improve. But this improvement is
not for the sake of these vegetative functions. Brain and muscle
demand vastly more fuel, and produce vastly more waste which must be
removed. At almost the close of the series the reproductive system
undergoes a modification which is almost revolutionary in its
results. But we shall find that this modification is necessitated by
the smaller amount of material which can be spared for this
function; not by its increasing importance, still less its dominance
for its own worth. The vertebrate is like an old Roman; everything
is subordinated to mental and physical power. He is the world
conqueror.

The important changes from fish upward affect the following organs:
1. The skeleton. A light, solid framework must be developed for the
body. 2. The appendages start as fins, and end as the legs and arms
of man. 3. The circulatory and respiratory systems developed so as
to carry with the utmost rapidity and certainty fuel and oxygen to
the muscular and nervous high-pressure engines. Or, to change the
figure, they are the roads along which supplies and munitions can be
carried to the army suddenly mobilized at any point on the frontier.
4. Above all, the brain, especially the cerebrum, the crown and goal
of vertebrate structure. The improvement is now practically
altogether in the animal organs of locomotion and thought. Still,
among these animal organs, the lower systems will lead in point of
time. The brain must to a certain extent wait for the skeleton.

1. The skeleton. The axial skeleton consists, in the lowest fish, of
the notochord, a cylindrical unsegmented rod of cartilage running
nearly the length of the body. This is surrounded by a sheath of
connective tissue, at first merely membranous, later becoming
cartilaginous or gristly. Pieces of cartilage extend upward over the
spinal marrow, and downward around the great aortic artery, forming
the neural and haemal arches. These unite with the masses of
cartilage surrounding the notochord to form cartilaginous vertebrae,
which may be stiffened by an infiltration of carbonate of lime. The
vertebral column of sharks has reached this stage. Then the
cartilaginous vertebrae ossify and form a true backbone. I have
described the process as if it were very simple. But only the
student of comparative osteology can have any conception of the
number of experiments which were tried in different groups before
the definite mode of forming a bony verteb