With the loss of water and of air, all possibility of development departs. Not only must every organism die, but even the inorganic can no longer change its state. In the extinction thus not only of inhabitants but of the habitat that made them possible, occurs a curious inversion of the order we are familiar with in the life history of organisms. In planets it is the grandchildren that die first, then the children, and lastly their surviving parent. And this is not accidental, but inevitably consequent upon their respective origins. For the offspring, as we may spell it with a hyphen, of any cosmic mass is of necessity smaller than that from which it issued. Being smaller, it must age quicker. In the natural order of events, then, its end must be reached first. Such has been the course taken, or still taking, by the bodies of our solar family. The latest generation has already succumbed to this ebbing of vitality with time. Every one of the satellites of the planets—those of Neptune, Uranus, Saturn, Jupiter, and our own Moon—is practically dead; born so the smaller which never were alive. Our own Moon carries its decrepitude on its face. To all intents and purposes its life is past; and that it had at one time a very fiery existence, the great lunar craters amply testify. It is now, for all its flooding with radiance our winter nights, the lifeless statue of its former self. The same inevitable end, in default of others, is now overtaking the planetary group. Its approach is stamped on the face of Mars. There we see a world dying of exhaustion. The signs of it are legible in the markings we descry. How long before its work is done, we ignore. But that it is a matter of time only, our study of the laws of the inexorable lead us to conclude. Mars has been spared the fate of Mercury and Venus to perish by this other form of planetary death. Last in our enumeration of the causes by which the end of a world may be brought about, because the last to occur in order of time, is the extinction of the Sun itself. Certain to come and conclude the solar system’s history as the abode of life, if all the others should by any chance fail to precede it, it fittingly forms the climax, grand in its very quietude, of all that went before. By the same physical laws that caused our Earth once to be hot, the Sun shines to-day. Only its greater size has given it a life and a brilliancy denied to smaller orbs. The falling together of the scattered particles of which it is composed, caused, and still is causing, the dazzling splendor it emits. And so long as it remains gaseous, its temperature must increase, in spite of its lavish expenditure of heat, as Homer Lane discovered forty years ago.