THE PRINCIPLES OF GEOGRAPHY Any part of geography may serve as a starting point from which by a series of approximations the whole field of the science may be surveyed. The best line of approach for an advanced student leads from simple fundamental facts step by step to the most com plex interactions, and the following scheme may be taken as ex pressing the views of modern geographers.
These simple conditions do not occur in fact. The lithosphere is composed of diverse rocks, each kind of which has its own specific heat and conductivity so that the rate of heating and cool ing varies and different temperatures result under the same radia tion. The surface of the lithosphere is not smooth but ridged into heights and hollows of every size and shape and the process of elevation and depression is always at work. Alternate heating and cooling by radiation from the sun and to space splinter the outer surface of the rocks and the pull of gravity causes the detritus to slip down the steep slopes and accumulate below. The earth is not all solid lithosphere but is partially enveloped in a hydrosphere which gathers in the great hollows and leaves the crests of the great ridges projecting thus producing the division of the surface into sea and land. Solar radiation falling on the mobile water alters its density and so sets up a circulation between the heated tropics and the chilled polar areas, while the rotation of the earth causes a deviation of the slow moving currents towards the right in the Northern hemisphere and towards the left in the Southern hemisphere ; the coast lines and the inequalities of the sea-bed also direct the movement of the currents so that they carry warmth from the tropics far towards the polar regions, along certain coasts, and coolness from the polar regions far towards the tropics along others, destroying the uniformity of the mathematical zones of temperature in the sea.
The differential attraction of the sun and moon sets up tidal waves in the hydrosphere which like the currents impinge upon and rub against the shores, and a similar though less noticeable strain is also produced in the lithosphere. The terraqueous globe is com pletely enveloped by the atmosphere of air which rests in uneasy equilibrium on land and sea alike. The air, interpenetrated in its lower layers by water-vapour from the hydrosphere, is set into lively circulation by solar radiation which is greatest in the tropics and by terrestrial radiation which preponderates in the polar regions, the general flow to north and south being deviated by the earth's rotation so as to have a westerly component in the pole seeking winds and an easterly component in the equator-seeking winds. Since heat is more readily added to and removed from air by contact with heated and cooled surfaces than by direct radia tion, the atmosphere is subject to a more powerful stimulus over land than over the sea as land heats and cools the more rapidly and to a greater degree. On the other hand the varied surface of the land opposes more friction than the smooth water surfaces, and the theoretical circulation of the air is more characteristically developed on the oceans than on land. Thus land or continental climates are extreme, while sea or oceanic climates are equable. Wind, in virtue of the very high velocity it may attain, is a power ful agency for wearing the rocks and for carrying fine particles to great distances, and it exercises still greater power as the vehicle of water-vapour the precipitation of which as rain is closely condi tioned by the configuration of the land over which it is carried. Water, falling as rain on the higher slopes descends by gravity, col lecting into runnels which eventually converge to rivers. Running water, aided by the detritus it carries, cuts for itself ravines or valleys and finally it spreads the material (well ground in the journey) as alluvium on the plains and silt on the shore of the sea. River action is the chief agent in carving the scenery of the land from the uplifted masses of rock and next to it comes the action of glacial ice: Wind also drives before it the surface water of the sea, checking, diverting or reinforcing the slow creep of the waters of the hydrosphere with the well marked currents which largely affect the temperature of the air and modify the crude extremes of astronomical climate zones on land as well as on sea. Waves raised by the wind sharpen the sea's edge to gnaw and wear away the margins of the land. The lower lands are sheathed in soil ground from the rocks of the mountains and the surface of most of the land is moistened by graduated rainfall and warmed or cooled by the prevailing wind according to the way in which the land-forms guide its flow.
To this point the earth can be treated as the product of chemi cal and physical processes which leave it as a finished house built, furnished, warmed and lighted in readiness for its occupation by life. The distribution of plant life on land and in the sea depends primarily on climate, especially on temperature and sunlight. On land the presence of moisture is at least an equal factor, vegeta tion being stunted to herbs and shrubs in the cold moist lands bordering the polar and Alpine zones, strengthened into deciduous forests and grasslands in the temperate zones, reduced almost to nothing in the rainless parts of the tropical zones, and brought to the height of luxuriant profusion in the rainy forests of the equa tor. The species of plants associated in the various regions are separated into special groups by barriers of mountain, desert or sea. The distribution of animals is conditioned in turn by their entire dependence for food on plants, directly in the her bivora, indirectly in the carnivora. Their species are equally con ditioned by environment and natural barriers. Both plants and animals have the power in some degree of modifying the environ ment which controls them as exemplified by the change of lagoons into swamps and ultimately into meadows by plants and the f orma tion of coral reefs by animals.
A far more complicated set of conditions governs the distribu tion of mankind over the land and the association of races and com munities with particular regions. Primitive races are conditioned by their environment almost as much as animals. They are de pendent on climate, plucking wild fruits or hunting game for food and completely isolated one from another by natural barriers. The rising degrees of civilization exhibit a growing superiority to the thraldom of such conditions until the white race of the present day may be looked upon as escaping from geographical control which no longer raises barriers to movement or settle ment.
Races of mankind grow to be associated with particular regions and aggregate into nations occupying separate countries. Each na tion, retentive of differing languages and conflicting religious ideals, creates and maintains boundaries natural or artificial, the protection or the alteration of which throughout the ages makes it necessary to call in history to explain the geography of to-day. Still mote complex are the conditions governing the discovery, production, transport and exchange of useful and desirable things between people of different nationality.