Progressive change has always, doubtless, been the universal law of creation, and the great ocean of space is, and ever has been, and ever will be the highway through which perpetually plough the great caravels which bear the fortunes of creative energy, laden with life and light and heat, in their eternal progression. The creative impulse once given, if it, too, was not primeval in the eternal past, must have gone on from development to development, like the transmission of life, from age to age and from realm to realm. "The mills of the gods grind slowly;" in these vast areas time is absolutely nothing; the processes we see are but as the dip of a swallow's wing compared with an inconceivable futurity; but all our energies, and all the energies of planets and suns and systems and galaxies, and of whatever other and wider created forms may stretch onward to infinity, came forth from the ocean of space, and to this ocean all these energies continue to return again in ceaseless circuit. Can we indicate any relationship of periodicity for the genesis of solar systems from space? There is a remarkable example of a somewhat similar periodicity in organic life for the rupture of tensions, so common that its analogous character and perfect regularity are scarcely even thought of. Among the highest species of mammalia we find that, in a state of health, whether resident of the heights of the Andes, the deserts of Africa, the jungles of India, or the most densely populated centers of London; among rich or poor, high or low, idle or industrious, virtuous or vicious, ancient or modern, civilized or barbarous, black, white, red, or yellow, the ovum of the mature female rises to the surface of the ovary, and at intervals, almost uniform, of twenty-eight days, organic excitement ensues, the enclosing vesicle is ruptured, and the ovum escapes. The remarkable feature is not that these processes continuously succeed each other; but that under such diverse conditions and opposite circumstances, and with two separate ovaries operating at the same time, simultaneously or successively, this almost miraculous interval of no more and no less than twenty-eight days between the successive ruptures of tension and their attendant phenomena, should constantly persist. For its ultimate cause we must look back to the vis a tergo to which we have already alluded; and there may be, and doubtless is, a similarly acting remote cause which regulates the periodical development of solar systems or of galaxies, periods of intense activity, followed by intervals of exhaustion and recuperation, and again succeeded by another period of activity, and so on perpetually, for space is perpetual, infinite, and inexhaustible.