In both these examples, the polished surfaces of the teeth, worn away by mutual action, afford striking evidence of the lapse of time. Some one may possibly object, however, to this: "What right have you to assume that these teeth were worn away at the moment of its creation, admitting the animal to have been created adult? May they not have been entire?" I reply, Impossible: the Hippopotamus's teeth would have been perfectly useless to him, except in the ground-down condition: nay, the unworn canines would have effectually prevented his jaws from closing, necessitating the keeping of the mouth wide open until the attrition was performed; long before which, of course, he would have starved. In a natural condition the mutual wearing begins as soon as the surface of the teeth come into contact with each other; that is, as soon as they have acquired a development which constitutes them fit for use. The degree of attrition is merely a question of time. There is no period that can be named, supposing the existence of the perfected teeth at all, in which the evidence of this action would not be visible. How distinct an evidence of past action, and yet, in the case of the created individual, how illusory! [Illustration: SKULL OF HIPPOPOTAMUS.] "Trampling his path through wood and brake, And canes, which, crackling, fall before his way, And tassel-grass, whose silvery feathers play O'ertopping the young trees,-- On comes the Elephant, to slake His thirst at noon, in yon pellucid springs. Lo! from his trunk upturn'd, aloft he flings The grateful shower: and now Plucking the broad-leaf'd bough Of yonder plane, with waving motion slow, Fanning the languid air, He waves it to and fro." We will not be content with admiring the vast size of the fine Dauntelah, and the majesty of his air and movement, and the intelligence manifested in all the actions of the "half-reasoning" beast, as he explores the amoenities of the young world to which he has but this morning been introduced. We are out on another sort of scent: let us try if we can glean any light from him on our present question. And, first, we cannot fail to notice his fine pair of tusks curving upwards almost to a semicircle. Each tusk is composed of a vast number of thin cones of ivory, superimposed one on another; ever increasing by new ones formed within the interior at the base, and moulded upon the vascular pulp which fills the cavity, and by which the solid ivory is constantly secreted and deposited. Each new cone pushes further and further out those previously deposited, and thus the tusk ever grows in length as it increases in age. [Illustration: SKULL OF ELEPHANT.]