Kitabı oku: «The Evolutionist at Large», sayfa 6

XIV.
DOG'S MERCURY AND PLANTAIN

The hedge and bank in Haye Lane are now a perfect tangled mass of creeping plants, among which I have just picked out a queer little three-cornered flower, hardly known even to village children, but christened by our old herbalists 'dog's mercury.' It is an ancient trick of language to call coarser or larger plants by the specific title of some smaller or cultivated kind, with the addition of an animal's name. Thus we have radish and horse-radish, chestnut and horse-chestnut, rose and dog-rose, parsnip and cow-parsnip, thistle and sow-thistle. On the same principle, a somewhat similar plant being known as mercury, this perennial weed becomes dog's mercury. Both, of course, go back to some imaginary medicinal virtue in the herb which made it resemble the metal in the eyes of old-fashioned practitioners.

Dog's mercury is one of the oddest English flowers I know. Each blossom has three small green petals, and either several stamens, or else a pistil, in the centre. There is nothing particularly remarkable in the flower being green, for thousands of other flowers are green and we never notice them as in any way unusual. In fact, we never as a rule notice green blossoms at all. Yet anybody who picked a piece of dog's mercury could not fail to be struck by its curious appearance. It does not in the least resemble the inconspicuous green flowers of the stinging-nettle, or of most forest trees: it has a very distinct set of petals which at once impress one with the idea that they ought to be coloured. And so indeed they ought: for dog's mercury is a degenerate plant which once possessed a brilliant corolla and was fertilised by insects, but which has now fallen from its high estate and reverted to the less advanced mode of fertilisation by the intermediation of the wind. For some unknown reason or other this species and all its relations have discovered that they get on better by the latter and usually more wasteful plan than by the former and usually more economical one. Hence they have given up producing large bright petals, because they no longer need to attract the eyes of insects; and they have also given up the manufacture of honey, which under their new circumstances would be a mere waste of substance to them. But the dog's mercury still retains a distinct mark of its earlier insect-attracting habits in these three diminutive petals. Others of its relations have lost even these, so that the original floral form is almost completely obscured in their case. The spurges are familiar English roadside examples, and their flowers are so completely degraded that even botanists for a long time mistook their nature and analogies.

The male and female flowers of dog's mercury have taken to living upon separate plants. Why is this? Well, there was no doubt a time when every blossom had both stamens and pistil, as dog-roses and buttercups always have. But when the plant took to wind fertilisation it underwent a change of structure. The stamens on some blossoms became aborted, while the pistil became aborted on others. This was necessary in order to prevent self-fertilisation; for otherwise the pollen of each blossom, hanging out as it does to the wind, would have been very liable to fall upon its own pistil. But the present arrangement obviates any such contingency, by making one plant bear all the male flowers and another plant all the female ones. Why, again, are the petals green? I think because dog's mercury would be positively injured by the visits of insects. It has no honey to offer them, and if they came to it at all, they would only eat up the pollen itself. Hence I suspect that those flowers among the mercuries which showed any tendency to retain the original coloured petals would soon get weeded out, because insects would eat up all their pollen, thus preventing them from fertilising others; while those which had green petals would never be noticed and so would be permitted to fertilise one another after their new fashion. In fact, when a blossom which has once depended upon insects for its fertilisation is driven by circumstances to depend upon the wind, it seems to derive a positive advantage from losing all those attractive features by which its ancestors formerly allured the eyes of bees or beetles.

