Natural barrier

“ Its internal relations, whether friendly or hostile, are affected by these. Natural barriers, such as mountains, seas, or great lakes and rivers, are often political frontiers exerting protecting or isolating influence. ”

“ Each species broadens its range as far as it can. It struggles knowingly or not to overcome the barriers of ocean or river, of mountain or plain, of woodland or desert, of moisture or drought, of cold or heat, of lack of food or abundance of enemies, whatever these barriers may be. Were it not for these barriers, every species would become what only man now is, practically cosmopolitan. Man is pre-eminently the barrier-crossing animal. ”

“ Save on the seaward terraces of stark rock, with their tide-marked base of weed-covered boulders, the densest vegetation known to mankind imposed everywhere a first barrier to human progress far more unconquerable than the awesome regions beyond. ”

“ Such boundaries mark the limits of that movement which is common to all animate things. Every living form spreads until it meets natural conditions in which it can no longer survive, or until it is checked by the opposing expansion of some competing form. ”

“ A topographic feature or a physical or a biological agency that restricts or prevents invasions is a barrier. Topographic features are usually permanent and produce permanent barriers. Biological ones are often temporary and exist for a few years or even a single season. Temporary barriers are often recurrent, however. Barriers are complete or incomplete with respect to the thoroughness of their action. They may affect invasion either by limiting migration or by preventing ecesis. ”

“ What if the base of the great ice barrier rested upon the ocean bed? What if it were not a floating chain of ice mountains, as he believed, but an immovable line of cliffs, their icy feet gripping the sandy bed of the Polar Sea?
Such might easily be the case; and if so, what then?
Ay! what then?
The scientist answered the question for himself.
A humiliating retreat from the barrier which had battled them; a still more humiliating return to their native shore, there to endure the scoffs and sneers of every dabbler in science who could put pen to paper. ”

“ This was an allusion to the fashion we had to employ in picking our steps on the lookout for holes. In this region the fair face of nature is distorted in every conceivable way with holes and ditches, some of the holes big enough to engulf a house, and it is no mere desire to avoid the water in these holes that compels us to pick our steps in this hell-swept part of the world; it is the first law of nature, self preservation, for many a poor lad has been done to death in them by drowning. ”

“ A similar process can be identified in our present relation to the physical environment. Many things mediate between us and the natural environment: our homes, clothes, the food processing industry. Even natural artifacts, such as gardens, lakes, or water channels, are a buffer against nature, an insertion between us and nature. Constituted in our language are experiences of survival and adaptation: the vocabulary of hunting, fishing, agriculture, animal husbandry, coping with changes in weather and climate, and coping with natural catastrophes such as floods and earthquakes. ”

“ Is the environment more likely to preserve or to destroy him, on account of this or that peculiarity with which he may be born? In giving the name of "accidental variations" to those peculiarities with which an animal is born, Darwin does not for a moment mean to suggest that they are not the fixed outcome of natural law. If the total system of the universe be taken into account, the causes of these variations and the visible environment which preserves or destroys them, undoubtedly do, in some remote and round-about way, hang together. ”

“ The degree of hindrance offered by any barrier to the extension of species is only relative. That which constitutes an impassable barrier to some groups is a high-road to others. The river which opposes the passage of the monkey or the cat would be the king's highway to the frog or the turtle. The waterfall which checks the ascent of the fish would be the chosen home of the ouzel.
In spite of the great variety among the barriers existing on the earth, we may divide the globe roughly into five realms or areas of distribution, having their boundaries in the sea or in differences of climate. ”

“ We have thus far spoken of species as limited by temperature alone, but they are limited also by barriers. If, then, there be an east and west barrier, such as a high mountain-range, or a wide sea or desert, there will be no shading or gradation of any kind, because the barrier prevents overlapping, interpenetration, and struggle on the margins. For example: The species north and south of the Himalayas, or north and south of Sahara, are widely different. ”

“ In other countries, dunes have spontaneously clothed themselves with forests, and the rapidity with which their surface is covered by various species of sand-plants, and finally by trees, where man and cattle and burrowing animals are excluded from them, renders it highly probable that they would, as a general rule, protect themselves, if left to the undisturbed action of natural causes. ”

“ The area which a race or people occupies is the resultant of the expansive force within and the obstacles without, either physical or human. Insurmountable physical obstacles are met where all life conditions disappear, as on the borders of the habitable world, where man is barred from the unpeopled wastes of polar ice-fields and unsustaining oceans. ”

“ Moreover, we find that the more terrestrial an organism, or the greater the difficulty it has in traversing a wide reach of ocean, the greater is the modifying influence of such a barrier upon that type. ”

“ In the case of land and fresh-water organisms the sea is the chief barrier; in the case of marine organisms, the land. Differences in land forms do not exert great influence on the distribution of living creatures directly, but indirectly such land forms as mountain ranges and internal drainage basins are very potent through their action on soil and climate. A snow-capped mountain ridge or an arid desert forms a barrier between different forms of life which is often more effective than an equal breadth of sea. ”

“ When, however, the return of the tide has closed the gates, and prevented this, though the space within is sufficiently large for pleasure and safety, the idea of constraint and danger sets them on effecting their deliverance. The wall is examined in every part; and when the water is near the summit, efforts are made to throw themselves over, by which they are not uncommonly left on the bank, to their own destruction. ”

“ Each was entirely aware of the mental and spiritual barrier, which made it absolutely impossible for them to approach each other in the sense of reality. A barrier infinitely more forbidding than any material one of stone or iron. ”

“ Wherever Nature left a flat it is made rugged and scarcely passable by enormous and fallen trunks, accumulated by the storms of ages, and forming, by their slow decay, a moss-covered soil, the haunt of rabbits and lizards. ”

