§ 103. Yet one more condition to be specified, is the state of agitation in which the constituents of an aggregate are kept. A familiar expedience will introduce us to this condition. When a vessel has been filled to the brim with loose fragments, shaking the vessel causes them to settle down into less space, so that more may be put in. And when among these fragments, there are some of much greater specific gravity than the rest, these will, in the course of a prolonged shaking, find their way to the bottom. What now is the meaning of these two results, when expressed in general terms? We have a group of units acted on by an incident force—the attraction of the Earth. So long as these units are not agitated, this incident force produces no changes in their relative positions; agitate them, and immediately their loose arrangement passes into a more compact arrangement. Again, so long as they are not agitated, the incident force cannot separate the heavier units from the lighter; agitate them, and immediately the heavier units begin to segregate. By these illustrations, a rude idea will be conveyed of the effect which vibration has in facilitating those re-arrangements which constitute Evolution. What here happens with visible units subject to visible oscillations, happens also with invisible units subject to invisible oscillations. One or two cases in which these oscillations are of mechanical origin, may first be noted. When a bar of steel is suspended in the magnetic meridian, and repeatedly so struck as to send vibrations through it, it becomes magnetized: the magnetic force of the Earth, which does not permanently affect it while undisturbed, alters its internal state when a mechanical agitation is propagated among its particles; and the alteration is believed by physicists, to be a molecular re-arrangement. It may be fairly objected that this re-arrangement is hypothetical; and did the fact stand alone, it would be of little worth. It gains significance, however, when joined with the fact that in the same substance, long-continued mechanical vibrations are followed by molecular re-arrangements that are abundantly visible. A piece of iron which, when it leaves the workshop, is fibrous in structure, will become crystalline if exposed to a perpetual jar. Though the polar forces mutually exercised by the atoms, fail to change their disorderly arrangement into an orderly arrangement while the atoms are relatively quiescent, these forces produce this change when the atoms are kept in a state of intestine disturbance.