Part 4

We have adopted an easy and slovenly way of dividing all rocks into primary, secondary, and tertiary, which veils from us the real chronological relations of evolving life in the different periods. The lias is ranked by geologists among the earliest secondary formations: but if we were to distribute all the sedimentary rocks into ten great epochs, each representing about equal duration in time, the lias would really fall in the tenth and latest of all. So very misleading to the ordinary mind is our accepted geological nomenclature. Nay, even commonplace geologists themselves often overlook the real implications of many facts and figures which they have learned to quote glibly enough in a certain off-hand way. Let me just briefly reconstruct the chief features of this scarcely recognised world's chronology as I sit on this piece of fallen chalk at the foot of the mouldering cliff, where the stream from the meadow above brought down the newest landslip during the hard frosts of last December. First of all, there is the vast lapse of time represented by the Laurentian rocks of Canada. These Laurentian rocks, the oldest in the world, are at least 30,000 feet in thickness, and it must be allowed that it takes a reasonable number of years to accumulate such a mass of solid limestone or clay as that at the bottom of even the widest primæval ocean. In these rocks there are no fossils, except a single very doubtful member of the very lowest animal type. But there are indirect traces of life in the shape of limestone probably derived from shells, and of black lead probably derived from plants. All these early deposits have been terribly twisted and contorted by subsequent convulsions of the earth, and most of them have been melted down by volcanic action; so that we can tell very little about their original state. Thus the history of life opens for us, like most other histories, with a period of uncertainty: its origin is lost in the distant vistas of time. Still, we know that there _was_ such an early period; and from the thickness of the rocks which represent it we may conjecture that it spread over three out of the ten great æons into which I have roughly divided geological time. Next comes the period known as the Cambrian, and to it we may similarly assign about two and a half æons on like grounds. The Cambrian epoch begins with a fair sprinkling of the lower animals and plants, presumably developed during the preceding age; but it shows no remains of fish or any other vertebrates. To the Silurian, Devonian, and Carboniferous periods we may roughly allow an æon and a fraction each: while to the whole group of secondary and tertiary strata, comprising almost all the best-known English formations--red marl, lias, oolite, greensand, chalk, eocene, miocene, pliocene, and drift--we can only give a single æon to be divided between them. Such facts will sufficiently suggest how comparatively modern are all these rocks when viewed by the light of an absolute chronology. Now, the first fishes do not occur till the Silurian--that is to say, in or about the seventh æon after the beginning of geological time. The first mammals are found in the trias, at the beginning of the tenth æon. And the first known bird only makes its appearance in the oolite, about half-way through that latest period. This will show that there was plenty of time for their development in the earlier ages. True, we must reckon the interval between ourselves and the date of this blue mud at many millions of years; but then we must reckon the interval between the lias and the earliest Cambrian strata at some six times as much, and between the lias and the lowest Laurentian beds at nearly ten times as much. Just the same sort of lessening perspective exists in geology as in ordinary history. Most people look upon the age before the Norman Conquest as a mere brief episode of the English annals; yet six whole centuries elapsed between the landing of the real or mythical Hengst at Ebbsfleet and the landing of William the Conqueror at Hastings; while under eight centuries elapsed between the time of William the Conqueror and the accession of Queen Victoria. But, just as most English histories give far more space to the three centuries since Elizabeth than to the eleven centuries which preceded them, so most books on geology give far more space to the single æon (embracing the secondary and tertiary periods) which comes nearest our own time, than to the nine æons which spread from the Laurentian to the Carboniferous epoch. In the earliest period, records either geological or historical are wholly wanting; in the later periods they become both more numerous and more varied in proportion as they approach nearer and nearer to our own time.

So too, in the days when Mr. Darwin first took away the breath of scientific Europe by his startling theories, it used confidently to be said that geology had shown us no intermediate form between species and species. Even at the time when this assertion was originally made it was quite untenable. All early geological forms, of whatever race, belong to what we foolishly call 'generalised' types: that is to say, they present a mixture of features now found separately in several different animals. In other words, they represent early ancestors of all the modern forms, with peculiarities intermediate between those of their more highly differentiated descendants; and hence we ought to call them 'unspecialised' rather than 'generalised' types. For example, the earliest ancestral horse is partly a horse and partly a tapir: we may regard him as a _tertium quid_, a middle term, from which the horse has varied in one direction and the tapir in another, each of them exaggerating certain special peculiarities of the common ancestor and losing others, in accordance with the circumstances in which they have been placed. Science is now perpetually discovering intermediate forms, many of which compose an unbroken series between the unspecialised ancestral type and the familiar modern creatures. Thus, in this very case of the horse, Professor Marsh has unearthed a long line of fossil animals which lead in direct descent from the extremely unhorse-like eocene type to the developed Arab of our own times. Similarly with birds, Professor Huxley has shown that there is hardly any gap between the very bird-like lizards of the lias and the very lizard-like birds of the oolite. Such links, discovered afresh every day, are perpetual denials to the old parrot-like cry of 'No geological evidence for evolution.'

