Any of my readers who possess skates will find, on examination, that the greater part of the blade is, in reality, soft iron, the steel, which comes upon the ice, being scarcely a fifth of an inch in length. The hardened steel allows the blade to take the necessary edge, while the soft iron preserves the steel from snapping.
Should the skate have been neglected and allowed to become a little rusty, the line of demarcation between the steel and the iron can be distinctly seen. Similarly, in the beaver and the water-rat, the orange-yellow colour of the enamel facing causes it to be easily distinguished from the rest of the tooth. In most of the rodents the enamel is white, and the line of demarcation is scarcely visible.
Now we have to treat of a question of mechanics.
If two substances of different degrees of hardness be subjected to the same amount of friction, it follows that the softer will be worn away long before the harder. It is owing to this principle that the edges of the rodent teeth preserve their chisel-like form. Being continually employed in nibbling, the softer backing of the teeth is rapidly worn away, while the hard plate of enamel upon the front of the tooth is but slightly worn, the result being the bevelled shape which is so characteristic of these teeth.
As all know, who have kept rabbits or white mice, the animals are always engaged in gnawing anything which will yield to their teeth, and unless the edges of their feeding troughs be protected by metal, will nibble them to pieces in a few days. Indeed, so strong is this instinct, that the health of the animals is greatly improved by putting pieces of wood into their cages, merely for the purpose of allowing them to exercise their chisel-edged teeth. Even when they have nothing to gnaw, the animals will move their jaws incessantly, just as if they were eating, a movement which gave rise to the idea that they chewed the cud.
It is worthy of remark that other animals, which, though not rodents, need to possess chisel-edged incisor teeth, have a similar habit. Such is the hippopotamus, and such is the hyrax, the remarkable rock-haunting animal, which in the authorised translation of the Scriptures is called the "coney," and which in the Revised Version is allowed in the margin to retain its Hebrew name, "shaphan."
The enamel also has an important part to play in the structure of the molar teeth. Each tooth is surrounded with the enamel plate, which is so intricately folded that the tooth looks as if it were made of a series of enamel triangles, each enclosing the tooth matter.
This structure is common to all the members of the group to which the water-rat belongs. It is the more remarkable because we find a somewhat similar structure in the molar teeth of the elephants, which, like the rodents, have the incisor teeth largely developed and widely separated from the molars.
There is nothing in the appearance of the water-rat which gives any indication of its aquatic habits.
For example, we naturally expect to find that the feet of swimming animals are webbed. The water-loving capybara of South America, the largest existing rodent, has its hoof-like toes partially united by webs, so that its aquatic habits might easily be inferred even by those who were unacquainted with the animal. Even the otter, which propels itself through the water mostly by means of its long and powerful tail, has the feet furnished with webs. So has the aquatic Yapock opossum of Australia, while the feet of the duck-bill are even more boldly webbed than those of the bird from which it takes its popular name. The water-shrews (whom we shall presently meet) are furnished with a fringe of stiff hair round the toes which answers the same purpose as the web.