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 Gamopetalse — 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 

 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 extra- 

 ordinary, 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 explana- 

 tion of this anomaly has been given by Mr. Ball, who supposes 



