La Place long ago based a theory of the originally gaseous condition of the solar system on the relation of the planets to each other, and to the sun, on their planes of revolution, the direction of their revolution, and that of their satellites. On these grounds he inferred that the solar system had been formed out of a nebulous mass by the mutual attraction of its parts. This view was further strengthened by the discovery of nebulae, which it might be supposed were undergoing the same processes by which the solar system was produced. This nebular theory, as it was called, was long very popular. It was subsequently supposed to be damaged by the fact that some of the nebulæ which had been regarded as systems in progress of formation were found by improved telescopes to be really clusters of stars, and it was inferred that the others might be of like character. The spectroscope has, however, more recently shown that some nebulæ are actually gaseous; and it has even been attempted to demonstrate that they are probably undergoing change fitting them to become systems. This has served to revive the nebular hypothesis, which has been further strengthened by the known fact that the sun is still an incandescent globe surrounded by an immense luminous envelope of vapours rising from its nucleus and condensing at its surface. On the other hand, while the sun may be supposed, from its great magnitude, to remain intensely heated, and while it will not be appreciably less powerful for myriads of years, the moon seems to be a body which has had time to complete the whole history of geological change, and to become a dry, dead, and withered world, a type of what our earth would in process of time actually become. [Illustration: _Figs. 1 to 5._--_Ideal sections illustrating the Genesis of the Earth._ Fig. 1. A vaporous world. Fig. 2. A world with a central fluid nucleus (_b_) and a photosphere (_a_). Fig. 3. The photosphere darkened, and a solid crust (_c_) and solid nucleus (_d_) formed. Fig. 4. Water (_e_) deposited on the crust, forming a universal ocean. Fig. 5. The crust crumpled by shrinkage, land elevated, and the water occupying the intervening depressions. The figures are all of uniform size; but the circle (A) shows th diameter of the globe when in the state of fig. 1, and that marked (B) its diameter when in the state of fig. 5. In all the figures (_a_) represents vapour or air; (_b_) liquid rock; (_c_) solid rock as a crust; (_d_) solid nucleus; (_e_) water.]