Here, again, on the roadside is a bit of plantain. Everybody knows its flat rosette of green leaves and its tall spike of grass-like blossom, with long stamens hanging out to catch the breeze. Now plantain is a case exactly analogous to dog's mercury. It is an example of a degraded blossom. Once upon a time it was a sort of distant cousin to the veronica, that pretty sky-blue speedwell which abounds among the meadows in June and July. But these particular speedwells gave up devoting themselves to insects and became adapted for fertilisation by the wind instead. So you must look close at them to see at all that the flowering spike is made up of a hundred separate little four-rayed blossoms, whose pale and faded petals are tucked away out of sight flat against the stem. Yet their shape and arrangement distinctly recall the beautiful veronica, and leave one in little doubt as to the origin of the plant. At the same time a curious device has sprung up which answers just the same purpose as the separation of the male and female flowers on the dog's mercury. Each plantain blossom has both stamens and pistils, but the pistils come to maturity first, and are fertilised by pollen blown to them from some neighbouring spike. Their feathery plumes are admirably adapted for catching and utilising any stray golden grain which happens to pass that way. After the pistils have faded, the stamens ripen, and hang out at the end of long waving filaments, so as to discharge all their pollen with effect. On each spike of blossoms the lower flowerets open first; and so, if you pick a half-blown spike, you will see that all the stamens are ripe below, and all the pistils above. Were the opposite arrangement to occur, the pollen would fall from the stamens to the lower flowers of the same stalk; but as the pistils below have always been fertilised and withered before the stamens ripen, there is no chance of any such accident and its consequent evil results. Thus one can see clearly that the plantain has become wholly adapted to wind-fertilisation, and as a natural effect has all but lost its bright-coloured corolla.

Common groundsel is also a case of the same kind; but here the degradation has not gone nearly so far. I venture to conjecture, therefore, that groundsel has been embarked for a shorter time upon its downward course. For evolution is not, as most people seem to fancy, a thing which used once to take place; it is a process taking place around us every day, and it must necessarily continue to take place to the end of all time. By family the groundsel is a daisy; but it has acquired the strange and somewhat abnormal habit of self-fertilisation, which in all probability will ultimately lead to its total extinction. Hence it does not need the assistance of insects; and it has accordingly never developed or else got rid of the bright outer ray-florets which may once have attracted them. Its tiny bell-shaped blossoms still retain their dwarf yellow corollas; but they are almost hidden by the green cup-like investment of the flower-head, and they are not conspicuous enough to arrest the attention of the passing flies. Here, then, we have an example of a plant just beginning to start on the retrograde path already traversed by the plantain and the spurges. If we could meet prophetically with a groundsel of some remote future century, I have little doubt we should find its bell-shaped petals as completely degraded as those of the plantain in our own day.

The general principle which these cases illustrate is that when flowers have always been fertilised by the wind, they never have brilliant corollas; when they acquire the habit of impregnating their kind by the intervention of insects, they almost always acquire at the same time alluring colours, perfumes, and honey; and when they have once been so impregnated, and then revert once more to wind-fertilisation, or become self-fertilisers, they generally retain some symptoms of their earlier habits, in the presence of dwarfed and useless petals, sometimes green, or if not green at least devoid of their former attractive colouring. Thus every plant bears upon its very face the history of its whole previous development.

XV.
BUTTERFLY PSYCHOLOGY

A small red-and-black butterfly poises statuesque above the purple blossom of this tall field-thistle. With its long sucker it probes industriously floret after floret of the crowded head, and extracts from each its wee drop of buried nectar. As it stands just at present, the dull outer sides of its four wings are alone displayed, so that it does not form a conspicuous mark for passing birds; but when it has drunk up the last drop of honey from the thistle flower, and flits joyously away to seek another purple mass of the same sort, it will open its red-spotted vans in the sunlight, and will then show itself off as one among the prettiest of our native insects. Each thistle-head consists of some two hundred separate little bell-shaped blossoms, crowded together for the sake of conspicuousness into a single group, just as the blossoms of the lilac or the syringa are crowded into larger though less dense clusters; and, as each separate floret has a nectary of its own, the bee or butterfly who lights upon the compound flower-group can busy himself for a minute or two in getting at the various drops of honey without the necessity for any further change of position than that of revolving upon his own axis. Hence these composite flowers are great favourites with all insects whose suckers are long enough to reach the bottom of their slender tubes.

