We have now a great blank of fifteen centuries—the dark ages of human progress—after which the era of observation and experiment began, and for the first time men really set themselves to study nature, thus laying the foundation for all the great theoretical advances of our time. As leading to the next great step in theories of evolution, we must note the life-long observations by Tycho Brahe of the apparent motions of the planets; the grand discovery of Kepler that all these apparently erratic motions were due to their revolution round the sun in elliptic orbits, with a fixed relation between their distance from the sun and their periods of revolution; and Newton’s epoch-making theory of universal gravitation by which all these facts and many others since discovered were harmonized and explained. But all this implied no law of development, and it was long thought that the solar system was fixed and unchangeable—that some altogether unknown or miraculous agency must have set it going, and that it had in itself no principle of change or decay, but might continue as it now is to all eternity. It was at the very end of the eighteenth century that Laplace announced his “Nebular Hypothesis,” the first attempt ever made to explain the origin of the solar system under the influence of the known laws of motion, gravitation, and heat, acting upon an altogether different antecedent condition of things—a true process of evolution. Laplace supposed that the whole matter of the solar system was once in a condition of vapor, and that it formed an enormous nebulous mass many times larger than the then known dimensions of the planetary sphere. He showed how, under the influence of gravitation, this nebula would condense, and that such irregularities of motion and density as would be sure to exist would lead to rotation of the mass. Under the law of gravitation this would lead to outer rings being left behind by the contraction of the central mass, which rings would at a later period become drawn together at some point of initial greater density and thus form planets. The whole process is admitted to be mathematically demonstrable, given the initial conditions; but recent extensions of our knowledge of the interplanetary and interstellar spaces have shown that the supposed void is really full of invisible solid matter, ranging from the bulk of the smaller planets down to the finest dust, and it is very difficult to imagine any possible causes which would keep all the solid matter of the system in a state of vapor, when subject, on the confines of the mass, to the cold of interstellar space. The antecedent condition of our system is now thought to have been either wholly or partially meteoritic, but in either case we have a genuine theory of its evolution which has now been so extended as to include the appearance of comets and meteors, of nebulæ, and star clusters, of temporary, periodic, and colored stars, and many other phenomena of the stellar universe.