400 
DARWINISM 
CHAP. 
meet with monocotyledons in the Carboniferous and Permian 
formations. The character of these fossils was long disputed, 
but is now believed to be well established ; and the sub¬ 
class continues to be present in small numbers in all succeeding 
deposits, becoming rather plentiful in the Upper Cretaceous, 
and very abundant in the Eocene 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 remark¬ 
able thing is that we here find the sub-class fully developed 
and in great luxuriance of types, all the three divisions— 
Apetalse, Polypetalse, and Gamopetake—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 dicoty¬ 
ledons 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 
dowering 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 extra¬ 
ordinary, because in the earlier Mesozoic formations we appear 
to have a fair representation of the dora of the period, 
including such varied forms as ferns, equisetums, cycads, 
conifers, and monocotyledons. The only hint at an explana¬ 
tion of this anomaly has been given by Mr. Ball, who supposes 