The butterfly's view of life is doubtless on the whole a cheerful one. Yet his existence must be something so nearly mechanical that we probably overrate the amount of enjoyment which he derives from flitting about so airily among the flowers, and passing his days in the unbroken amusement of sucking liquid honey. Subjectively viewed, the butterfly is not a high order of insect; his nervous system does not show that provision for comparatively spontaneous thought and action which we find in the more intelligent orders, like the flies, bees, ants, and wasps. His nerves are all frittered away in little separate ganglia distributed among the various segments of his body, instead of being governed by a single great central organ, or brain, whose business it always is to correlate and co-ordinate complex external impressions. This shows that the butterfly's movements are almost all automatic, or simply dependent upon immediate external stimulants: he has not even that small capacity for deliberation and spontaneous initiative which belongs to his relation the bee. The freedom of the will is nothing to him, or extends at best to the amount claimed on behalf of Buridan's ass: he can just choose which of two equidistant flowers shall first have the benefit of his attention, and nothing else. Whatever view we take on the abstract metaphysical question, it is at least certain that the higher animals can do much more than this. Their brain is able to correlate a vast number of external impressions, and to bring them under the influence of endless ideas or experiences, so as finally to evolve conduct which differs very widely with different circumstances and different characters. Even though it be true, as determinists believe (and I reckon myself among them), that such conduct is the necessary result of a given character and given circumstances – or, if you will, of a particular set of nervous structures and a particular set of external stimuli – yet we all know that it is capable of varying so indefinitely, owing to the complexity of the structures, as to be practically incalculable. But it is not so with the butterfly. His whole life is cut out for him beforehand; his nervous connections are so simple, and correspond so directly with external stimuli, that we can almost predict with certainty what line of action he will pursue under any given circumstances. He is, as it were, but a piece of half-conscious mechanism, answering immediately to impulses from without, just as the thermometer answers to variations of temperature, and as the telegraphic indicator answers to each making and breaking of the electric current.

In early life the future butterfly emerges from the egg as a caterpillar. At once his many legs begin to move, and the caterpillar moves forward by their motion. But the mechanism which set them moving was the nervous system, with its ganglia working the separate legs of each segment. This movement is probably quite as automatic as the act of sucking in the new-born infant. The caterpillar walks, it knows not why, but simply because it has to walk. When it reaches a fit place for feeding, which differs according to the nature of the particular larva, it feeds automatically. Certain special external stimulants of sight, smell, or touch set up the appropriate actions in the mandibles, just as contact of the lips with an external body sets up sucking in the infant. All these movements depend upon what we call instinct – that is to say, organic habits registered in the nervous system of the race. They have arisen by natural selection alone, because those insects which duly performed them survived, and those which did not duly perform them died out. After a considerable span of life spent in feeding and walking about in search of more food, the caterpillar one day found itself compelled by an inner monitor to alter its habits. Why, it knew not; but, just as a tired child sinks to sleep, the gorged and full-fed caterpillar sank peacefully into a dormant state. Then its tissues melted one by one into a kind of organic pap, and its outer skin hardened into a chrysalis. Within that solid case new limbs and organs began to grow by hereditary impulses. At the same time the form of the nervous system altered, to suit the higher and freer life for which the insect was unconsciously preparing itself. Fewer and smaller ganglia now appeared in the tail segments (since no legs would any longer be needed there), while more important ones sprang up to govern the motions of the four wings. But it was in the head that the greatest changes took place. There, a rudimentary brain made its appearance, with large optic centres, answering to the far more perfect and important eyes of the future butterfly. For the flying insect will have to steer its way through open space, instead of creeping over leaves and stones; and it will have to suck the honey of flowers, as well as to choose its fitting mate, all of which demands from it higher and keener senses than those of the purblind caterpillar. At length one day the chrysalis bursts asunder, and the insect emerges to view on a summer morning as a full-fledged and beautiful butterfly.

