iar
aspect to the flora. As only 200 or 300 species of plants are known in
all the rocks ranging from the Trias to the Oolite inclusive, our data
are too scanty as yet to affirm whether the vegetation of this second
epoch was or was not on the whole of a simpler organization than that of
our own times.

In the Lower Cretaceous formation, near Aix-la-Chapelle, the leaves of a
great many dicotyledonous trees have lately been discovered by Dr.
Debey, establishing the important fact of the coexistence of a large
number of angiosperms with cycadeae, and with that rich reptilian fauna
comprising the ichthyosaur, plesiosaur, and pterodactyl, which some had
supposed to indicate a state of the atmosphere unfavorable to a
dicotyledonous vegetation.

The number of plants hitherto obtained from _tertiary_ strata of
different ages is very limited, but is rapidly increasing. They are
referable to a much greater variety of families and classes than an
equal number of fossil species taken from secondary or primary rocks,
the angiosperms bearing the same proportion to the gymnosperms and
acrogens as in the present flora of the globe. This greater variety may,
doubtless, be partly ascribed to the greater diversity of stations in
which the plants grew, as we have in this case an opportunity, rarely
enjoyed in studying the secondary fossils, of investigating inland or
lacustrine deposits accumulated at different heights above the sea, and
containing the memorials of plants washed down from adjoining mountains.

In regard, then, to the strata from the cretaceous to the uppermost
tertiary inclusive, we may affirm that we find in them all the principal
classes of living plants, and during this vast lapse of time four or
five complete changes in the vegetation occurred, yet no step whatever
was made in advance at any of these periods by the addition of more
highly organized species.

If we next turn to the fossils of the animal kingdom, we may inquire
whether, when they are arranged by the geologists in a chronological
series, they imply that beings of more highly developed structure and
greater intelligence entered upon the earth at successive epochs, those
of the simplest organization being the first created, and those more
highly organized being the last.

Our knowledge of the Silurian fauna is at present derived entirely from
rocks of marine origin, no fresh-water strata of such high antiquity
having yet been met with. The fossils, h