In these various applications of physical theory to the industrial arts, countless minds, of a class that formerly were not reached by scientific reasoning at all, are now brought into daily contact with complex and subtle operations of matter, and their habits of thought are thus notably modified. Meanwhile, in the higher regions of chemistry and molecular physics the progress has been such that no description can do it justice. When we reflect that a fourth generation has barely had time to appear on the scene since Priestley discovered that there was such a thing as oxygen, we stand awestruck before the stupendous pile of chemical science which has been reared in this brief interval. Our knowledge thus gained of the molecular and atomic structure of matter has been alone sufficient to remodel our conceptions of the universe from beginning to end. The case of molecular physics is equally striking. The theory of the conservation of energy, and the discovery that light, heat, electricity, and magnetism are differently conditioned modes of undulatory motion transformable each into the other, are not yet fifty years old. In physical astronomy we remained until 1839 confined within the limits of the solar system, and even here the Newtonian theory had not yet won its crowning triumph in the discovery of the planet Neptune. To-day we not only measure the distances and movements of many stars, but by means of spectrum analysis are able to tell what they are made of. It is more than a century since the nebular hypothesis, by which we explain the development of stellar systems, was first propounded by Immanuel Kant, but it is only within thirty years that it has been generally adopted by astronomers; and among the outward illustrations of its essential soundness none is more remarkable than its surviving such an enlargement of our knowledge. Coming to the geologic study of the changes that have taken place on the earth's surface, it was in 1830 that Sir Charles Lyell published the book which first placed this study upon a scientific basis. Cuvier's classification of past and present forms of animal life, which laid the foundations alike of comparative anatomy and of palæontology, came but little earlier. The cell-doctrine of Schleiden and Schwann, prior to which modern biology can hardly be said to have existed, dates from 1839; and it was only ten years before that the scientific treatment of embryology began with Von Baer. At the present moment, twenty-six years have not elapsed since the epoch-making work of Darwin first announced to the world the discovery of natural selection.