These, in conjunction with other considerations, lead to the conviction that the universe is in a state of evolution, and that the solar system at one time existed in a nebular state. But whether Laplace’s description of the series of events through which the original nebula passed is the true one or not is a very different matter; it presents so many difficulties that scarcely any student now supports it. [Illustration: In the beginning, it is supposed that the earth was part of a vast nebula of gaseous matter and meteorites, resembling the nebula of Argo, illustrated above. Later, as the cooling process advanced, the nebula assumed a rotatory movement in the form of a spiral. The nebula of Andromeda affords an excellent illustration of this. Another stage would be as in the annular nebula of Aquaris, the mass forming into a ball with the outer ring attached. HOW THE HEAVENS TELL THE STORY OF THE ORIGIN OF THE EARTH] [Illustration: Or, like the nebula of Cygni, with the central sun well formed and the gaseous ring far removed, the earth would begin to shape, and the ring would roll up to form the moon. Jupiter, which is in a molten state, wreathed in thick vapour, with the “great red spot” indicating the beginning of the solidifying process, shows what the earth was like before it assumed its present solid condition. This shows the earth and the moon in their relative sizes; while the diagram below it illustrates the distance apart. ] [Sidenote: Laplace’s Theory Abandoned] A fundamental difficulty is the extreme tenuity of the gas which is assumed to have formed the planetary rings. A second difficulty, which has been emphasised by Professors Chamberlin and Moulton, is to be found in the comparatively small amount of rotational energy which the system at present possesses, for this is less than 1/200 of that which, on the most favourable assumption, must have been contained within the original nebula. Less fundamental, but equally fatal, is the fact that one of the satellites of Saturn revolves round its primary in a direction opposed to that of the rotation of the planet itself. [Recently Mr. Stratton, following out a suggestion of Professor W. H. Pickering, has shown that this is quite consistent, and, indeed, is a natural deduction from Laplace’s hypothesis.] Hence for these and other reasons we are reluctantly compelled to abandon an hypothesis which for over a century has exercised an influence on our conception of the cosmos not less profound, penetrating, and far-reaching than that of the famous Darwinian doctrine of natural selection, now on its trial. [Sidenote: What are the Nebulæ?]