In the very beginning, so far as the mental vision of the astronomer can dimly pierce with hypothetical glance the abyss of ages, the matter which now composes the material universe seems to have existed in a highly diffuse and nebulous condition. The gravitative force, however, with which every atom of the whole vast mass was primarily endowed, caused it gradually to aggregate around certain fixed and definite centres, which became in time the rallying-points or nuclei of future suns. The primitive potential energy of separation in the atoms of the mass was changed into actual energy of motion as they drew closer and closer together about the common centre, and into molecular energy or heat as they clashed with one another in bodily impact around the hardening core. Thus arose stars and suns, composed of fiery atomic clouds in a constant state of progressive concentration, ever gathering-in the hem of their outer robes on the surface of the solid globe within, and ever radiating off their store of associated energy to the impalpable and hypothetical surrounding ether. This, in necessarily brief and shadowy abstract, is the nebular theory of Kant and Laplace, as amended and supplemented by the modern doctrine of the correlation and conservation of energies. Applied to the solar system, of which our own planet forms a component member, the evolutionary doctrine (in its elder shape) teaches us to envisage that minor group as the final result of a single great diffuse nebula, which once spread its faint and cloud-like mass with inconceivable tenuity, at least as far from its centre, now occupied by the sun's body, as the furthest point in the orbit of Neptune, the outermost of the yet known planets. From this remote and immense periphery it has gradually gathered itself in, growing denser and denser all the time, towards its common core, and has left behind, at irregular intervals, concentric rings or belts of nebulous matter, which, after rupturing at their weakest point, have hardened and concentrated round their own centre of gravity into Jupiter, Saturn, the Earth, or Venus. The main central body of all, retreating ever within as it dropped in its course the raw material of the planetary masses, has formed, at last, the sun, the great ruler and luminary of our system. Much as this primitive evolutionary concept of the development and history of the solar system has been modified and altered of late years by recent researches into the nature of comets and meteors and of the sun's surface, it still remains for all practical purposes of popular exposition the best and simplest mental picture of the general type of astronomical evolution. For the essential point which it impresses upon the mind is the idea of the planets in their several orbits and with their attendant satellites as due, not to external design and special creation, in the exact order in which we now see them, but to the slow