y by
common consent placed at the bottom of the class. And this suited
well their general structure, while in particular characteristics
they were often more highly organized than higher groups of the same
class.

The palaeontologist found that the oldest fossil forms belonged to
these generalized groups, and that more highly specialized
forms--that is, those in which the special class distinctions were
more sharply and universally marked--were of later geological
origin. Thus the oldest fish were most like our present ganoids and
sharks, though differing much from both. Our common teleost fish,
like perch and cod, appeared much later. The oldest bird, the
archaeopteryx, had a long tail like that of a lizard, and teeth; and
thus stood in many respects almost midway between birds and
reptiles. And most of the earliest forms were "comprehensive,"
uniting the characteristics of two or more later groups. Thus as the
classification became more natural, based on a careful comparison of
the whole anatomy of the animals, its order was found to coincide in
general with that of geological succession.

Then the zooelogist began to ask and investigate how the animal grew
in the egg and attained its definite form. And this study of
embryology brought to light many new and interesting facts. Agassiz
especially emphasized and maintained the universality of the fact
that there was a remarkable parallelism between embryos of later
forms and adults of old or fossil groups. The embryos of higher
forms, he said, pass through and beyond certain stages of structure,
which are permanent in lower and older members of the same group.

You remember that the fin on the tail of a fish is as a rule
bilobed. Now the backbone of a perch or cod ends at a point in the
end of the tail opposite the angle between the two lobes, without
extending out into either of them. In the shark it extends almost to
the end of the upper lobe. Now we have seen that sharks and ganoids
are older than cod. In the embryo of the cod or perch the backbone
has, at an early stage, the same position as in the shark or ganoid;
only at a later stage does it attain its definite position.

So Agassiz says the young lepidosteus (a ganoid fish), long after it
is hatched, exhibits in the form of its tail characters thus far
known only among the fossil fishes of the Devonian period. The
embryology of turtles throws light upon the fossil chelonians. It is
already known that the em