The idea of the progressive development of the earth in its greater features throughout all geologic time by the action of forces resident in the earth itself preceded the acceptance of the evolution of organic forms. We have said that the fundamental idea of geology is that of the evolution of the earth through all time. Now, it was Dana who first studied geology wholly from this point of view. For him geology was the development of the earth as a unit. Before him, doubtless, geology was a kind of history--i. e., a chronicle of thrilling events--but Dana first made it a philosophic history. Before Dana, geology was an account of the succession of formations and their fossil contents. Dana made it an account of the evolution of earth forms and the concomitant and resulting evolution of organic forms. It is true that first and for a long time his evolutional conception was incomplete. It is true that while he attributed the evolution of earth forms to natural causes and processes, he still shrank from applying similar causes to the changes in life forms, but this was the almost necessary result of the then universal belief in the supernatural origin and the unchangeableness of organic forms. He lived to make his conception of evolution as a natural process, both of the earth and of organic forms, complete. _Ocean Basins and Continents._--If we divide geological causes and processes into two general kinds as to their origin--viz., internal, or earth-derived, and external, or sun-derived--evidently the former is the original and fundamental kind. These determine earth forms, while the other only modify them; these determine the great features, the other only the lesser features; the former rough-hews the earth features, the latter shapes them. It is the effects of these interior earth forces which are the most important to study. And among these effects the most fundamentally important of all is the formation of those greatest features--the ocean basins and continental arches. The most probable view is that they are formed by unequal radial contraction in the secular cooling of the earth. The earth was certainly at one time an incandescently hot mass, which gradually cooled and contracted to its present temperature and size. Now, if it were perfectly homogeneous both in density and in conductivity in all parts, then, cooling and contracting equally in every part, it would retain its symmetric oblate-spheroid form, though diminishing in size. But if there were any, the least, heterogeneity either in density or especially in conductivity over large areas, then the more conductive areas, contracting more rapidly toward the center radially, would form hollows or basins, and the less conductive areas would stand out as higher arches. Thus were formed the oceanic basin and the continental arches of the lithosphere. The same causes which produced would continue to increase them, and thus the ocean basins would increase in