For a minute or two it stands and waits till the air it breathes has filled out its wings, and till the warmth and sunlight have given it strength. For the wings are by origin a part of the breathing apparatus, and they require to be plimmed by the air before the insect can take to flight. Then, as it grows more accustomed to its new life, the hereditary impulse causes it to spread its vans abroad, and it flies. Soon a flower catches its eye, and the bright mass of colour attracts it irresistibly, as the candle-light attracts the eye of a child a few weeks old. It sets off towards the patch of red or yellow, probably not knowing beforehand that this is the visible symbol of food for it, but merely guided by the blind habit of its race, imprinted with binding force in the very constitution of its body. Thus the moths, which fly by night and visit only white flowers whose corollas still shine out in the twilight, are so irresistibly led on by the external stimulus of light from a candle falling upon their eyes that they cannot choose but move their wings rapidly in that direction; and though singed and blinded twice or three times by the flame, must still wheel and eddy into it, till at last they perish in the scorching blaze. Their instincts, or, to put it more clearly, their simple nervous mechanism, though admirably adapted to their natural circumstances, cannot be equally adapted to such artificial objects as wax candles. The butterfly in like manner is attracted automatically by the colour of his proper flowers, and settling upon them, sucks up their honey instinctively. But feeding is not now his only object in life: he has to find and pair with a suitable mate. That, indeed, is the great end of his winged existence. Here, again, his simple nervous system stands him in good stead. The picture of his kind is, as it were, imprinted on his little brain, and he knows his own mates the moment he sees them, just as intuitively as he knows the flowers upon which he must feed. Now we see the reason for the butterfly's large optic centres: they have to guide it in all its movements. In like manner, and by a like mechanism, the female butterfly or moth selects the right spot for laying her eggs, which of course depends entirely upon the nature of the young caterpillars' proper food. Each great group of insects has its own habits in this respect, may-flies laying their eggs on the water, many beetles on wood, flies on decaying animal matter, and butterflies mostly on special plants. Thus throughout its whole life the butterfly's activity is entirely governed by a rigid law, registered and fixed for ever in the constitution of its ganglia and motor nerves. Certain definite objects outside it invariably produce certain definite movements on the insect's part. No doubt it is vaguely conscious of all that it does: no doubt it derives a faint pleasure from due exercise of all its vital functions, and a faint pain when they are injured or thwarted; but on the whole its range of action is narrowed and bounded by its hereditary instincts and their nervous correlatives. It may light on one flower rather than another; it may choose a fresher and brighter mate rather than a battered and dingy one; but its little subjectivity is a mere shadow compared with ours, and it hardly deserves to be considered as more than a semi-conscious automatic machine.

XVI.
BUTTERFLY ÆSTHETICS

The other day, when I was watching that little red-spotted butterfly whose psychology I found so interesting, I hardly took enough account, perhaps, of the insect's own subjective feelings of pleasure and pain. The first great point to understand about these minute creatures is that they are, after all, mainly pieces of automatic mechanism: the second great point is to understand that they are probably something more than that as well. To-day I have found another exactly similar butterfly, and I am going to work out with myself the other half of the problem about him. Granted that the insect is, viewed intellectually, a cunning bit of nervous machinery, may it not be true at the same time that he is, viewed emotionally, a faint copy of ourselves?

