by Grant Allen
It is possible that the light of fire-flies and glow-worms may be similarly of use for the guidance of the sexes, though Mr. Belt believes it to be a warning mark of inedibility. However this may be, it is certain that butterflies and insects generally are so constituted that they can distinguish their own mates unerringly from all others, even though the difference between the species be almost microscopical. Whether this discrimination be due to sight, or, as some authors maintain, to smell, it shows equally that a minute correspondence exists between the senses of the insect and its habits of life. And as this correspondence, so far as each separate species is concerned, must have had a beginning at some time or other, and consequently a cause, there seems no sufficient ground for doubting that conspicuousness of colour formed one of the determining conditions. Indeed, I cannot myself see why Mr. Wallace, who allows the attractive nature of colouring in flowers, should deny its attractive nature in the question of sex.
It is allowed on all hands, I believe, that the special odours of insects, as well as their stridulating noises, are guides to the sexes in their search for one another. It does not appear that colour stands on any different footing in this respect.
Furthermore, many insects have two sets of colours, apparently for different purposes, the one set protective from the attacks of enemies, the other set attractive for the opposite sex. Thus several butterflies have the lower sides of their wings protectively coloured, so as to preserve them from the notice of birds or lizards, while they sit with folded wings on a flower or leaf; whereas the upper sides are attractively coloured, and displayed in the open sunlight as they flit about in search of mates. Moths, again, whose habits of folding the wings are exactly opposite, often have their upper surfaces imitative or protective, while the under sides are bright and beautiful. Sometimes the union of protective and attractive features in the same insect is very striking. Thus Mr. Wallace himself mentions a leaf-like butterfly, Kallima paralekta, whose wings are purple and orange above, but exactly mimic dead foliage when closed; this insect always rests among dead or dry leaves, and imitates every stage of decay, being even apparently spotted with small fungi. So, too, Mr. Bates tells us of a grasshopper, Pterochroza, whose sheath-like fore-wings similarly resemble a green leaf, while its hind-wings, usually covered except during flight, are “decorated with gaily-coloured eyelike spots.” Again, Mr. Belt observes that the males of some butterflies which mimic the Heliconiidae are coloured with black and white, quite unlike the mimicking females, while some South African species show perfectly marvellous differences in this respect. In all these cases, one cannot but believe that while the one form of colouring has been acquired for the sake of protection, the other must differ from it for some sufficient functional purpose.
Once more, there seems to be a pretty constant connection between the general beauty of the flora in any particular district and the general beauty of its insect inhabitants. Of course, it has long been noted that where few or no flying insects exist, few or no bright-coloured flowers are found. But what I wish to point out here is the converse fact that where bright blossoms are common, insects are brilliant, while where most blossoms are inconspicuous, most insects are dingy. On this head the mass of evidence, though difficult to quote, is overwhelming. A few of the more striking instances may, however, be briefly given here. On the whole, the brightest flowers grow among the tropics, and on the whole, tropical insects are unusually beautiful. The flora and the minor fauna of Madagascar are equally remarkable for their splendid hues. Sir Joseph Hooker notices the extreme magnificence of the Himalayan flora; and he also mentions the singular loveliness of the butterflies. Sir Emerson Tennent speaks continually of the beauty of the “brilliant flowering shrubs” in Ceylon, and he likewise speaks of the beauty of its butterflies in large numbers. In Mr. Bates’s work on Brazil, I notice almost on every page the conjunction of pretty insects with striking flowers, or the absence of both together. Especially does he note the beauty of the Longicorn beetles found on flowers at Caripi, while a few pages before he remarks upon the abundance of exquisite blossoms at the same place. I cannot help interpolating here, though out of proper order, the remark that, just in like manner, he seldom mentions the capture of a handsome bird without adding that it was shot in a fruit-tree. Almost the only bright butterfly which I ever observed in large numbers in Jamaica was the Callidryas eubule, feeding on the abundant yellow cactus blossom, whose hue it exactly resembled, and which is the only common and conspicuous large entomophilous flower in the colony. Both Mr. Wallace himself and Lord George Campbell, an excellent non-scientific observer, remark upon the beauty of the insects and flowers of the Ké Islands. Similarly, the flowers of Amboyna are paralleled by its gorgeous beetles, butterflies, and birds. On the other hand, in New Zealand “there are scarcely any gay flowers and blossoms; but few herbaceous plants, nothing but shrubs and trees; shrubs with obscure green flowers;” while at the same time “the butterflies are distinguished neither by size nor by richness of colour.” Oceanic islands, which have few or no bright flowers, are remarkable for the absence of bright insects; and Mr. Darwin mentions of the Galapagos group both the fact that he “did not see one beautiful flower,” and also the universal dinginess of the whole fauna. But this question is one on which it is difficult to quote positive authorities: it must rather suffice to mention that a considerable search into the general impressions of travellers — the best evidence, after all, on so indefinite a point — has convinced me that such a general relation does actually obtain. It is no answer to say that the insects are necessary for the production of the flowers: the real point at issue is this — why are insects bright where bright flowers exist in numbers, and dull where flowers are rare or inconspicuous? We can hardly explain this wide coincidence otherwise than by supposing that a taste for colour is produced through the constant search for food among entomophilous blossoms, and that this taste has reacted upon its possessors through the action of unconscious sexual selection.
