Geomorphology, though not accepted without demur as a definite branch of science in itself, has at last come to be generally recognized as a convenient term to connote the study of terrestrial relief. Elie de Beaumont in 1852 enunciated with too great precision the theory that similarity of orientation was a standard test of similarity in the age and origin of the great mountain chains. Lowthian Green in 1875 proposed his tetrahedral theory of the disposition of the continents and the ocean basins, on the ground that a sphere undergoing contraction tends to assume the form of a tetrahedron, or body enclosed by four equal equilateral triangles. He applied this theory to the form of the spherical earth at its present stage of contraction, indicated how far it accounted for the present distribution of land and sea, and attempted to give reasons for its failure to do so in certain respects. Professor C. Lapworth in 1892 stated his theory of folding, according to which the continents are the arches of vast folds in the crust of the earth, and the ocean basins the troughs between them. E. Suess has modified this view in his treatise _Das Antlitz der Erde_ (The Face of the Earth), 1885–1901. Sir George Darwin invoked the effects of tidal strain upon the crust, associating this with the form of the continents. The subject, which has also been dealt with by Professors J. W. Gregory and A. E. H. Love, M. Bertrand, A. de Lapparent, and A. Supan, among others, has thus been approached from both the purely physical and the mathematical standpoint, but the problem has not reached its solution. We have already given sufficient indication that the exact scope of geography has not been found easy to define by common consent; that fact does not lighten the task of tracing its development in the nineteenth century. It is not inconceivable that on one view of the subject this volume should have concluded with the preceding paragraph. On the other hand, the new value attaching to the geographical studies of distribution and environment makes it imperative to carry the story further. These studies have not only been systematized in themselves, but have become complementary of other sciences, and thus we find the term “geography” incorporated in certain scientific compounds--zoogeography or zoological distribution; anthropogeography, the distribution of mankind; biogeography, the distribution of living things generally--or perhaps more mercifully treated in such phrases as “plant geography.” Zoogeography and plant geography are concerned with the division of the earth’s surface into regions possessing individual characteristics in regard to their fauna or flora. The principle of regional division, indeed, has become a leading principle of geographical research, in regard not only to fauna and flora, but to man as well; to the physical characters of the land, and to climatic conditions.