That, where no other circumstances interfere, the order in which different uniformities are established varies as their complexity, is manifest. The geometry of straight lines was understood before the geometry of curved lines; the properties of the circle before the properties of the ellipse, parabola and hyperbola; and the equations of curves of single curvature were ascertained before those of curves of double curvature. Plane trigonometry comes in order of time and simplicity before spherical trigonometry; and the mensuration of plane surfaces and solids before the mensuration of curved surfaces and solids. Similarly with mechanics: the laws of simple motion were generalized before those of compound motion; and those of rectilinear motion before those of curvilinear motion. The properties of equal-armed levers, or scales, were understood before those of the lever with unequal arms; and the law of the inclined plane was formulated earlier than that of the screw, which involves it. In chemistry, the progress has been from the simple inorganic compounds, to the more involved organic ones. And where, as in most of the other sciences, the conditions of the exploration are more complicated, we still may clearly trace relative complexity as one of the determining circumstances. The progression from concrete relations to abstract ones, and from the less abstract to the more abstract, is equally obvious. Numeration, which in its primary form concerned itself only with groups of actual objects, came earlier than simple arithmetic: the rules of which deal with numbers apart from objects. Arithmetic, limited in its sphere to concrete numerical relations, is alike earlier and less abstract than Algebra, which deals with the relations of these relations. And in like manner, the Infinitesimal Calculus comes after Algebra, both in order of evolution and in order of abstractness. In Astronomy, the progress has been from special generalizations, each expressing the motions of a particular planet, to the generalizations of Kepler, expressing the motions of the planets at large; and then to Newton’s generalization, expressing the motions of all heavenly bodies whatever. Similarly with Physics, Chemistry and Biology, there has ever been an advance from the relations of particular facts and particular classes of facts, to the relations presented by still wider classes—to truths of a high generality or greater abstractness.