VIII.

_CUCKOO-PINT._

In the bank which supports the hedge, beside this little hanger on the flank of Black Down, the glossy arrow-headed leaves of the common arum form at this moment beautiful masses of vivid green foliage. 'Cuckoo-pint' is the pretty poetical old English name for the plant; but village children know it better by the equally quaint and fanciful title of 'lords and ladies.' The arum is not now in flower: it blossomed much earlier in the season, and its queer clustered fruits are just at present swelling out into rather shapeless little light-green bulbs, preparatory to assuming the bright coral-red hue which makes them so conspicuous among the hedgerows during the autumn months. A cut-and-dry technical botanist would therefore have little to say to it in its present stage, because he cares only for the flowers and seeds which help him in his dreary classifications, and give him so splendid an opportunity for displaying the treasures of his Latinised terminology. But to me the plant itself is the central point of interest, not the names (mostly in bad Greek) by which this or that local orchid-hunter has endeavoured to earn immortality.

This arum, for example, grows first from a small hard seed with a single lobe or seed-leaf. In the seed there is a little store of starch and albumen laid up by the mother-plant, on which the young arum feeds, just as truly as the growing chick feeds on the white which surrounds its native yolk, or as you and I feed on the similar starches and albumens laid by for the use of the young plant in the grain of wheat, or for the young fowl in the egg. Full-grown plants live by taking in food-stuffs from the air under the influence of sunlight: but a young seedling can no more feed itself than a human baby can; and so food is stored up for it beforehand by the parent stock. As the kernel swells with heat and moisture, its starches and albumens get oxidised and produce the motions and rearrangements of particles that result in the growth of a new plant. First a little head rises towards the sunlight and a little root pushes downward towards the moist soil beneath. The business of the root is to collect water for the circulating medium--the sap or blood of the plant--as well as a few mineral matters required for its stem and cells; but the business of the head is to spread out into leaves, which are the real mouths and stomachs of the compound organism. For we must never forget that all plants mainly grow, not, as most people suppose, from the earth, but from the air. They are for the most part mere masses of carbon-compounds, and the carbon in them comes from the carbonic acid diffused through the atmosphere around, and is separated by the sunlight acting in the leaves. There it mixes with small quantities of hydrogen and nitrogen brought by the roots from soil and water; and the starches or other bodies thus formed are then conveyed by the sap to the places where they will be required in the economy of the plant system. That is the all-important fact in vegetable physiology, just as the digestion and assimilation of food and the circulation of the blood are in our own bodies.

The arum, like the grain of wheat, has only a single seed-leaf; whereas the pea, as we all know, has two. This is the most fundamental difference among flowering plants, as it points back to an early and deep-seated mode of growth, about which they must have split off from one another millions of years ago. All the one-lobed plants grow with stems like grasses or bamboos, formed by single leaves enclosing another; all the double-lobed plants grow with stems like an oak, formed of concentric layers from within outward. As soon as the arum, with its sprouting head, has raised its first leaves far enough above the ground to reach the sunlight, it begins to form fresh starches and new leaves for itself, and ceases to be dependent upon the store laid up in its buried lobe. Most seeds accordingly contain just enough material to support the young seedling till it is in a position to shift for itself; and this, of course, varies greatly with the habits and manners of the particular species. Some plants, too, such as the potato, find their seeds insufficient to keep up the race by themselves, and so lay by abundant starches in underground branches or tubers, for the use of new shoots; and these rich starch receptacles we ourselves generally utilise as food-stuffs, to the manifest detriment of the young potato-plants, for whose benefit they were originally intended. Well, the arum has no such valuable reserve as that; it is early cast upon its own resources, and so it shifts for itself with resolution. Its big, glossy leaves grow apace, and soon fill out, not only with green chlorophyll, but also with a sharp and pungent essence which makes them burn the mouth like cayenne pepper. This acrid juice has been acquired by the plant as a defence against its enemies. Some early ancestor of the arums must have been liable to constant attacks from rabbits, goats, or other herbivorous animals, and it has adopted this means of repelling their advances. In other words, those arums which were most palatable to the rabbits got eaten up and destroyed, while those which were nastiest survived, and handed down their pungency to future generations. Just in the same way nettles have acquired their sting and thistles their prickles, which efficiently protect them against all herbivores except the patient, hungry donkey, who gratefully accepts them as a sort of _sauce piquante_ to the succulent stems.

