of comparative anatomy, ornithology, ichthyology, entomology and botany, beside treating of the connection which exists between man and the lower animals, and the reciprocal influence of civilisation and animal life.

In the first place, let us define our animal.

What is a water-rat, and where is its place in zoological systems of the present day? Its name in science is Arvícola amphíbius. This title tells its own story.

Though popularly called a rat, the animal has no right to the name, although, like the true rat, it is a rodent, and much resembles the rat in size and in the length and colour of its fur. The likeness, however, extends no further.

The rats are long-nosed and sharp-snouted animals, whereas the water-rat has a short, blunt nose. Then, the ears of the rats are large and stand out boldly from the head, while those of the water-rat are small, short, and rounded. Again, the tail of the rat is long and slender, while that of the water-rat is comparatively short. Place the two animals side by side, and you will wonder how anyone could mistake the one for the other.

The teeth, too, are quite different.

Instead of being white, like those of the rat, the incisor teeth are orange-yellow, like those of the beaver. Indeed, the water-rat possesses so many beaver-like characteristics, that it was ranked near the beaver in the systematic lists.

Now, however, the Voles, as these creatures ought rightly to be called, are thought to be of sufficient importance to be placed by themselves, and separated from the true beavers.

The voles constitute quite a large group of rodents, including several animals which are popularly ranked among the mice.

One very remarkable characteristic of the voles is the structure of their molar teeth.

Being rodents, they can have but two incisor teeth in each jaw, these teeth being rootless, and so set in their sockets that they are incessantly worn away in front, and as incessantly grow from the base, take the curved form of their sockets, and act much like shears which have the inestimable property of self-sharpening when blunted, and self-renewal when chipped or actually broken off by coming against any hard substance. Were the teeth to be without this power, the animal would run a great risk of dying from hunger, the injured tooth not being able either to do its own work, or to aid its companion of the opposite jaw. Either tooth alone would be as useless as a single blade of a pair of scissors.

There is another notable characteristic of these incisor teeth. If you will examine the incisors of any rodent, whether it be a rat, a mouse, a rabbit, or a beaver, you will see that the tips are "bevelled" off just like the edge of a chisel. This shape is absolutely necessary to keep the tooth in working order. How is this object to be attained?

In the solution of this problem we may see one of the many links which connect art and nature.

Should our readers know anything of carpentering, let them examine the structure of their chisels. They are not made wholly of hard steel, as in that case they would be liable to snap, just as does the blade of a foil when undue pressure is brought to bear upon it. Moreover, the operation of sharpening would be extremely difficult.

So the blade of the chisel is merely faced with a thin plate of hardened steel, the remainder being of softer material.

Now, it is not at all likely that the unknown inventor of the modern chisel was aware of the analogy between art and nature, and would probably have been very much surprised if anyone had stated that he had borrowed his idea from the incisor teeth of the water-rat.

Yet he might have done so, for these teeth are almost wholly formed of ordinary tooth matter, and are faced with a thin plate of hard enamel, which exactly corresponds with the hardened steel facing of a chisel.

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.

But the structure of the water-rat gives no indication of its habits, so that no one who was unacquainted with the animal would even suspect its swimming and diving powers. Watch it as long as you like, and I do not believe that you will see it eating anything of an animal nature.

I mention this fact because it is often