The lifetime of a world like our earth may be truly said to be a lifetime of cooling. Beginning in the glowing vaporous condition which we see in the sun and stars, an orb in space passes gradually to the condition of a cool, non-luminous mass, and thence, with progress depending chiefly on its size (slower for the large masses and quicker for the small ones), it passes steadily onward toward inertness and death. Regarding the state in which we find the earth to be as the stage of a planet’s mid-life—viz., that in which the conditions are such that multitudinous forms of life can exist upon its surface, we may call that stage death in which these conditions have entirely disappeared. Now, among the conditions necessary for the support of life in general are some which are unfavorable to individual life. Among these may be specially noted the action of those subterranean forces by which the earth’s surface is continually modeled and remodeled. It has been remarked with great justice, by Sir John Herschel, that since the continents of the earth were formed, forces have been at work which would long since have sufficed to have destroyed every trace of land, and to have left the surface of our globe one vast limitless ocean. But against these forces counteracting forces have been at work, constantly disturbing the earth’s crust, and, by keeping it irregular, leaving room for ocean in the depressions, and leaving the higher parts as continents and islands above the ocean’s surface. If these disturbing forces ceased to work, the work of disintegrating, wearing away, and washing off the land would go on unresisted. In periods of time such as to us seem long, no very great effect would be produced; but such periods as belong to the past of our earth, even to that comparatively short part of the past during which she has been the abode of life, would suffice to produce effects utterly inconsistent with the existence of life on land. Only by the action of her vulcanian energies can the earth maintain her position as an abode of life. She is, then, manifesting her fitness to support life in those very throes by which, too often, many lives are lost. The upheavals and downsinkings, the rushing of ocean in great waves over islands and seaports, by which tens of thousands of human beings, and still greater numbers of animals, lose their lives, are part of the evidence which the earth gives that within her frame there still remains enough of vitality for the support of life during hundreds of thousands of years to come.