What was the origin of the fish? Here we are in much the same
position as we were in regard 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.
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