But while many of the stars are probably in a similar stage of biogenetic development to that of our earth (for the last one hundred million years at least), others have advanced far beyond this stage, and, in their planetary old age, are hastening towards their end--the same end that inevitably awaits our own globe. The radiation of heat into space gradually lowers the temperature until all the water is turned into ice; that is the end of all organic life. The substance of the rotating mass contracts more and more; the rapidity of its motion gradually falls off. The orbits of the planets and of their moons grow narrower. At length the moons fall upon the planets, and the planets are drawn into the sun that gave them birth. The collision again produces an enormous quantity of heat. The pulverized mass of the colliding bodies is distributed freely through infinite space, and the eternal drama of sun-birth begins afresh. The sublime picture which modern astrophysics thus unveils before the mind's eye shows us an eternal birth and death of countless heavenly bodies, a periodic change from one to the other of the different cosmogenetic conditions, which we observe side by side in the universe. While the embryo of a new world is being formed from a nebula in one corner of the vast stage of the universe, another has already condensed into a rotating sphere of liquid fire in some far distant spot; a third has already cast off rings at its equator, which round themselves into planets; a fourth has become a vast sun whose planets have formed a secondary retinue of moons, and so on. And between them are floating about in space myriads of smaller bodies, meteorites, or shooting-stars, which cross and recross the paths of the planets apparently like lawless vagabonds, and of which a great number fall onto the planets every day. Thus there is a continuous but slow change in the velocities and the orbits of the revolving spheres. The frozen moons fall onto the planets, the planets onto their suns. Two distant suns, perhaps already stark and cold, rush together with inconceivable force and melt away into nebulous clouds. And such prodigious heat is generated by the collision that the nebula is once more raised to incandescence, and the old drama begins again. Yet in this "perpetual motion" the infinite substance of the universe, the sum total of its matter and energy, remains eternally unchanged, and we have an eternal repetition in infinite time of the periodic dance of the worlds, the metamorphosis of the cosmos that ever returns to its starting-point. Over all rules the law of substance. II.--PROGRESS OF GEOLOGY