we see together a Dog, a Cat, a Bear, a Horse, a Cow, and a Deer. The
first feature that strikes us as common to any two of them is the horn in
the Cow and Deer. But how shall we associate either of the others with
these? We examine the teeth, and find those of the Dog, the Cat, and the
Bear sharp and cutting, while those of the Cow, the Deer, and the Horse
have flat surfaces, adapted to grinding and chewing, rather than cutting
and tearing. We compare these features of their structure with the habits
of these animals, and find that the first are carnivorous, that they seize
and tear their prey, while the others are herbivorous or grazing animals,
living only on vegetable substances, which they chew and grind. We compare
farther the Horse and Cow, and find that the Horse has front teeth both in
the upper and lower jaw, while the Cow has them only in the lower; and
going still farther and comparing the internal with the external features,
we find this arrangement of the teeth in direct relation to the different
structure of the stomach in the two animals,--the Cow having a stomach
with four pouches, adapted to a mode of digestion by which the food is
prepared for the second mastication, while the Horse has a simple stomach.
Comparing the Cow and the Deer, we find that the digestive apparatus is
the same in both; but though they both have horns, in the Cow the horn is
hollow, and remains through life firmly attached to the bone, while in the
Deer it is solid and is shed every year. With these facts before us, we
cannot hesitate to place the Dog, the Cat, and the Bear in one division,
as carnivorous animals, and the other three in another division as
herbivorous animals,--and looking a little farther, we perceive, that, in
common with the Cow and the Deer, the Goat and the Sheep have cloven feet,
and that they are all ruminants, while the Horse has a single hoof, does
not ruminate, and must therefore be separated from them, even though, like
them, he is herbivorous.

This is but the simplest illustration, taken from the most familiar