ene and Miocene. In the
latter formation 272 species have been discovered; but the 116 species
in the Eocene form a larger proportion of the total vegetation of the
period.

True dicotyledons appear very much later, in the Cretaceous period, and
only in its upper division, if we except a single species from the
Urgonian beds of Greenland. The remarkable thing is that we here find
the sub-class fully developed and in great luxuriance of types, all the
three divisions--Apetalae, Polypetalae, and Gamopetalae--being
represented, with a total of no less than 770 species. Among them are
such familiar forms as the poplar, the birch, the beech, the sycamore,
and the oak; as well as the fig, the true laurel, the sassafras, the
persimmon, the maple, the walnut, the magnolia, and even the apple and
the plum tribes. Passing on to the Tertiary period the numbers increase,
till they reach their maximum in the Miocene, where more than 2000
species of dicotyledons have been discovered. Among these the
proportionate number of the higher gamopetalae has slightly increased,
but is considerably less than at the present day.

_Possible Cause of sudden late Appearance of Exogens._

The sudden appearance of fully developed exogenous flowering plants in
the Cretaceous period is very analogous to the equally sudden appearance
of all the chief types of placental mammalia in the Eocene; and in both
cases we must feel sure that this suddenness is only apparent, due to
unknown conditions which have prevented their preservation (or their
discovery) in earlier formations. The case of the dicotyledonous plants
is in some respects the most extraordinary, because in the earlier
Mesozoic formations we appear to have a fair representation of the flora
of the period, including such varied forms as ferns, equisetums, cycads,
conifers, and monocotyledons. The only hint at an explanation of this
anomaly has been given by Mr. Ball, who supposes that all these groups
inhabited the lowlands, where there was not only excessive heat and
moisture, but also a superabundance of carbonic acid in the
atmosphere--conditions under which these groups had been developed, but
which were prejudicial to the dicotyledons. These latter are supposed to
have originated on the high table-lands and mountain ranges, in a rarer
and drier atmosphere in which the quantity of carbonic acid gas was much
less; and any deposits formed in lake beds at high altitudes and at such
a remot