits, could be transferred to syrup, sugar, honey, bon-bons, cakes, and puddings; just as we saw that the human liking for dainty perfumes, formed upon strawberries and oranges, could be transferred to hyacinths and heliotropes: so, I believe, the love for colour, formed upon the natural food of the various species, can be transferred to the choice of beautiful mates, and, strengthened by this transference, can be handed down by heredity to mankind till it results at last in the disinterested delight in the sunset and the autumn hues, in the flowers of our gardens, the varying tints of our landscapes, and the exquisite harmony of our Guidos and our Rossettis. Let us see, then, how far the facts of nature will bear out the theory on this subject which we have framed from the analogy of our other senses.
CHAPTER IX.
THE DIRECT REACTION OF THE COLOUR-SENSE UPON THE ANIMAL INTEGUMENTS.
If any unscientific person were asked to name the two most beautiful classes of animals in the world, he would unhesitatingly answer, “Butterflies and humming-birds.” It is a significant fact that these are perhaps the most exclusively flower-haunting of all invertebrate or vertebrate creatures respectively. And if he were asked to name any other birds, besides the single family above mentioned, which are specially conspicuous for their brilliant colouration, he would probably reply, “Parrots and their allies.” It is an equally significant fact that these birds are fruit-eaters. Following up the hint thus given us, we may run through the chief instances of brilliant species in both great divisions of articulates and vertebrates, in order to discover whether there is any constancy of connection between the nature of the food and the colouration.
As before, we may narrow down our consideration of the articulates to the great group of insects, because we know too little about the habits of their marine congeners to argue with any certainty as to their traits; while the other land-articulates are relatively unimportant for our present purpose. Now amongst the insects, the most brilliant order are the Lepidoptera, including both the butterflies and moths, which, it need hardly be said, feed upon flowers. Of course, it may be readily objected that the amount of food eaten by the perfect winged insects is relatively small, and that the caterpillars live for the most part upon the green portions of plants. Indeed, some butterflies possess no mouths at all, and pass the whole of their short lives by the expenditure of energy laid up in the larval condition. Yet this objection does not really invalidate the general conclusion; for the eyes of the perfect insects have been evidently adapted to the colours of flowers, and the main object of their winged state is the perpetuation of the species; so that we can easily understand how the tastes ancestrally formed in their last stage should dominate the selection of their mates. Hence we find that the colours of caterpillars are mostly protective, being due to natural selection alone, while those of butterflies are mostly attractive, being largely due to sexual selection.
Furthermore, if we examine the Lepidoptera in detail, we shall find similar conclusions thrust upon us. They may be divided into two great sections, — the moths and the butterflies, — of which the former are mainly nocturnal or crepuscular, while the latter are mainly diurnal. Now there can be no comparison as to brilliancy between the vast majority of these two groups. The moths vary for the most part from dingy grey to dusky black, while the butterflies revel in every shade of golden yellow, splendid crimson, and metallic blue. Again, the eyes of these two divisions differ in structure in a manner which suggests the inference that the diurnal insects are much better provided with optical discrimination than their nocturnal allies, especially when the facts are compared with certain exactly similar or corresponding peculiarities in the nerve-terminals in the eyes of owls and bats.
Nor does the argument stop here. Certain species or families of moths fly by day, and these (e.g., the crimson-speckled Deiopeia pulchella, Callimorpha dominula, the Agaristidæ. Ægeriidæ, Zygænidæ, &c.) are as brightly tinted as any butterflies. Mr. Bates mentions a Brazilian Urania, “a beautiful tailed and gilded moth, whose habits are those of a butterfly;” and I know by personal experience the Jamaican species of similar tastes, whose wings are exquisitely dappled with black, green, and gold. Indeed, it may be stated generally that most brilliant insects are fond of displaying themselves in the open sunlight; while conversely most insects which frequent dark places or fly by night alone are dusky and ugly. By the side of these facts it is well to remember that diurnal flowers, which appeal to bees and butterflies, have corollas in every variety of red, blue, orange, and purple; while nocturnal flowers, which appeal to moths, are generally white or pale yellow in hue.
If we compare the carrion-feeding and omnivorous flies with the flower-haunting Lepidoptera, we see at once the difference of taste, as exhibited in the presence or absence of sexual selection. The flies are generally dark and inconspicuous, with thin transparent wings; and whatever beauty they possess is due to mere surface-play of interference-colours, not to the existence of distinct pigments. Nothing in the nature or appearance of their ordinary food-stuffs would lead us to credit them with any ancestral love for pure and beautiful hues.
There are, however, some striking exceptions amongst the dipterous insects, which fully bear out our general conclusion. The tribe of Brachystomatidæ are “large flies, adorned with brilliant colours, which for the most part haunt flowers, living upon honey.” The Notacanthæ “are also frequently brilliantly coloured. They generally frequent flowers.” The Conopidæ, too, “are elegantly variegated in their colours. They may be found in great abundance during the summer, hovering upon their powerful wings over flowers in gardens and elsewhere.” The invariability of this conjunction will hardly allow us to regard it as accidental.
