This marked scientific movement is allied with the psychological tendency in its improvement of method, and with the sociological in its emphasis upon human welfare. [Sidenote: Remarkable achievements during past two centuries.] [Sidenote: Hutton, Agassiz, Darwin, and others.] =The Development of the Natural Sciences in Modern Times.=--We have already (chapter XV) witnessed the growth of the natural sciences and the beginning of their introduction into the curriculum toward the close of the seventeenth century. This tendency was also greatly stimulated by Rousseau, who, we have seen (pp. 218-222), may be held to advocate the scientific, as well as the sociological and psychological movements. And during the past two centuries this development has become most rapid and extensive. The desire for scientific investigation steadily grew throughout the eighteenth and nineteenth centuries until its ideals, methods, and results became patent in every department of human knowledge. The strongholds of ignorance, superstition, and prejudice were rapidly stormed and taken through new discoveries or new marshallings of facts already discovered. But evident as this movement has been, it is scarcely possible here even to mention the more important scientific achievements, or to outline the broad sweep of progress in astronomy, geology, biology, physiology, chemistry, physics, and other sciences within a century. The Newtonian theory has been confirmed by the investigations of Lagrange and Laplace and by the discovery of Neptune by mathematical reasoning from the effects of its gravitation. Hutton’s ‘Plutonic’ theory of continents and Agassiz’s hypothesis of a universal ice-age have been formulated; the doctrine of evolution of Darwin (Fig. 51) and Mendel’s law of inheritance have been established; Liebig and others have thrown light upon the process of digestion and the functioning of the lungs and liver; atoms, molecules, and ions have been defined; Joule and Mayer have demonstrated the conservation of energy; and the periodic law of chemical elements has been discovered by Newlands. [Sidenote: During nineteenth century science applied] [Sidenote: to problems of labor, transportation, communication, comfort, and hygiene.]