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besides many illustrating the geology of other countries, particularly North America, India, and Australia, the results of private labour, and of our colonial or foreign geological surveys.
Such is the intimate connection between physical features and geological structure, that to the geologist a glance at the geological map of a country would enable him to obtain a very good idea of the character of its scenery. Denudation and the origin of scenery are most interesting subjects ; how hard and soft rocks are acted upon by rain, river, and sea; how our bays and gulfs, islands and straits, cliffs and gorges, hills and valleys, were formed, are subjects now much discussed: they link geology with physical geography. Elevation gave the plan; denudation did the work. Gedlogy is but the “physical geography of past ages ;” we must interpret the past by the light of the present.‘ Everything betokens change, and we cannot but sigh when we think of the vast amount of denudation that is going on. At this rate, by atmospheric agencies alone, a mass of land as large as Europe must, according to a rough estimate of Professor Geikie, disappear in about 4,000,000 years I What use, then, will our geological maps be? However, if we have thrown any light upon them, and pointed out some of their uses, so that they may be more fully appreciated at the present day, we may feel happy that our labour has not been lost. '
VERY day, every hour, there is going on around us a veritable death-struggle. It excites little attention. People would be in no hurry to read the telegraphic despatches 'concerning it from the seat of war, even if there were any to read. Special correspondents there are, but their letters are appreciated but by a few. Nevertheless, it cannot be said that mankind in general is not interested in the result of the struggle. On the contrary, little as the affair is heeded, it is of very serious import‘ to the human race. Our food-supplies depend on it; the well-being of our flocks and herds is essentially dependent on it; the building of our houses, the fabrication of our raiment, are to a large extent contingent on it; nay, the soil beneath our feet, and the very sky above our heads, are materially, very materially, influenced by the result of the contest of which we are about to speak. Edward Forbes was wont to say that the movement of a periwinkle over a rock might be of greater consequence to the human race than the progress of an Alexander; and the results of the wars of the plants are assuredly of no less importance, seeing that the very
existence of an Alexander depends in no slight degree upon '
them. The campaigns we speak of are real; they are not mental figments, or allegorical illustrations. Success in the practice of horticulture, of agriculture, of forestry, depends on the action we men take towards the combatants. If we remain neutral, the weakest goes to the wall, overpowered by the stronger; if we interfere, we exert a very powerful influence for the time; but immediately we cease to exert our power, the combat begins again, and with enhanced violence. The essence of successful cultivation often consists almost entirely in the removal of the plant from the influence of that hostile “ environment” to which, under natural circumstances, it would be subjected. It is this that accounts, in a great
measure, though of ~course not wholly, for the oft-observed fact
that certain plants, flowers, and fruits, attain far greater perfection in our gardens than they ever do in their native countries.
That 'a war of extermination is thus going on around us may strike some with surprise. They are so accustomed to associate flowers and plants with peace and repose, that they are astonished to find that other far less amiable ideas may, with even more justice, be associated with them. And yet a moment’s reflection, or a'passing glance at the nearest hedgerow or pasture, will show the reality of the struggle. All that beautiful disorder, that apparently careless admixture of divers fonns and‘ colours—the sweeping curves of the brambles, the entwining coils of the honeysuckle, the creeping interlacement of the ground ivy or the pennywort—all are but indications of the fray that is constantly going on. It would seem as if the weakest must succumb, must be overpowered by the strongergrowing plants, and so they are at certain places and at certain times; but, under other conditions, the victory may be with the apparently weaker side, just as the slow-going tortoise may outrun the fleeter hare. In any case the success is often only temporary; the victor becomes in time the vanquished; the vanquished, in its turn, regains its former conquest; and so on.
It is proposed in the following notes to give a few illustrations of the nature and effects of this conflict, of the way in which it is carried on, and of the circumstances which favour it. '
Agriculturists had long been practically conversant with the_ advantages derivable from the practice of not growing the same crop on the same soil for too long a period. The advantages consequent on this so-called rotation of crops are due to more than one cause; but it was Dureau de la Malle who, in 1825, called attention to the phenomenon of natural rotation. From long observation of what takes place in woods and pasture-lands, he established the fact that an alternation of growth, as he called it, occurs as a natural phenomenon. In pasture-lands, for instance, the grasses get the upper hand at One time, the leguminous plants at another; so that, in the course of thirty years, the author whose observations we are citing was witness of five or six such alternations.
It follows from all this that a plant, as was pointed out by the late Dean Herbert, does not necessarily grow in the situation best adapted for it, but where it can best hold its own against its hostile neighbours, and best sustain itself against unfavourable conditions generally.
