Page:Popular Science Monthly Volume 31.djvu/521

Rh during the course of the last half-century. The science of the earth's crust no longer stands isolated as a study by itself: it falls into its proper place in the hierarchy of knowledge as the science of the secondary changes, induced under the influence of internal forces and incident energies, on the cooling and corrugated surface of a once incandescent and more extended planet. I know no better gauge of the widening which comes over the thoughts of men with the processes of the suns than to turn from the rudis indigestaque moles of the "Principles" and the "Elements" (great as they both were in their own day) to the luminous, lucid, and comprehensive arrangement of Geikie's splendid and systematic "Text-Book. The one is an agreeable and able dissertation on a number of isolated and floating geological facts; the other is a masterly and cosmically-minded account of the phenomena observable on the outer shell of a cooling world, duly considered in all their relations, and fully co-ordinated with all the chief results of all elder and younger sister sciences.

The battle of uniformitarianism itself, however, was but a passing episode in the great evolutionary movement. That movement began along several distinct lines toward the close of the previous century, and only at last consciously recognized its own informing unity of purpose some thirty-five years ago. From another point of view—in connection with its influence upon thought at large—the evolutionary crisis has been treated elsewhere in this review by a philosophic thinker: but in its purely scientific aspect it must also be briefly considered here, forming, as it does, the acknowledged mainspring of all living and active contemporary science.

Evolution is not synonymous with Darwinism. The whole immensely exceeds the part. Darwinism forms but a small chapter in the history of a far vaster and more comprehensive movement of the human mind. In its astronomical development evolution had already formulated itself with perfect distinctness before the period with which we have here especially to deal. The nebular theory of Kant and Laplace was the first attempt to withdraw the genesis of the cosmos from the vicious circle of metaphysical reasoning, and to account for it by the continuous action of physical and natural principles alone. Our own age has done much to cast doubt upon the unessential details of Kant's rough conception, but, in return, it has made clearer than ever the fundamental truth of its central idea—the idea that stars, and suns, and solar systems, consist of materials once more diffusely spread out through space and now aggregated around certain fixed and definite nuclei by the gravitative force inherent in their atoms and masses. As these masses or atoms drew closer together in union around the common center, their primitive potential energy of separation (frankly to employ the terminology of our own time) was changed, first into the kinetic energy of molar motion in the act of union, and then into the kinetic energy of molecular motion or heat,