In the evolution of our solar system from the primæval nebula, this operation of reducing the movements to the same plane and of requiring that all the movements shall take place in the same direction, having had play for unmeasured ages, has in the main accomplished its end. All the important bodies of the system do go round in the same direction; that much, at least, has been attained. All of them also go round in planes which are nearly coincident, but, as we have already noted, they are not yet absolutely coincident. The greatest planets have, however, very nearly become reconciled, so far as the planes of their orbits are concerned, to the condition which dynamics imposes. The same is true of the rotation of the sun on its axis. That axis is inclined at an angle of eighty-three degrees to the plane of the ecliptic, so that the sun’s equator would have to be shifted only through an angle no greater than seven degrees, if it were to be placed in the plane in which it should be situated, if the condition of the smallest quantity of energy for a given amount of moment of momentum was to be realised. We find a greater discrepancy in the plane of the earth’s equator. This is inclined by about twenty-three degrees to the plane of the ecliptic. Here there is some energy which might yet be expended without a diminution of the amount of moment of momentum in the system; for if the earth’s axis were to be made perpendicular to the plane of the ecliptic, then the velocity of rotation of the earth about its axis might undergo a corresponding abatement, and yet keep up the requisite moment of momentum. We thus see that even with the older planets the conditions which would be enforced, if the moment of momentum was to be sustained with the least quantity of energy, are not absolutely complied with; which simply means that there has not yet been time enough for our system to arrive at the perfect state, to which it must be approximating.