Finally, it would seem that Mr. Wallace’s own theory of “typical colours” really allows all that is here required. For Mr. Wallace speaks distinctly of the “need of recognition and identification by others of the same species” as one among the determining causes of such colours, not only in the case of birds, but also in that of butterflies, four families of which he specifies by name. Now, conspicuousness of hue is certainly a very simple means of identification: and I think we must allow that it acts as an allurement to the eye in the case of flower-feeding species. I am quite disposed to accept Mr. Wallace’s belief that the actual disposition of the stripes, spots, and lines, is a matter of special typical arrangement; but even here one would naturally suppose that some minute cause must at first have led to the preference for one arrangement over another. Briefly, to sum up the whole question, after making full allowance for warning colours and for mimetic or other protective colours, there seems to remain a large margin of cases in which brilliancy exists for purely attractive purposes: while often the attractive function is combined with more or less of protective device. Anybody who watches our own English butterflies on a sunny day can hardly doubt that display forms a part of the object for which their yellow, orange, or crimson-spotted wings have been developed, and that such display makes them an easier mark for their scattered mates. Above all, it is necessary to remember that the winged condition in these insects is hardly more itself than a sexual device for the perpetuation of the various species.
And now let us pass on to consider the fuller evidence afforded us by vertebrates.
To begin with fishes, it must be allowed that our present knowledge of their habits scarcely justifies us in making any distinct inferences from their colouring. Nevertheless, a few facts may perhaps be gleaned even here. The mass of lower marine animals are brilliantly coloured with what seem to be purely adventitious or protective colours; and we had occasion already to remark upon the similarity between their hues and those of fungi, saprophytes, and other like
vegetal organisms. As a few examples may be mentioned sea-anemones, star-fish, echini, medusæ, ascidians, sea-slugs, and corals. When we reach the Mollusca, the colouring begins to assume a different type, but it would be difficult to assign any sufficient cause for its occurrence. Amongst the marine Articulata, and especially the crustaceans, many species exhibit a regularity of hue, and minute arrangement of spots and lines, which seems to bespeak a certain amount of sexual selection. These doubtful instances, however, we must pass over, not because they are less interesting, but because they are so very uncertain; and in a brief examination like the present, we must necessarily confine our attention to the most salient points. Now, the abundance of coloured organisms, both animal and vegetal, in the sea, affords a fair ground for belief that fishes may have acquired a colour-sense, and a taste for bright hues. We know that they (as well as the crustaceans) can be attracted by crimson or scarlet rags, and that glistening objects like metals or artificial baits rapidly seize upon their attention. As to the brilliancy, beauty, and regularity of their colouring, the reader must be referred to Mr. Darwin’s description in the “Descent of Man,” where he will find a full account of the principal facts which go to prove the existence among them of sexual selection. I must content myself here by saying that for gorgeous colouring and variety of patterns they are nowhere surpassed in the whole animal kingdom; and that metallic sheen is especially conspicuous among the devices whereby they insure the attention of their mates.
