arches be of hard bone, or of
cartilage, or even of a softer substance; every Vertebrate has the
brain, the spinal marrow or spinal cord, and the organs of the senses in
the upper cavity, and the organs of digestion, respiration, circulation,
and reproduction in the lower one; every Vertebrate has four locomotive
appendages built of the same bones and bearing the same relation to the
rest of the organization, whether they be called pectoral and ventral
fins, or legs, or wings and legs, or arms and legs. Notwithstanding
the rudimentary condition of these limbs in some Vertebrates and their
difference of external appearance in the different groups, they are all
built of the same structural elements. These are the typical characters
of the whole branch, and exist in all its representatives.

What now are the different modes of expressing this structural plan that
lead us to associate certain Vertebrates together in distinct classes?
Beginning with the lowest class,--the Fishes are cold-blooded, they
breathe through gills, and they are egg-laying; in other words, though
they have the same general structure as the other Vertebrates, they
have a special mode of circulation, respiration, and reproduction. The
Reptiles are also cold-blooded, though their system of circulation is
somewhat more complicated than that of the Fishes; they breathe through
lungs, though part of them retain their gills through life; and they lay
eggs, but larger and fewer ones than the Fishes, diminishing in number
in proportion to their own higher or lower position in their class. They
also bestow greater care upon their offspring than most of the
Fishes. The Birds are warm-blooded and air-breathing, having a double
circulation; they are egg-laying like the two other classes, but their
eggs are comparatively few in number, and the young are hatched by the
mother and fed by the parent birds till they can provide for themselves.

The Mammalia are also warm-blooded and breathe through lungs; but
they differ from all other Vertebrates in their mode of reproduction,
bringing forth living young which they nurse with milk. Even in the
lowest members of this highest group of the Vertebrates, at the head
of which stands Man himself, looking heavenward it is true, but
nevertheless rooted deeply in the Animal Kingdom, we have the dawning
of those family relations, those intimate ties between parents and
children, on which the whole social organization of the hu