And now the arum begins its great preparations for the act of flowering. Everybody knows the general shape of the arum blossom--if not in our own purple cuckoo-pint, at least in the big white 'Æthiopian lilies' which form such frequent ornaments of cottage windows. Clearly, this is a flower which the plant cannot produce without laying up a good stock of material beforehand. So it sets to work accumulating starch in its root. This starch it manufactures in its leaves, and then buries deep underground in a tuber, by means of the sap, so as to secure it from the attacks of rodents, who too frequently appropriate to themselves the food intended by plants for other purposes. If you examine the tuber before the arum has blossomed, you will find it large and solid; but if you dig it up in the autumn after the seeds have ripened, you will see that it is flaccid and drained; all its starches and other contents have gone to make up the flower, the fruit, and the stalk which bore them. But the tuber has a further protection against enemies besides its deep underground position. It contains an acrid juice like that of the leaves, which sufficiently guards it against four-footed depredators. Man, however, that most persistent of persecutors, has found out a way to separate the juice from the starch; and in St. Helena the big white arum is cultivated as a food-plant, and yields the meal in common use among the inhabitants.

When the arum has laid by enough starch to make a flower it begins to send up a tall stalk, on the top of which grows the curious hooded blossom known to be one of the earliest forms still surviving upon earth. But now its object is to attract, not to repel, the animal world; for it is an insect-fertilised flower, and it requires the aid of small flies to carry the pollen from blossom to blossom. For this purpose it has a purple sheath around its head of flowers and a tall spike on which they are arranged in two clusters, the male blossoms above and the female below. This spike is bright yellow in the cultivated species. The fertilisation is one of the most interesting episodes in all nature, but it would take too long to describe here in full. The flies go from one arum to another, attracted by the colour, in search of pollen; and the pistils, or female flowers, ripen first. Then the pollen falls from the stamens or male flowers on the bodies of the flies, and dusts them all over with yellow powder. The insects, when once they have entered, are imprisoned until the pollen is ready to drop, by means of several little hairs, pointing downwards, and preventing their exit on the principle of an eel-trap or lobster-pot. But as soon as the pollen is discharged the hairs wither away, and then the flies are free to visit a second arum. Here they carry the fertilising dust with which they are covered to the ripe pistils, and so enable them to set their seed; but, instead of getting away again as soon as they have eaten their fill, they are once more imprisoned by the lobster-pot hairs, and dusted with a second dose of pollen, which they carry away in turn to a third blossom.

As soon as the pistils have been impregnated, the fruits begin to set. Here they are, on their tall spike, whose enclosing sheath has now withered away, while the top is at this moment slowly dwindling, so that only the cluster of berries at its base will finally remain. The berries will swell and grow soft, till in autumn they become a beautiful scarlet cluster of living coral. Then once more their object will be to attract the animal world, this time in the shape of field-mice, squirrels, and small birds; but with a more treacherous intent. For though the berries are beautiful and palatable enough they are deadly poison. The robins or small rodents which eat them, attracted by their bright colours and pleasant taste, not only aid in dispersing them, but also die after swallowing them, and become huge manure heaps for the growth of the young plant. So the whole cycle of arum existence begins afresh, and there is hardly a plant in the field around me which has not a history as strange as this one.

IX.

_BERRIES AND BERRIES._

This little chine, opening toward the sea through the blue lias cliffs, has been worn to its present pretty gorge-like depth by the slow action of its tiny stream--a mere thread of water in fine weather, that trickles down its centre in a series of mossy cascades to the shingly beach below. Its sides are overgrown by brambles and other prickly brushwood, which form in places a matted and impenetrable mass: for it is the habit of all plants protected by the defensive armour of spines or thorns to cluster together in serried ranks, through which cattle or other intrusive animals cannot break. Amongst them, near the down above, I have just lighted upon a rare plant for Southern Britain--a wild raspberry-bush in full fruit. Raspberries are common enough in Scotland among heaps of stones on the windiest hillsides; but the south of England is too warm and sickly for their robust tastes, and they can only be found here in a few bleak spots like the stony edges of this weather-beaten down above the chine. The fruit itself is quite as good as the garden variety, for cultivation has added little to the native virtues of the raspberry. Good old Izaak Walton is not ashamed to quote a certain quaint saying of one Dr. Boteler concerning strawberries, and so I suppose I need not be afraid to quote it after him. 'Doubtless,' said the Doctor, 'God _could_ have made a better berry, but doubtless also God never did.' Nevertheless, if you try the raspberry, picked fresh, with plenty of good country cream, you must allow that it runs its sister fruit a neck-and-neck race.

