UNSTABLE ENVIRONMENT.
Where living creatures are in harmony with their surroundings,—where, in other words, they are adapted to their environment,—and where, further, this environment is apparently in a state of equilibrium; there we find the fewest and least marked variations in the living creatures. To the casual observer the face of nature maintains the same guise from year to year. The earth seems solid and unyielding; the mountains appear to be everlasting; the restless waters of rivers and brooks seemingly move and throb in the same channel; the tides ebb and flow in apparently unchanging ocean beds; the birds and flowers and woodlands look alike from year to year; and all the varied phenomena of nature appear completed and permanent, as if the present world were constructed in an unyielding mold.
But nothing is fixed and rigid in nature. The earth itself travels rapidly through space and brings in due season spring, summer, autumn and winter; revolves upon its axis and alternates the starry night with sunshine; and periodically changes its orbit so that at one time the northern pole has a temperate climate where water lilies may grow, and at another period presents an arctic climate with impassable barriers of ice. Ice and frost and other forces are breaking up the rocks of mountains, making larger and smaller fragments and even powder; the rains descend and the mountain brooks are swollen to resistless torrents which carry the fragments and mud to the rivers, and these latter take the mud on to the ocean.
Thus, by degrees, the mountains, hills and all the earth are being eroded and the great bulk of the detritus carried by the rivers to the sea and deposited along the sea margins. Thus sedimentation goes along with erosion, and gradually marginal sea bottoms of immense thickness are formed, which will in time be consolidated into rocks and uplifted as dry land. The ceaseless grinding of waves and tides erodes the coast line and adds débris to the marginal sea bottom. The finest sediment is carried out by the tides so far as to reach the ocean currents, and thus is strewn broadcast over portions of deep-sea bottom, and will also in due time be consolidated into rock.
Myriads of animals that form calcareous shells live and die in the ocean. The shells of the dead animals are falling like a perpetual shower on certain ocean bottoms, year after year, so that immense accumulations of calcareous substances occur there, which will also in time be consolidated into limestone rocks, and uplifted as dry land.
Deltas are forming at present at the mouths of certain rivers, and estuaries at others. Lands are now gradually emerging from the sea in some places and at others sinking into the sea. The entire coast of Scandinavia, both on the Baltic and Atlantic sides, is rising out of the sea, and has been doing so for a long time. It is rising at the rate of more than two feet in a hundred years. During an immense period of time there has been a gradual elevation of all the southern part of the South American continent. Sometimes a large elevation takes place rapidly. In 1822, and again in 1835, the southwest coast of South America, after severe earthquakes, was elevated several feet along a distance of several hundred miles. It is known that the coast of Greenland, for five hundred miles, is subsiding. From a study of coral barriers and atolls it is believed that an area of the mid-Pacific sea bottom covering over ten million square miles is sinking and has been doing so for a long time.
The foregoing facts tend to illustrate the truth that nothing is permanent in the environment of living creatures at present. All surroundings are perpetually changing, though apparently ever so slowly. The changes that are now going on were also taking place yesterday, last week, last century, last æon, and so on throughout geologic time. Gradual oscillations of the earth’s crust on a grand scale and affecting whole continents, but usually so slowly as to escape popular observation, have been taking place ceaselessly through inconceivable ages. These oscillations have produced all the great inequalities of the earth’s surface, such as ocean basins, continents and mountain chains. The oscillations are probably due to the slow cooling and unequal shrinking of the whole earth which has been progressive through all geologic time.
The state of the contest between the eroding and the uplifting agencies of the world at any time determines the height of mountains and continents, the depths of seas, the distribution of land and water, for that period.
Fig. 11.—Archæan North America. The white part of the drawing indicates the emerged land; the dark shading indicates the submerged land covered by a shallow sea; the light shading indicates the deep sea.
From Shaler’s First Book in Geology. By courtesy of the publishers, D. C. Heath & Co.
Knowing that the present physical agencies at work on the globe have been acting through long ages,[6] it can readily be appreciated how small effects have been accumulated and low elevations, for instance, have become immense, high mountain ranges. The growing mountain ranges alter the climate and the meteorological conditions. The rainfall on one side differs from that on the other. The temperature varies with the altitude, and so on.
Although continents have gradually and steadily grown from the earliest times, there have been many local alterations of land and sea. Marginal sea bottoms have become great mountain ranges. Islands have appeared and sunk from view. Lakes have been gradually converted into solid land or into peat bogs. Fresh water bodies have become brackish. Dry lands have become marshes, and forests have been buried beneath the waves. Geologic changes have caused great alterations in climate at given times and in given areas.
These statements may be illustrated and emphasized by a brief reference to the development of the Continent of North America. This Continent has grown from comparatively a small beginning to its present great proportions. In doing so it has passed through eras of stupendous duration. These eras in the order of their occurrence are as follows:
(1) Archæan era; (2) Palæozoic era (subdivided into Cambrian, Silurian, Devonian and Carboniferous periods); (3) Mesozoic era (subdivided into Triassic, Jurassic and Cretaceous periods); (4) Cenozoic era (subdivided into Tertiary and Quaternary periods); the Tertiary is subdivided into Eocene, Miocene and Pliocene epochs; the Quaternary is subdivided into Glacial, Champlain and Terrace epochs; (5) Psychozoic era, or recent epoch.
