these adjoining zooelogical faunae,
each epoch is divided, as we have seen, into a number of periods,
occupying successive levels one above another, and differing
specifically from each other in time as zooelogical provinces differ from
each other in space. In short, every epoch is to be looked upon from two
points of view: as a unit, complete in itself, having one character
throughout, and as a stage in the progressive history of the world,
forming part of an organic whole.

* * * * *

As the Jurassic epoch was ushered in by the upheaval of the Jura, so its
close was marked by the upheaval of that system of mountains called the
Cote d'Or. With this latter upheaval began the Cretaceous epoch, which
we will examine with special reference to its subdivision into periods,
since the periods in this epoch have been clearly distinguished, and
investigated with especial care. I have alluded in the preceding article
to the immediate contact of the Jurassic and Cretaceous epochs in
Switzerland, affording peculiar facilities for the direct comparison of
their organic remains. But the Cretaceous deposits are well known, not
only in this inland sea of ancient Switzerland, but in a number of
European basins, in France, in the Pyrenees, on the Mediterranean
shores, and also in Syria, Egypt, India, and Southern Africa, as well as
on our own continent. In all these localities, the Cretaceous remains,
like those of the Jurassic epoch, have one organic character, distinct
and unique. This fact is especially significant, because the contact of
their respective deposits is in many localities so immediate and
continuous that it affords an admirable test for the development-theory.
If this is the true mode of origin of animals, those of the later
Jurassic beds must be the progenitors of those of the earlier Cretaceous
deposits. Let us see now how far this agrees with our knowledge of the
physiological laws of development.

Take first the class of Fishes. We have seen that in the Jurassic
periods there were none of our common Fishes, none corresponding to our
Herring, Pickerel, Mackerel, and the like,--no Fishes, in short, with
thin membranous scales, but that the class was represented exclusively
by those with hard, flint-like scales. In the Cretaceous epoch, however,
we come suddenly upon a horde of Fishes corresponding to our smaller
common Fishes of the Pickerel and Herring tribes, but principally of the