When the comparative anatomy of fish, amphibia, and reptiles had
been carefully studied it was evident that the amphibia stood far
nearer the fish in general structure, while the higher reptiles
closely approached birds. Then it was noticed that our common fish
formed a fairly well-defined group, but that the ganoids, including
the sturgeons, gar-pikes, and some others, had at least traces of
amphibian characteristics. Such generalized forms, with the
characteristics of the class less sharply marked, were usually 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 palæontologist 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
archæopteryx, 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 zoölogist began to ask and investigate how the animal grew
in the egg and attained its definite form. And this study of