The Girls Own Paper, Vol. VIII, No. 355, October 16, 1886 | Page 4

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,