Beetles or Coleoptera show us like results. The carrion-feeders are for the most part black and unattractive, as are also the nocturnal species and those which live in water. But the brilliant species are often flower-feeders, and fly much in the sunlight, exhibiting their exquisite metallic sheen, and displaying their beauty to their mates. If we take the Lamellicorn beetles in particular, we shall find a very instructive difference between two of their closely-allied families. The cockchafers feed on leaves, and they are some of the dingiest creatures of their class; but the Cetoniadæ feed upon flowers, as their English name of “rose-beetles” implies, and they are conspicuous for the beauty of their colouring, including “a vast number of the most brilliant exotic species.” It is a significant fact, too, that their mandibles have been specially modified, so as to enable them to lick up honey, which clearly shows a long persistence in flower-haunting habits, quite sufficient to account for the formation of a definite taste for colour. “Those species,” says Latreille, speaking of Lamellicorns generally, “which live in the perfect state upon vegetable substances, are remarkable for the brilliancy of the metallic colours with which they are adorned. But the majority of the other species, which subsist on decomposing vegetation, manure, tan, or excrementitious matter, are generally of a uniform black or brown hue.” The magnificent Buprestidæ are also, in many cases at least, flower-haunters. The tetramerous beetles, including the gorgeous Longicorns, may be regarded as mainly flower-feeding or plant-haunting insects, and their colours, as a rule, are very brilliant. Similarly, among heteromerous beetles, the Trachelia of Professor Westwood are active diurnal animals, most of which live upon the leaves or suck the
honey of flowers, and they are often adorned with beautiful colours; but the Atrachelia, nocturnal in their habits and foul feeders, are generally black and dingy in hue. Altogether, though it would be difficult to sum up so very varied a group as the Coleoptera in a single sentence, I think a careful examination will convince the inquirer that here, too, a general connection exists between brilliancy of hue and flower-feeding or fruit-eating habits.
When we turn to the Hymenoptera, or bee and wasp tribe, a great difficulty at first sight arises in our way. It would seem as though some of these insects ought to be of all others the most gorgeously arrayed, and yet for the most part they are but plain and inconspicuous creatures. However, a closer view dispels the doubt. Only one tribe of the Hymenoptera, that of the Anthophila, or bees, is specially adapted for feeding on flowers. Now these fall into two classes, the social and the solitary; and the habits of the former class, of course, place them almost entirely outside the sphere of sexual selection. The queen or mother-bee, a prisoner for life, does not herself seek honey among flowers, and those bees which do so have no power of transmitting their tastes or habits to descendants. Indeed, the whole question of heredity in these interesting animals remains involved in so much mystery, that it would be useless to base any arguments upon it in either direction.
On the other hand, the solitary bees are often beautifully coloured, as in the well-known case of the carpenter-bee. The Nomadæ, or cuckoo-bees, are also very brilliant insects. The omnivorous wasps do not exhibit equal beauty; and the almost wingless, highly social, and mainly carnivorous ants are quite inconspicuous animals, probably possessing colour-perception in a very slight degree. But the Chrysididæ, a family of lower Hymenoptera, are also solitary flower-haunters, and “in the richness of their colours they vie with humming-birds.”
Of course, it cannot be denied that a few less notable classes of insects which do not haunt flowers are nevertheless more or less brilliant in their colouring. But this does not interfere with the general truth of our inference that flower-feeders are specially noticeable for their bright hues. If we can find ground for believing that those species which habitually seek their food in gay blossoms have developed a peculiar love for colour, which is shown in their choice of mates, we shall have done quite as much as is needful. Besides, other sources exist from which a love for colour may be derived as well as from flowers. For example, the Orthopterous family of Mantidæ, or praying-insects, are noticeable in many instances for their bright tints; but as they live by devouring other insects, a taste of the sort may have been generated indirectly in their case from the nature of their food. Still, most of the Mantidæ seem rather to be deceptively coloured like their surroundings, so as to escape the notice both of their prey and of their enemies among birds. A similar explanation must be given in the case of the Phasmidæ, or leaf and stick insects, whose colouring, though sometimes comparatively striking when seen in a cabinet, is purely imitative of the foliage or fallen sticks around them. Many flower-haunting spiders, too (to travel for a moment outside the limits of the true insects), are “exquisite gems” of ruby or sapphire colouration; yet we must rather attribute their magnificent hues to the need for imitating the petals on which they creep than to sexual selection. Such instances, however, in no way militate against our main conclusion; they only show that other causes at work have sometimes produced similar results to those which we are contemplating, though in a different manner. Thus it is quite possible that the beauty of the tiger-beetles may be due to their habit of hunting other bright-coloured insects in the open sunlight.
