The Primary epoch, which comes next, comprises the Devonian or Old Red Sandstone, the Carboniferous or Coal system, and the Permian, the average thickness of the three together amounting to about 42,000 feet. In these the family of Ferns predominates, the remains of which constitute the bulk of the large strata of coal, forming in modern times our great resource for obtaining the energy which, in a transformed shape, does so much of our work. Pines begin to appear, though sparingly, in this epoch. The Secondary epoch comprises the Triassic, the Jurassic, and the Cretaceous or Chalk formation, the average thickness of the three amounting to about 15,000 feet. In this era a higher species of vegetation predominates, that of the Gymnosperms, or plants having naked seeds, of which the pines, or Coniferæ, and the palm-ferns, or Cycadeæ, are the two principal classes. As in the case of the former epoch, traces of the approaching higher organisation in the form of leaf-bearing trees began to appear towards its close. The Tertiary period extends from the end of the Chalk to the commencement of the Quaternary or modern period. It is divided into the Eocene or older, the Miocene or middle, and the Pliocene or newest Tertiary system; though the division is somewhat arbitrary, depending on the number of existing species, mostly of shellfish, which have been found in each. The average thickness of the three together is about 3,000 feet. In this formation a still higher class of vegetation of the same order as that now existing, which made its first appearance in the Chalk period, has become predominant. It is that of Angiosperms, or plants with covered seeds, forming leafy forests of true trees. This group is divided into the two classes of monocotyledons or single-seed-lobed plants, and dicotyledons or plants with double seed-lobes. The monocotyledons spring from a single germ leaf, and are of simpler organisation than the other class. They comprise the grasses, rushes, lilies, irids, orchids, sea-grasses, and a number of aquatic plants, and in their highest form develop into the tree-like families of the palms and bananas. The dicotyledons include all forms of leaf-bearing forest trees, almost all fruits and flowers, in fact by far the greater part of the vegetable world familiar to man, as coming into immediate relation with it, except in the case of the cultivated plants, which are developments of the monocotyledon grasses. We see, therefore, in the geological record a confirmation of the evolution over immense periods of time of the more complex and perfect from the simple and primitive.