THE most important principles established in physical geography during the century are that the description of the earth's surface features must be accompanied by explanation, and that the surface features must be correlated with their inhabitants. During the establishment of these evolutionary principles, exploration at home and abroad has greatly increased the store of recorded facts; the more civilized countries have been in large part measured and mapped; the coasts of the world have been charted; the less civilized continents have been penetrated to their centers. This harvest of fact has been an indispensable stimulus to the study of physical geography; yet it can not be doubted that the spirit which has given life to the letter of the subject is the principle of evolution—inorganic and organic. This is especially true of the geography of the lands.
The century has seen the measurement of higher peaks in the Himalayas than had been previously measured in the Andes. The Nile has been traced to its source in the lakes of equatorial Africa, verifying the traditions of the ancients; and the Kongo has been found to cross the equator twice on its way to the sea. Facts without number have been added to the previous sum of knowledge. But at the same time, it has been discovered that the valleys of mountain ranges are the work of erosion; that the product of valley erosion is often seen in extensive piedmont fluviatile plains; that waterfalls are retrogressively worn away until they are reduced to the smooth grade of a maturely established river; and that interior basins are slowly filling with the waste that is washed in from their rims upon their floors. Here are explanatory generalizations, involving, yet going far beyond matter of direct observation. Such generalizations in geography correspond to the recognition in astronomy that planetary movements exemplify the law of gravitation; they are the Newton as against the Kepler of the subject.
The sufficient justification of the demand that has now arisen for explanation and correlation in the study of land forms is found in the repeated experience that until an explanatory description of a region can be given, one may be sure that some of its significant elements pass unnoticed; and until the controls that it exerts on living forms are studied, one may be confident that its geographical value is but half