There still remains a margin of inexplicable cases, as might naturally be expected, for the study of these questions is yet in its infancy, and only a few isolated endeavours have hitherto been made to account at all for the external appearance of animals. Among such may be mentioned the gorgeous tropical locusts, the dragon-flies (which, however, prey upon many brilliant species), and several of the Longicorn beetles. But all these instances cannot blind us to the fact that if we look at the flower-haunting insects in the mass they are by far the most conspicuous for beauty of their kind. It is not necessary to explain in detail the colouring of every individual species — an endless task, which would demand far more competent treatment than I could give: it is quite sufficient if we find a general coincidence between bright food and bright hues in the feeder, without pretending at once to account for every apparent exception.
And now, before we pass on to examine the vertebrate world in the same manner as we have here examined the articulate, we must pause a moment to meet, or rather to touch lightly, a powerful objection which has been urged against the whole theory of sexual selection by no less a writer than Mr. A. R. Wallace. In his work on “Tropical Nature,” that ingenious evolutionist endeavours entirely to overthrow Mr. Darwin’s laborious superstructure, raised with so much toil and skill in the “Descent of Man,” and to substitute for the doctrine of voluntary choice, which the older naturalist there advanced, a number of minor principles, whose joint action may be supposed to have produced the existing colours of the animal world. Mr. Wallace has urged his objections with even more than his usual ingenuity; and I may frankly confess that he has attacked the theory of sexual selection with such judicious vigour that I felt inclined on first reading his essays to abandon entirely all that part of the present work which was based on the original doctrine enunciated by Mr. Darwin. On fuller consideration, however, I have determined, though with much hesitation, to retain it, in hopes that the few suggestions which I have to make upon the question may possibly contribute to a clearer comprehension of its issues, and to its ultimate settlement in one direction or the other. I cannot for a moment pretend to meet a distinguished specialist like Mr. Wallace on his own ground; nor do I wish to dispute the force and accuracy of many among his criticisms; yet I trust I may be able to add my small quotum of facts and inferences to the whole data for a final opinion, and I believe that the very generalisation which it is the object of the present chapter roughly to establish, may be useful in showing some additional basis for the theory of sexual selection. For if we find that fruit-eating and flower-feeding animals do really exhibit unusually beautiful colours, then we shall have some further ground for believing that they do exert some vague sort of choice or preference in the search for mates.
Accordingly, I shall jot down in passing, under each head, such points as occur in relation to this vexed question.
In the first place, it is well to remember that sexual selection does not necessarily imply a deliberate exercise of will, or comparison between the rival charms of various possible mates, which seems hardly probable in the case of insects at least. We must guard against the error of transferring our own highly-developed notions of beauty to the simple half-conscious minds of beetles or butterflies. With us, beauty is a very complex idea, compounded of numerous presentative, representative, and re-representative elements; and our choice of mates is a conscious selection, guided more or less deliberately by many complicated considerations, often too numerous for analysis even in our own minds. But, without attributing to the butterfly any such highly-evolved feelings, we may well believe that certain individuals, whose brilliant colours contrasted strikingly with the green foliage about them, might more readily succeed in attracting the attention of mates than their dingier compeers. We know that the eyes of insects are allured by the colours of flowers, which have been developed for this very purpose: and there is therefore nothing improbable in the supposition that they are also allured by bright hues in their fellows. In short, I am inclined to suggest that conspicuousness rather than beauty in our human sense is aimed at by the butterfly’s wing. To some this will doubtless appear equivalent to a surrender of the whole position; but a little reflection will probably remove such an apprehension. For the sense of beauty in its simplest form, as Mr. Darwin rightly puts it, is nothing more than “the reception of a peculiar kind of pleasure from certain colours, forms, and sounds.” Now we hav
e seen reason to believe that the insect feels some slight pleasure in the perception of colour in flowers; and we may also conclude that the pleasure is equally felt from the similar stimulation of a brilliant mate. In either case, it seems probable that a semi-automatic action is set up by the sight of the bright hue, which leads on the insect instinctively to the blossom or the opposite sex alike. Such a selective process does not seem to me at all to transcend the narrow faculties of a beetle or a butterfly. Mates on this theory are not chosen on account of their brilliancy, but their brilliancy renders them the most natural objects to choose.
The familiar instance of the moth and the candle shows us this automatic tendency in its fullest form. In that case it would seem as though the intensity of the visual stimulus set up a motor activity of the wings, which would become more and more powerful the more directly the eyes of the insect were turned towards the light. Accordingly, any random movements in that direction would be followed by more and more rapid gyrations, ending, as we know, in the central flame, whenever the eyes both pointed straight toward that quarter. We must conclude here that in the natural circumstances of moths few bright objects would occur around them, except flowers, and so the eye has probably been connected with the motor system in such a manner that the reception of a light-stimulus acts immediately upon the wings. In the presence of such a rare and unpremeditated object as a candle, the hereditary instinct or organised habit becomes a bad guide, finds itself at fault, and finally results in the insect’s death. But the case is very much like that of a human child, who knowing that bright red berries are usually sweet and innocuous, poisons itself with those of the cuckoo-pint or the yam. The instinctive pleasure can only have been adapted to the usual environment of the race, not to special and exceptional circumstances like artificial lights and poisonous berries.