“ He urges, that these constant differences cannot be attributed to natural selection, because they occur in different valleys on the same side of the mountain, where food, climate, and enemies are the same ”

“ Transition forms of relief between highlands and lowlands.
In view of the barrier character of mountains, a fact of immense importance to the distribution of man and his activities is the rarity of abrupt, ungraded forms of relief on the earth's surface. The physiographic cause lies in the elasticity of the earth's crust and the leveling effect of weathering and denudation. Everywhere mountains are worn down and rounded off, while valleys broaden and fill up to shallow trough outlines. Transition forms of relief abound. Human intercourse meets therefore few absolute barriers on the land ”

“ Physical barriers are typically permanent in character, while biological ones are either permanent or temporary, depending upon the permanence of the formation and the constancy of the physical factors which determine it. ”

“ But the longer the tunnel the better would it serve in the above-described means of escape; therefore natural selection would tend to preserve the birds which made the longest tunnels, until the utmost benefit that length of tunnel could give had been attained.
And similarly, I believe, all the host of animal instincts may be fully explained by the joint operation of these two causes—intelligent adjustment and survival of the fittest. For now I may draw attention to another fact which is of great importance, viz., that instincts admit of being modified as modifying circumstances may require. ”

“ This unfortunate condition is partly accounted for by the nature of the land, broken by ravines, mountain torrents, lakes, and marshes, and bristling with hedges or earth-works which make a sort of citadel of every field; without roads, without canals, and at the mercy of prejudices which scorn our modern agriculture. ”

“ While they bring out clearly the effect of mountain barriers, indented coasts, and easy routes whether by land or water, they scarcely touch on the more subtle relationships between man on the one hand and the climate, plants, and animals which form the dominant features of his physical environment on the other hand. ”

“ Our hardy animals scrambled like chamois along, over the edges of the steep precipices, and carried us with safety past the terrible abyss, at the bottom of which the stream leapt, with a frightful roaring, from rock to rock. ”

“ The channels of great rivers become unnavigable, their estuaries are choked up, and harbors which once sheltered large navies are shoaled by dangerous sand-bars. The earth, stripped of its vegetable glebe, grows less and less productive, and, consequently, less able to protect itself by weaving a new network of roots to bind its particles together, a new carpeting of turf to shield it from wind and sun and scouring rain. ”

“ If you try to control the natural flow of the river through all these artificial means, building a dam, so to speak, you will inundate and destroy the whole thing. That is why you find thoughts welling up inside you despite your efforts to control, observe and be aware of them. ”

“ We inhabit these ruins however with a perfect security. The various generations of men, animals, and plants, succeed each other without interruption; the earth produces fully sufficient for their subsistence; the sea has its limits ”

“ When the universal inundation takes place, the whole surface of the earth is covered with water, and all animal life is destroyed; after which, nature is renewed and subsists as before, till the element of fire, becoming prevalent in its turn, dries up all the moisture, converts every substance into its own nature, and at last, by a universal conflagration, reduces the world to its pristine state. ”

“ You know that even now many tracts of the Earth’s surface are still covered with putrid morasses and impenetrable forests, the cold and damp atmosphere of which gives birth to noxious insects, and breathes forth devastating epidemics; which are almost entirely the dwelling-place of wild animals, and only afford to the few creatures in human form who are to be found in them the means of dragging on a dull and joyless existence, without freedom, usefulness, or dignity. ”

“ They have a look of individuality, like a crowd of big rough men pushing and elbowing one another for room, and you can see how this haphazard condition has come about when you stumble by chance on a huge mass of rock thrusting up out of the earth among the houses. ”

“ In countries with a humid sky, or moderately undulating surface and an equable temperature, a small extent of forest, enough to serve as a mechanical screen against the action of the wind in localities where such protection is needed, suffices. But most of the territory occupied by civilized man is exposed, by the character of its surface and its climate, to a physical degradation which cannot be averted except by devoting a large amount of soil to the growth of the woods. ”

“ But the comparative exemption of the American people from the terrible calamities which the overflow of rivers has brought on some of the fairest portions of the Old World, is, in a still greater degree, to be ascribed to the fact that, with all our thoughtless improvidence, we have not yet bared all the sources of our streams, not yet overthrown all the barriers which nature has erected to restrain her own destructive energies. ”

“ Had it not been for these convulsive disturbances of the surface, the earth would have presented an almost uniform plain, and it would have been ill-adapted for the abode of man. The hills raised by the disturbances of nature, and the valleys worn by the storms of ages, minister especially to his wants, and afford him the means of enjoyment which he could not possess had the surface been otherwise formed. ”

“ It is as if the whole social form were breaking down, and the human element swarmed within the disintegration, like maggots in cheese. The roads, the railways are built, the mines and quarries are excavated, but the whole organism of life, the social organism, is slowly crumbling and caving in, in a kind of process of dry rot, most terrifying to see. ”

“ We no longer live in the days when terrible swarms within made irruptions, when one heard beneath his feet the obscure course of a dull rumble, when indescribable elevations from mole-like tunnels appeared on the surface of civilization, where the soil cracked open, where the roofs of caverns yawned, and where one suddenly beheld monstrous heads emerging from the earth. ”

“ But he maintains, I imagine wilfully, that there is a tendency in all the mammalia to become smaller on being transported to the new world, and refers the fact to the quality of the earth, the condition of the climate, the degrees of heat and humidity, to the height of mountains, amounts of running or stagnant waters, extent of forest, and above all to the brutal condition of nature in a new country, which he evidently regards with true aristocratic abhorrence. ”