§ 79. On contemplating our Solar System the first fact which strikes us, is, that all its members are in motion; and that their motion is of a two-fold, or rather of a three-fold, kind. Each planet and satellite has a movement of rotation and a movement of translation; besides the movement through space which all have in common with their rotating primary. Whence this unceasing change of place? The hypothesis of Evolution supplies us with an answer. Impossible as it is to assign a reason for the pre-existence of matter in the diffused form supposed; yet assuming its pre-existence in that form, we have in the gravitation of its parts a cause of motion adequate to the results. So far too as the evidence carries us, we can perceive some quantitative relation between the motions produced, and the gravitative forces expended in producing them. The planets formed from that matter which has travelled the shortest distance towards the common centre of gravity, have the smallest velocities: the uniform law being that in advancing from the outermost to the innermost planets, the rate of orbital motion progressively increases. It may indeed be remarked that this is explicable on the teleological hypothesis; since it is a condition to equilibrium. But without dwelling on the fact that this is beside the question, it will suffice to point out that the like cannot be said of the planetary rotations. No such final cause can be assigned for the rapid axial movement of Jupiter and Saturn, or the slow axial movement of Mercury. But if in pursuance of the doctrine of correlation we look for the antecedents of these gyrations which all planets exhibit, the theory of Evolution furnishes us with equivalent ones; and ones which bear manifest quantitative relations to the motions displayed. For the planets that turn on their axes with extreme rapidity, are those having great masses and large orbits—those, that is, of which the once diffused elements moved to their centres of gravity through immense spaces, and so acquired high velocities. While, conversely, there has resulted the smallest axial movement where the orbit and the mass are both the smallest.