Man is a _mammal_. He belongs to the most brilliant and influential
of the five classes of vertebrates—the class to which belong so
many of his associates and victims, the class to which belong the
horse, the dog, the deer, the ox, the sheep, the swine, the
squirrel, the camel, the unattenuated elephant, and the
timid-hearted hare. To this class belong also the lion, the tiger,
the kangaroo, the beaver, the bear, the bat, the monkey, the mole,
the wolf, the ornithorhynchus, and the whale—in short, _all
animals that have hair_. Fishes and reptiles have scales; birds have
feathers; all mammals are covered to a greater or less extent with
hair. The aquatic habits of whales render hair of no use to them.
Hence, while the unborn of these animals still cling to the
structural traditions of their ancestors and are covered with hair,
the adults are almost hairless. The sartorial habits of human beings
and the selective influences of the sexes have had a similar effect
on the hairy covering of the human body. Hair exists all over the
human body surface, excepting on the soles of the hands and feet,
but in a greatly dwarfed condition. It is only on the scalp and on
the faces of males, where it is scientifically assisted for purposes
of display, that it grows luxuriantly. It is by no means certain
that even the hair on the masculine scalp will last forever. For if
the hermetical derby and other deadly devices worn by men continue
their devastations as they have in the past, we may expect to have,
in the course of generations, men with foreheads reaching regularly
to the occiput. Most animals lay eggs. Man does not. Like the dog,
the horse, the squirrel, and the bat, man is viviparous, the eggs
hatching within the parental body. Human young are born helpless,
and are sustained during the period of their infancy by the
secretions of the milk glands. So are all the sons and daughters of
mammals. Whether they come into the world among the waters or among
the desert sands, in the hollow of a tree, in a hole in the earth,
or in a palace, the children of mammals are frail and pitiful, and
they survive to grow and multiply only because they are the object
of the loving and incessant sacrifices of a mother.
Mammals are distinguished from all other animals by the possession
of two kinds of skin glands—the sweat glands and the oil
glands—and by the development of certain of these glands in the
female into organs for the nourishing of the young. Among reptiles
and birds the lower jaw is suspended from the skull by a bone called
the quadrate bone. Among men and other mammals the lower jaw is
joined directly to the skull, the quadrate bone becoming, in the
vicissitudes of evolution, the hammer (malleus) of the mammalian
ear. Man has a four-chambered heart—two reservoirs which receive,
and two pumps which propel, the scarlet waters of the body. Fishes
have two-chambered hearts; frogs and most reptiles have
three-chambered hearts; all mammals and birds have four-chambered
hearts. The red corpuscles in the blood of fishes, frogs, reptiles,
and birds, are discs, double-convex, nucleated, and in shape oval or
triangular. In man and in all other mammals (except the archaic
camel) the red corpuscles are double-concave, non-nucleated, and
circular. ‘Man has a diaphragm dividing the body cavity into chest
and abdomen, and a shining white bridge of interlacing fibres,
called _corpus callosum_, uniting his cerebral hemispheres. And man
is a mammal because, like other mammals, he has, in addition to the
qualities already mentioned, these valuable and distinct
characteristics.