ra was attained. At the
same time the skull was developing in a somewhat similar manner. But
the skull is far more complex in origin and undergoes far more
numerous and important changes than the simpler vertebral column.
Into its history we have no time to enter.

And what shall we say of bone itself as a mere material or tissue,
with its admirable lightness, compactness, and flawlessness. And
every bone in our body is a triumph of engineering architecture. No
engineer could better recognize the direction of strain and stress,
and arrange his rods and columns, arches and buttresses, to suitably
meet them, than these problems are solved in the long bone of our
thigh. And they must be lengthened while the child is leaping upon
them. An engineer is justly proud if he can rebuild or lengthen a
bridge without delaying the passage of a single train. But what
would he say if you asked him to rebuild a locomotive, while it was
running even twenty miles an hour? And yet a similar problem had to
be solved in our bodies.

But the vertebral column is not perfected by fish. The vertebrae with
few exceptions are hollow in front and behind, biconcave; and
between each two vertebrae there is a large cavity still occupied by
the notochord. Thus these vertebrae join one another by their edges,
like two shallow wine-glasses placed rim to rim. Only gradually is
the notochord crowded out so that the vertebrae join by their whole
adjacent surfaces. Even in highest forms, for the sake of mobility,
they are united by washer-like disks of cartilage. Biconcave
vertebrae persisted through the oldest amphibia, reptiles, and
birds. But finally a firm backbone and skull were attained.

2. The appendages. Of these we can say but little. The fish has
oar-like fins, attached to the body by a joint, but themselves
unjointed. By the amphibia legs, with the same regions as our own
and with five toes, have already appeared. The development of the
leg out of the fin is one of the most difficult and least understood
problems of vertebrate comparative anatomy. The legs are at first
weak and scarcely capable of supporting the body. Only gradually do
they strengthen into the fore- and hind-legs of mammals, or into the
legs and wings of birds and old flying reptiles.

3. Changes in the circulatory and respiratory systems. The fish
lives altogether in the water and breathes by gills, but the dipnoi
among fishes breathes by lungs as well as gills. As long as