As regards the special question upon which we are now engaged, a few facts may be shortly set down. In the first place, the lower animals of tropical seas are on the whole much more brilliantly coloured than those of temperate climates, and the same remark holds good of the fishes. Mr. Darwin has noticed the extreme beauty of the shoals which played in and out among the brilliant organisms of the coral lagoons; and though Mr. Wallace objects that this may be due to protective causes, in order that they may escape notice among the bright creatures about them, yet as Mr. Darwin pertinently replies, he was struck by the obvious conspicuousness of their appearance rather than by their resemblance to environing objects. I have myself observed the same point frequently in the West Indian harbours, where the fish and the neighbouring creatures, seen through clear still water, all appeared equally beautiful and noticeable. But I cannot do better than quote Mr. Wallace’s own description of the harbour of Amboyna. “The bottom,” he says, “was absolutely hidden by a continuous series of corals, sponges, actiniæ, and other marine productions of magnificent dimensions, varied forms, and brilliant colours. . . . In and out among them moved numbers of blue and red and yellow fishes, spotted and banded and striped in the most striking manner, while great orange or rosy transparent medusæ floated along near the surface.” And elsewhere he observes, “The fishes (of Amboyna) are perhaps unrivalled for variety and beauty by those of any one spot on the earth.” These facts at least tend to show that our theory does not receive any active contradiction from the conditions of marine existence; and they are confirmed by numerous other like passages in several authors whom I think it superfluous to quote.
On the other hand, the larger predatory species, such as sharks and pikes, together with the majority of temperate fishes, are decidedly wanting in brilliant hues. Mr. Wallace observes that river fish, even of the tropics, rarely if ever have gay or conspicuous markings; and this is just what we would expect from the nature of their food, consisting as it does of worms, small flies, and other inconspicuous objects. However, we must allow that in this case Mr. Wallace has witnessed against himself with excessive fervour; for many river fish undoubtedly have very brilliant colours, as I can especially testify from having caught in my youth numbers of the Canadian sun-fish, perhaps the most exquisite creature of its class which I have ever seen. But this particular species lives in shallow marshy water, and may have derived its tastes from butterflies and dragon-flies. Professor Agassiz speaks much, too, concerning the beauty of the Amazonian fishes, which he describes as having a quasi-marine character; and in spite of a possible accusation for grotesqueness, I would venture to suggest that their colouring may perhaps be due to those numerous butterflies which Mr. Bates so often describes as flitting in numbers along the banks of that mighty river. However this may be, we must leave these somewhat inconclusive cases, and hurry on to the terrestrial vertebrates.
The amphibia yield little evidence in either direction. The beautiful colouring of tree-frogs, when compared with toads and common frogs, is more probably protective than attractive. Still, the newts show us very unmistakable signs of sexual selection in their crests; and we cannot say that the habits of amphibia may not have generated a love for colour.