To compare the structure of a raspberry with that of a strawberry is a very instructive botanical study. It shows how similar causes may produce the same gross result in singularly different ways. Both are roses by family, and both have flowers essentially similar to that of the common dog-rose. But even in plants where the flowers are alike, the fruits often differ conspicuously, because fresh principles come into play for the dispersion and safe germination of the seed. This makes the study of fruits the most complicated part in the unravelling of plant life. After the strawberry has blossomed, the pulpy receptacle on which it bore its green fruitlets begins to swell and redden, till at length it grows into an edible berry, dotted with little yellow nuts, containing each a single seed. But in the raspberry it is the separate fruitlets themselves which grow soft and bright-coloured, while the receptacle remains white and tasteless, forming the 'hull' which we pull off from the berry when we are going to eat it. Thus the part of the raspberry which we throw away answers to the part of the strawberry which we eat. Only, in the raspberry the separate fruitlets are all crowded close together into a single united mass, while in the strawberry they are scattered about loosely, and embedded in the soft flesh of the receptacle. The blackberry is another close relative; but in its fruit the little pulpy fruitlets cling to the receptacle, so that we pick and eat them both together; whereas in the raspberry the receptacle pulls out easily, and leaves a thimble-shaped hollow in the middle of the berry. Each of these little peculiarities has a special meaning of its own in the history of the different plants.

Yet the main object attained by all is in the end precisely similar. Strawberries, raspberries, and blackberries all belong to the class of attractive fruits. They survive in virtue of the attention paid to them by birds and small animals. Just as the wild strawberry which I picked in the hedgerow the other day procures the dispersion of its hard and indigestible fruitlets by getting them eaten together with the pulpy receptacle, so does the raspberry procure the dispersion of its soft and sugary fruitlets by getting them eaten all by themselves. While the strawberry fruitlets retain throughout their dry outer coating, in those of the raspberry the external covering becomes fleshy and red, but the inner seed has, notwithstanding, a still harder shell than the tiny nuts of the strawberry. Now, this is the secret of nine fruits out of ten. They are really nuts, which clothe themselves in an outer tunic of sweet and beautifully coloured pulp. The pulp, as it were, the plant gives in, as an inducement to the friendly bird to swallow its seed; but the seed itself it protects by a hard stone or shell, and often by poisonous or bitter juices within. We see this arrangement very conspicuously in a plum, or still better in a mango; though it is really just as evident in the raspberry, where the smaller size renders it less conspicuous to human sight.

It is a curious fact about the rose family that they have a very marked tendency to produce such fleshy fruits, instead of the mere dry seed-vessels of ordinary plants, which are named fruits only by botanical courtesy. For example, we owe to this single family the peach, plum, apricot, cherry, damson, pear, apple, medlar, and quince, all of them cultivated in gardens or orchards for their fruits. The minor group known by the poetical name of Dryads, alone supplies us with the strawberry, raspberry, blackberry, and dewberry. Even the wilder kinds, refused as food by man, produce berries well known to our winter birds--the haw, rose-hip, sloe, bird-cherry, and rowan. On the other hand, the whole tribe numbers but a single thoroughgoing nut--the almond; and even this nut, always somewhat soft-shelled and inclined to pulpiness, has produced by a 'sport' the wholly fruit-like nectarine. The odd thing about the rose tribe, however, is this: that the pulpy tendency shows itself in very different parts among the various species. In the plum it is the outer covering of the true fruit which grows soft and coloured: in the apple it is a swollen mass of the fruit-stalk surrounding the ovules: in the rose-hip it is the hollowed receptacle: and in the strawberry it is the same receptacle, bulging out in the opposite direction. Such a general tendency to display colour and collect sugary juices in so many diverse parts may be compared to the general bulbous tendency of the tiger-lily or the onion, and to the general succulent tendency of the cactus or the house-leek. In each case, the plant benefits by it in one form or another; and whichever form happens to get the start in any particular instance is increased and developed by natural selection, just as favourable varieties of fruits or flowers are increased and developed in cultivated species by our own gardeners.