The destructive agencies are so active that in a short time all the land would disappear and be distributed over the bed of the ocean were it not for the upheavals due to the contraction of the crust. From causes, concerning which all geologists do not agree, lines of weakness are developed and portions of the crust are forced up into mountain ranges or broad plat eaus, while other adjacent portions are de pressed. The outer rocks or those forming the slopes of a young mountain range are composed of the sediment of older rocks. They are the result of processes of destruction and recon struction, which are to be at once renewed. For no sooner are new areas exposed than the sun either by direct heat, or through the agency of wind, frost, rain, or running water, proceeds to carve the surface into new forms, carrying away the waste to cover the rocks with soil for the support of vegetation or to fill up the hol lows of the sea. Rain not only attacks the rocks by force of impact and solution, but by moving broken fragments from higher to lower levels it uses them as cutting tools. The sur face water and that flowing from springs form little streams, which uniting produce rivers. In its upper course, which is the steepest, the work of a stream is purely destructive. It is cutting a gorge or valley in the high land. As the slope becomes more gradual, it is alternately depositing its load of stones and gravel or sweeping them away to some lower point. In its final stretch through a nearly level area it is building up an alluvial plain. Deltas and many interior valleys, like that of the Missis sippi, are thus formed. Rivers carry away im mense quantities of solid material and are con tinually cutting down or extending their basins. The tendency of river work is to reduce the land to a dead level and a subdued surface is usually an indication of old age.
Relief, or the vertical aspect of a land area, is the result of the forces of upheaval and erosion, which are continually at work. The arrangement of mountains, plateaus and valleys varies in the different continents and upon the arrangement, climate, the possibilities of de velopment, and the history of a county largely depend. The principal mountain ranges are however, so disposed that the largest drain age areas are tributary to the Atlantic Ocean or its arms. Asia, the largest continent, has the greatest average elevation and the highest point of land.
The many changes in the inorganic world profoundly affect the organic world and the distribution of life. In turn the very processes of nature are largely influenced by living or ganisms. The spread of any species, animal or vegetable, is promoted or retarded by such geographical features as oceans, mountains, plains, deserts and climate, but marine animals, by withdrawing dissolved carbonate of lime from the sea water, have constructed mountains of limestone, and vegetation retards the work of denudation, regulates rainfall and modifies the rate of evaporation. Plants alone are able to construct organic from inorganic material and animal life is dependent directly or indirectly upon plants for food.
All plants and animals are adapted to cer tain environments and could not live if the essential conditions were changed, but every form of life does not of necessity reach every region suited to it. Natives of one country when carried to another frequently develop with amazing rapidity, even to the extent of crowd ing out indigenous species. In the tropics, the
forests contain a variety of forms and the large animals live singly or in families. In the temperate zones many plants and animals for mutual protection are gregarious. Forests often contain only one kind of tree and the variety is never great. Grasses do not grow singly as in hot climates, but form a sod. An imals move in herds and fish in shoals.
Natural distribution has been greatly modi fied by man. In some regions the large land animals have been exterminated and vast for est tracts have been removed. Domestic ani mals and cultivated plants have replaced the native species. Man alone of living creatures is able to rise superior to his environment, to conquer and control adverse geographical con ditions. This faculty is acquired and is the result of long development. Primitive man was undoubtedly as helpless as other animals. The same sort of barriers that retard the migration of species, have also affected the growth of na tions and no thorough conception of history, which is a record of man's development, can overlook the fundamental importance of geog raphy. It was not until the barriers were broken down and until isolation gave way to intercourse with other peoples that civilization made safe and permanent advances.
Political Geography deals merely with the distribution of the human race in different com munities, but an intelligent study of the boun daries of states involves an acquaintance with natural geographical conditions and the history of the inhabitants. It has been proposed to di vide history into three periods. The first, known as the Fluvial, includes the growth of nations developing in fertile river deltas, with such scanty means of intercourse as the streams afforded. The Mesopotamian nations of Assyria and Babylon, the Egyptians and the Hindus are well-known examples. As the sailor became more venturesome and skirted the shores of large inland seas, the Mediter ranean period succeeded the Fluvial. Columbus and Vasco da Gama inaugurated the last, or the Oceanic period. Such a view is at least not inappropriate in tracing the development of commerce.
Commercial Geography is the most prac tical branch of the subject. It means a knowl edge of the distribution of the world's prod ucts, of existing demands for these commodi ties and satisfactory means of transportation and exchange. The Phoenicians were the first great traders. Their horizon was practically limited by the shores of the Mediterranean, al though they sailed beyond its confines and many of the goods with which their ships were laden were brought to Syria by caravans. The Phoenicians were succeeded by the Cartha ginians and the Greeks, but the typical mer chants of the Mediterranean class were the Venetians. Their supremacy in the world's commerce was unquestioned until, at the close of the 15th century, the Atlantic succeeded the Mediterranean as the highway of trade. The centre of the world's commerce then passed in succession from Portugal to Spain and then on to Holland, the Hanseatic towns and London. To-day commerce is not a monopoly, but is world-wide. In volume of trade the United States is surpassed by no other nation, but competition is keen and the successful mer chant of the future must be well versed in that knowledge of which geography forms the basis.