The sources of success in the contest are manifold; they vary more or less in each individual case. Probably they are never exactly the same ; nevertheless, there are certain circumstances which must always be operative in conducing to the
victory. A few illustrations must sufiice. It is easy to understand why first comers, duly installed, should have an advantage over later visitants; why the more prolific should outnumber the less fertile; and how it is that a perennial plant has a better chance on any given spot, coeter'is paribus, than an annual whose progeny would find the ground occupied, and their chances of survival materially interfered with by their longer-lived neighbours.
Again, there is no difficulty in understanding why such plants as quitch (Triticum reports) or bearbine (Convoloulus Sepium) hold their own so tenaciously and so much to the prejudice of their neighbours. The long creeping underground stems rooting, or capable of rooting, at every joint give them an immense advantage over plants not so favourably organised. The ends of the shoots of the convolvulus, moreover, dilate into tubers, which are thrust into the ground to form in the succeeding spring fresh centres of vegetation. A great rooting power is obviously of great benefit; not less so is an extensive leaf surface. It is not only that the' copious feeding roots absorb the available nourishment from the soil, not only that the wide leaf surface avails itself of every ray of sunlight, every whiff of air that plays over it, and thus serves to build up the tissues of the plant to which the root or leaf respectively belong, but they practically oust other plants less favourably circumstanced than themselves. The roots occupy 'the soil, and rob the weaker plants of their share of its resources. The tree with dense foliage shuts off from its lowlier neighbour much of the light and air necessary for its existence; and hence, in a measure, the absence of vegetation in pine forests or under the shadow of dense woods.* Some plants there are specially organised to resist and overcome these hostile conditions. Among them are the climbers, the twining plants, and those with tendrils of one sort or another. The bramble or wild rose, with its slender, arching, hookbeset branches; the wild hop, with its coils of cord-like sprays; the clematis, clinging on firmly by means of its leaf-stalks to anything it can lay hold of; the ivy, grappling with the trunk of a tree—all these are, in some sense, weakly plants; they would be overweighted in the struggle with their stronger neighbours if it were not for the special adaptation of their structure just alluded to, and which enables them to bear their part bravely in the conflict.
‘ These struggles were not unknown to ancient naturalists, as witness tl e following passage from Pliny, “Nat. Hist." lib. xv. cap. 24z—“Necant invicem inter sese umbra vel densitate atque alimenti rapina . . . necnt at edera vinciens, nee viscum prodest et cytisus necatur e0 quod halimon.» vocant Graaci.” .
It is easy to understand how an alteration of the conditions under which plants grow influences very materially the struggle we have been alluding to. A very slight change in climatal conditions—produced, for instance, by the growth of sheltering trees, or by the drainage of the soil—may be followed by the growth of quite a different set of plants from those that occupied the ground previously. The altered conditions have been advantageous to the one and disadvantageous to the other set of plants.
As an illustration of the complexity of the checks and relations between organic beings struggling together, Darwin mentions the case of a barren heath which fell under his observation, part of which was left intact, while another portion had been enclosed and planted with Scotch fir. The change in the native vegetation of the planted part of the heath was most remarkable. “Not only the proportional numbers of the heath plants were wholly changed, but twelve species of plants, not counting grasses and carices, flourished in the plantations which could not be found on the heath.”
This sort of change was pointedly referred to by Dureau de la Malle, who relates how, after the felling of the timber in forests of a particular district of France, broom, foxglove, heaths, birch-trees, and aspens sprung up, replacing the oaks, the beech, and the ash felled by the woodman. After thirty years the birch and poplars were felled in their turn. Still very few of the original possessors of the soil, the oaks, &c., made their appearance; the ground was still occupied with young birch and poplar. It is not till after the third repetition of the coppicing—after an interval of ninety years—that the oaks and beech reconquer their original position. They retain it for a time, and then the struggle begins again.
Antiquarian researches also have proved that in the natural state of things, without any violent change in external conditions, the nature of forests becomes altered. The Hercynian forests, of which Caesar speaks, and which then consisted of deciduous-leaved trees, are now made up principally of conifers. A forest which, in the Middle Ages, was of beech, is now stocked with oak, and 'vice 've'rsd. Again, we have the evidence afforded by submerged forests and peat bogs, according to which certain plants, now extinct in particular localities, once flourished there. We are not alluding to plants that may have required a different climate from what they now experience, but to such cases as the silver fir, the Scotch fir, Pinus Mughus, &c., which are found in this partially fossilised condition in spots where there is apparently nothing to prevent them from growing now, where in fact they do grow well when planted. ‘