The principal part of the Chisel-blade is therefore made of soft iron, a very thin plate of steel running along the back. This plate answers the same purpose as the enamel in the tooth, while the soft iron takes the place of the soft bone. Axe-blades, which are, in fact, formed like two chisels placed back to back, are made on a similar principle, except that the steel plate occupies the centre of the blade, and the soft iron is on either side. Thus the thin plate of steel is easily brought to an edge, while the soft iron can be ground away without any difficulty. I do not mean to state that the inventor of this combination of thin steel and soft iron had taken his idea from the Rodent tooth, but only to show that the invention, beautiful, simple, and ingenious as it is, has its prototype in Nature. I may here mention that the Plane-iron, which is, in fact, a modified Chisel, is made in exactly the same fashion. * * * * * NEXT we come to the Adze. [Illustration: ADZE-TEETH OF HIPPOPOTAMUS.] [Illustration: STONE ADZE OF POLYNESIA. STEEL ADZE.] In some respects there is much resemblance between the blade of the Adze and the teeth of the Rodent, especially in their curve, which is almost identical in both. This form is seen in the structure of other teeth than those of Rodents. There is, for example, the tooth of the Hippopotamus, which is not only curved, like that of the Rodent, but bevelled off in a similar way at the tip. With these formidable teeth, one of which is now before me, the Hippopotamus makes terrible havoc among the herbage, mowing it down, so to speak, and stowing it away wholesale in its enormous stomach. A Hippopotamus indeed, when angered, has been known to sever a man’s body completely in two with a single bite, so trenchant are the teeth, and so powerful the jaws. Then there is a little animal called the Hyrax, or Rock-rabbit, which is the coney of Scripture. This creature is really one of the pachydermatous group, although its small size, hairy coat, its activity among the rocks, and its apparently rodent teeth, have induced many persons to place it among that group. These teeth, however, like those of the Hippopotamus, are bevelled off at their tips, and, as they perform a similar office, they take a similar curve. It is worthy of notice that in the Stone Adze the bevelled edge much more resembles the rodent tooth than does the Steel Adze, the reason being evidently that stone is more fragile than steel, and requires greater thickness. Still, the principle is the same in both, only the metal is more attenuated than the stone.