Here he stands on a purple thistle again, true, as usual, to the plant on which I last found him. There can be no doubt that he distinguishes one colour from another, for you can artificially attract him by putting a piece of purple paper on a green leaf, just as the flower naturally attracts him with its native hue. Numerous observations and experiments have proved with all but absolute certainty that his discrimination of colour is essentially identical with our own; and I think, if we run our eye up and down nature, observing how universally all animals are attracted by pure and bright colours, we can hardly doubt that he appreciates and admires colour as well as discriminates it. Mr. Darwin certainly judges that butterflies can show an æsthetic preference of the sort, for he sets down their own lovely hues to the constant sexual selection of the handsomest mates. We must not, however, take too human a measure of their capacities in this respect. It is sufficient to believe that the insect derives some direct enjoyment from the stimulation of pure colour, and is hereditarily attracted by it wherever it may show itself. This pleasure draws it on, on the one hand, towards the gay flowers which form its natural food; and, on the other hand, towards its own brilliant mates. Imprinted on its nervous system is a certain blank form answering to its own specific type; and when the object corresponding to this blank form occurs in its neighbourhood, the insect blindly obeys its hereditary instinct. But out of two or three such possible mates it naturally selects that which is most brightly spotted, and in other ways most perfectly fulfils the specific ideal. We need not suppose that the insect is conscious of making a selection or of the reasons which guide it in its choice: it is enough to believe that it follows the strongest stimulus, just as the child picks out the biggest and reddest apple from a row of ten. Yet such unconscious selections, made from time to time in generation after generation, have sufficed to produce at last all the beautiful spots and metallic eyelets of our loveliest English or tropical butterflies. Insects always accustomed to exercising their colour-sense upon flowers and mates, may easily acquire a high standard of taste in that direction, while still remaining comparatively in a low stage as regards their intellectual condition. But the fact I wish especially to emphasise is this – that the flowers produced by the colour-sense of butterflies and their allies are just those objects which we ourselves consider most lovely in nature; and that the marks and shades upon their own wings, produced by the long selective action of their mates, are just the things which we ourselves consider most beautiful in the animal world. In this respect, then, there seems to be a close community of taste and feeling between the butterfly and ourselves.

Let me note, too, just in passing, that while the upper half of the butterfly's wing is generally beautiful in colour, so as to attract his fastidious mate, the under half, displayed while he is at rest, is almost always dull, and often resembles the plant upon which he habitually alights. The first set of colours is obviously due to sexual selection, and has for its object the making of an effective courtship; but the second set is obviously due to natural selection, and has been produced by the fact that all those insects whose bright colours show through too vividly when they are at rest fall a prey to birds or other enemies, leaving only the best protected to continue the life of the species.

But sight is not the only important sense to the butterfly. He is largely moved and guided by smell as well. Both bees and butterflies seem largely to select the flowers they visit by means of smell, though colour also aids them greatly. When we remember that in ants scent alone does duty instead of eyes, ears, or any other sense, it would hardly be possible to doubt that other allied insects possessed the same faculty in a high degree; and, as Dr. Bastian says, there seems good reason for believing that all the higher insects are guided almost as much by smell as by sight. Now it is noteworthy that most of those flowers which lay themselves out to attract bees and butterflies are not only coloured but sweetly scented; and it is to this cause that we owe the perfumes of the rose, the lily-of-the-valley, the heliotrope, the jasmine, the violet, and the stephanotis. Night-flowering plants, which depend entirely for their fertilisation upon moths, are almost always white, and have usually very powerful perfumes. Is it not a striking fact that these various scents are exactly those which human beings most admire, and which they artificially extract for essences? Here, again, we see that the æsthetic tastes of butterflies and men decidedly agree; and that the thyme or lavender whose perfume pleases the bee is the very thing which we ourselves choose to sweeten our rooms.

Finally, if we look at the sense of taste, we find an equally curious agreement between men and insects; for the honey which is stored by the flower for the bee, and by the bee for its own use, is stolen and eaten up by man instead. Hence, when I consider the general continuity of nervous structure throughout the whole animal race, and the exact similarity of the stimulus in each instance, I can hardly doubt that the butterfly really enjoys life somewhat as we enjoy it, though far less vividly. I cannot but think that he finds honey sweet, and perfumes pleasant, and colour attractive; that he feels a lightsome gladness as he flits in the sunshine from flower to flower, and that he knows a faint thrill of pleasure at the sight of his chosen mate. Still more is this belief forced upon me when I recollect that, so far as I can judge, throughout the whole animal world, save only in a few aberrant types, sugar is sweet to taste, and thyme to smell, and song to hear, and sunshine to bask in. Therefore, on the whole, while I admit that the butterfly is mainly an animated puppet, I must qualify my opinion by adding that it is a puppet which, after its vague little fashion, thinks and feels very much as we do.