d to the origin of the higher Invertebrates. Once
the fish plainly appears upon the scene it is found to be undergoing a
process of evolution like all other animals. The vast majority of our
fishes have bony frames (or are Teleosts); the fishes of the Devonian
age nearly all have frames of cartilage, and we know from embryonic
development that cartilage is the first stage in the formation of bone.
In the teeth and tails, also, we find a gradual evolution toward the
higher types. But the earlier record is, for reasons I have already
given, obscure; and as my purpose is rather to discover the agencies
of evolution than to strain slender evidence in drawing up pedigrees, I
need only make brief reference to the state of the problem.

Until comparatively recent times the animal world fell into two clearly
distinct halves, the Vertebrates and the Invertebrates. There were
several anatomical differences between the two provinces, but the most
conspicuous and most puzzling was the backbone. Nowhere in living nature
or in the rocks was any intermediate type known between the backboned
and the non-backboned animal. In the course of the nineteenth century,
however, several animals of an intermediate type were found. The
sea-squirt has in its early youth the line of cartilage through the
body which, in embryonic development, represents the first stage of the
backbone; the lancelet and the Appendicularia have a rod of cartilage
throughout life; the "acorn-headed worm" shows traces of it. These are
regarded as surviving specimens of various groups of animals which, in
early times, fell between the Invertebrate and Vertebrate worlds, and
illustrate the transition.

With their aid a genealogical tree was constructed for the fish. It was
assumed that some Cambrian or Silurian Annelid obtained this stiffening
rod of cartilage. The next advantage--we have seen it in many cases--was
to combine flexibility with support. The rod was divided into connected
sections (vertebrae), and hardened into bone. Besides stiffening the
body, it provided a valuable shelter for the spinal cord, and its upper
part expanded into a box to enclose the brain. The fins were formed of
folds of skin which were thrown off at the sides and on the back, as
the animal wriggled through the water. They were of use in swimming, and
sections of them were stiffened with rods of cartilage, and became the
pairs of fins. Gill slits (as in some of the highest worms) appeared i