its, has usually been classed with the fishes; but, although
this animal exclusively inhabits the water, and is supplied with fins,
it nevertheless exhibits a striking alliance to quadrupeds. It has warm
blood, and produces its young alive; it nourishes them with milk, and,
for that purpose, is furnished with teats. It is also supplied with
lungs, and two auricles and two ventricles to the heart; all of which
bring it still closer into an alliance with the quadrupedal species of
the animal kingdom.

587. THE GENERAL CHARACTERISTICS OF THE MAMMALIA have been frequently
noticed. The bodies of nearly the whole species are covered with hair, a
kind of clothing which is both soft and warm, little liable to injury,
and bestowed in proportion to the necessities of the animal and the
nature of the climate it inhabits. In all the higher orders of animals,
the head is the principal seat of the organs of sense. It is there that
the eyes, the ears, the nose, and the mouth are placed. Through the last
they receive their nourishment. In it are the _teeth_, which, in most of
the mammalia, are used not only for the mastication of food, but as
weapons of offence. They are inserted into two movable bones called
jaws, and the front teeth are so placed that their sharp edges may
easily be brought in contact with their food, in order that its fibres
may readily be separated. Next to these, on each side, are situated the
canine teeth, or tusks, which are longer than the other teeth, and,
being pointed, are used to tear the food. In the back jaws are placed
another form of teeth, called grinders. These are for masticating the
food; and in those animals that live on vegetables, they are flattened
at the top; but, in carnivora, their upper surfaces are furnished with
sharp-pointed protuberances. From the numbers, form, and disposition of
the teeth, the various genera of quadrupeds have been arranged. The
_nose_ is a cartilaginous body, pierced with two holes, which are called
nostrils. Through these the animal is affected by the sense of smell;
and in some it is prominent, whilst in others it is flat, compressed,
turned upwards, or bent downwards. In beasts of prey, it is frequently
longer than the lips; and in some other animals it is elongated into a
movable trunk or proboscis, whilst, in the rhinoceros tribe, it is armed
with a horn. The _eyes_ of quadrupeds are generally defended by movable
lids, on the outer margins of which are fringes of