Among the reptiles, however, a good many facts may be quoted to our purpose. In the first place, the large water-haunting crocodiles and alligators are peculiarly dull and unsightly objects; while the whole order of Chelonia, including the turtles and tortoises, are as inconspicuous for colour as they well can be. On the other hand, the smaller saurians, many of which lead an arboreal life, and feed off varied food, sometimes fruit, sometimes insects and other small animals, are often noticeable for their beauty. Most iguanas, great jungle lizards, contrast strongly in hue with the crocodiles; but one species, Amblyrhynchus cristatus, which inhabits the dull-coloured Galapagos Islands, and has taken to a strangely abnormal marine life, is remarkable for the same sombre tints which characterise the other animals of that singular archipelago, being described by Mr. Darwin as of “a dirty black colour.” The lesser lizards give unmistakable proofs of sexual selection in the brilliant pouches which they protrude when sunning themselves, and whose exquisite colours have struck every observer. Their mechanism for changing the hue of their skin, by compressing or spreading the layers of pigment cells, has already been noticed; and it seems to betray a considerable sensitiveness to colour. Dr. Günther also sees reason to believe that the frugivorous lizards have the tongue as an organ of taste, while in the insectivorous species he considers it merely an organ for the prehension of prey. Now, a large number of all lizards are noticeable for their exquisite hues. Green, as might naturally be supposed, from protective reasons, forms the general groundwork of their colouring; but, as often happens under similar circumstances, many other shades are intermingled, apparently to perform the attractive function. Especially is this noticeable in the Anolis and other like genera, whose beautiful orange pouches consist of folds of the skin, which are concealed under ordinary circumstances, but protruded for display when the animal feels himself secure, and can sun himself at leisure on a dead branch. The family of Agamidæ include many of the most brilliant species, especially the exquisite Draco, whose beauty Mr. Darwin extols so highly; and these, says Dr. Günther, are arboreal in their mode of life, while the dull-coloured genera inhabit rocks or plains. It is true that Draco feeds on insects; but when we remember the beauty of many among these little tropical creatures, we see fair grounds for believing that its habits may have led it to form a taste for colour. The Geckos also eat moths; while Mr. Gosse found on dissection many pretty insects in the stomach of the lovely Venus lizard of the West Indies. Here, too, we may even see the indirect effect of flowers and fruits; for Mr. Wallace, after noticing the “abundance and varied colours of the little jumping spiders which abound on flowers and foliage (in the Aru Islands), and are often perfect gems of beauty,” goes on to say about the lizards in the same place, “Every rotten trunk or dead branch served as a station for some of these active little insect-hunters, who, I fear, to satisfy their gross appetites, destroy many gems of the insect world, which would feast the eyes and delight the heart of our more discriminating entomologists.”
Among snakes we find somewhat similar facts. While the arboreal species, still having green for their groundwork, “are characterised by their vivid colouration,” the ground-snakes, burrowing-snakes, and water-snakes are mostly dull and inconspicuous. Of the ground-colubrides, in particular, Dr. Günther says, “They live on the ground
, and are generally of not brilliant colouration; only a few, which frequent grassy plains, are of a bright green colour.” The Dendrophidæ eat lizards and like prey, and are usually very bright; their colours sometimes, as in the magnificent Chrysopelea ornata, being decidedly not protective. Of course, almost all arboreal snakes feed upon various foods, such as birds, smaller reptiles, or other brilliant animals, whose colours may have served to give them a taste in that direction. I confess I attach little importance to any of these cases; still I think it worth while prominently to call attention to the fact that most arboreal creatures are conspicuous for their exceptional brilliancy.
Doubtless much of the bright colouration in all these animals is more or less warning or protective. Thus, one may mention Mr. Belt’s “little frog that hops about in the day-time, dressed in a bright livery of red and blue.” Mr. Belt suspected this species to be inedible, from the staring nature of its hues; accordingly, he offered one to some ducks, but only succeeded in making one young duck bite it amongst some meat; and the bird “instantly threw it out of its mouth, and went about jerking its head as if trying to throw off some unpleasant taste.” Then, again, there are the coral-snakes of South America, for which Mr. Darwin has fully accounted. Once more Mr. Wallace, in his “Contributions to the Theory of Natural Selection,” has pointed out that green lizards frequent trees, while many Geckos are so marbled as to resemble the bark on which they crawl. Mr. Belt speaks also of a green Nicaraguan species, which looks exactly like the herbage among which it lurks, and has actually acquired leaf-like expansions to deceive its prey. Mr. Bates, too, notices a pale-green snake (Dryophis fulgida) so perfectly imitating the stem of a liana that it deceived even his practised eye at first sight; and we must, doubtless, refer to the same cause the verdant colour of the grass-snakes mentioned by Dr. Günther. But the noticeable point here, as in the case of the butterflies, is this, that while we find a prevailing imitative greenness, apparently for protective purposes, we so often find a mixture of crimson, blue, yellow, orange, or metallic iridescence, whose function seems to me purely attractive. We shall notice similarly, when we come to look at the parrots, that their prevailing ground-tint is likewise green, but that they indulge in every variety of brighter pigments in a decidedly conspicuous manner. My own observation of West Indian lizards would certainly lead me to say that their colours were far more likely to betray them than to protect them, even in their native haunts.