The physical geography of the continent at the close of that early geologic era known as the Archæan is shown on the map ([Fig. 11]). At this time the vast portion of the continent, whose outlines nevertheless existed, was submerged under a shallow sea, as indicated by the dark shading on the figure. The white V-shaped mass, starting just above the site of the great lakes and extending on the one hand in a northeasterly direction to Labrador, and on the other in a northwesterly direction to the Arctic Ocean, is the emerged land of this Archæan time. Smaller masses of Archæan land are also seen at the site of the Blue Ridge Mountains in the east and at that of the Rocky Mountains in the west. Around these lands as a nucleus the North American Continent has been built. Therefore, at the close of the Archæan or beginning of the next, or Palæozoic era, the whole interior portion of the continent was covered by a shallow sea which beat against the Canadian Archæan land on the north, the Blue Ridge Archæan land on the east, and the Rocky Mountain Archæan land on the west. This shallow sea is known as the Palæozoic Sea. Throughout the vast ages of the Palæozoic era, immense sediments were being deposited along the marginal sea bottoms. The deposition of these sediments was simultaneous with a further sinking of the submerged continent, so that the shallowness of the Palæozoic Sea was maintained; finally, the uplifting forces predominated, and the submerged land along the margins of the Canadian Archæan appeared as dry land, and thus increased the area of the infant continent. During all this period there was a steady and slow growth of the land southward from the Canadian Archæan, so that towards its close the visible continent had increased nearly, though not exactly, to the proportions attained in a still later (Cretaceous) period ([Fig. 12]).
Fig. 12.—Cretaceous North America. The white portion of the figure indicates emerged land—the growing continent.
From Shaler’s First Book in Geology. By courtesy of the publishers, D. C. Heath & Co.
At the close of the Palæozoic era the slow, steady changes that had been going on were replaced by more rapid and comparatively revolutionary changes, which caused great alterations in the physical geography and climate. Hitherto the continent had been comparatively low. Now the vast sedimentary accumulations constituting the marginal sea bottom of the eastern portion of the Palæozoic Sea, which had been accumulating through all Palæozoic time, were uplifted into the great Appalachian chain of mountains.
During the earlier ages (Silurian and Devonian) of this Palæozoic era, the place of the Appalachian chain of mountains was marginal sea bottom; but during the later ages (Carboniferous) it was, through repeated oscillations, in an uncertain state, being sometimes swamp land, sometimes covered with river sediment, and sometimes covered by the sea. It was during this Carboniferous age that the great coal measures were formed; at this time also the climate was probably very uniform, warm and moist, loaded with carbonic acid gas and deficient in oxygen. This period was undoubtedly a paradise for the great coal-forming plants, but was very unsuitable for the hot-blooded air-breathing animals, such as mammals and birds, none of which existed at that period. Throughout all geological time the excessive amount of moisture in the air has been gradually removed by the growth of continents in size and height; also the superabundant carbon dioxide in the atmosphere has been removed in many ways, especially by the plants in the coal period appropriating the carbon. Many ages later, at the close of the Jurassic period, the Sierra Nevada range of mountains was uplifted. Up to this time the site of these mountains was a marginal sea bottom receiving vast amounts of sediment, and the Pacific coast-line was east of the site of the Sierra range. Naturally vast changes in physical geography and climate occurred in consequence. During these and the following Cretaceous ages that the continent was growing, the great interior Palæozoic sea and what may be called the Gulf of Mexico were more and more restricted, as shown in the map of North America in the Cretaceous period of its growth ([Fig. 12]). This great inland sea, separating the continent into an eastern and western portion, is now called Cretaceous Sea instead of Palæozoic. This Cretaceous Sea covered the whole plains and plateau region of the continent, and extended from the Gulf of Mexico to the Arctic Ocean. At the end of the Cretaceous period of the continent this sea was obliterated by the gradual upheaval of this region and replaced by great lakes. At the same time the western marginal bottom of the sea was uplifted into the Wahsatch range of mountains; also at this time a line of islands in the Cretaceous Sea was uplifted into the Colorado mountains. All these events were entailing tremendous changes in physical geography and climate. [Fig. 13] is a representation of the map of North America in the early Tertiary period, the time succeeding the Cretaceous period. In this period, the continent continuing to uplift, the lakes that occupied the site of the Cretaceous Sea are obliterated; the Coast Range mountains of California and Oregon are uplifted from marginal sea bottom ([Fig. 13], dark shading); the Atlantic and Gulf borders are extended (dark shading), so that at the close of the Tertiary period the North American continent had attained its present form, except the southern portion of Florida and its keys. Since then the latter have grown and are still growing.
Fig. 13.—Early Tertiary North America.
From Shaler’s First Book in Geology. By courtesy of the publishers, D. C. Heath & Co.
A later epoch still in the history of the globe is known as the Quaternary period, the period that immediately preceded our present epoch. The great features of this period, which is divided into Glacial, Champlain and Terrace epochs, are the wide-spread oscillations of the earth’s crust in high latitudes towards the north and south poles, attended with great changes of climate from temperateness to extreme cold.
The Glacial epoch was characterized by upward crust movements, the land becoming over one thousand feet higher than at present. The land was covered with ice, and an arctic severity of climate extended almost to the Gulf of Mexico. The Champlain epoch was characterized by a downward movement of the coast until it became five hundred feet or more below the present level, so that many lower portions of the continent became covered with sea. At this time there was a moderation of the temperature, a melting of the vast sheets of ice, and consequently a flooding of rivers and lakes, with many icebergs floating in them. The last or Terrace epoch of the Quaternary period was characterized by a crust movement up to the present condition of things.
What is true of the instability of the North American Continent is true of all the continents of the globe. They have all grown from small beginnings to their present huge proportions, and are now undergoing slow but irresistible changes. When these facts are held in mind, one may form a faint conception of the colossal changes that have taken place throughout the sweep of bygone ages. Environment means a complexity of conditions almost infinite in their number and character, and almost infinite in their variations.