Next to the Ferns, the Club-Mosses (Lycopodiacae) seem to have contributed most largely to the marsh-forests.  They were characterized, then, as now, by the small size of the leaves growing close against the stem, so that the stem itself, though covered with leaves, looks almost naked, like the stem of the Cactus.  Beside these, there are the tree-like Equiseta, in which we find the articulations on the trunk corresponding exactly to those now so characteristic of those marsh-grasses which are the modern representatives of this family of plants, with cone-like fructifications on the summit of the stem.

I would merely touch here upon a subject which does not belong to my own branch of Natural History, but is of the greatest interest in botanical research, namely, the gradation of plants in the geological ages, and the combination of characters in some of the earlier vegetable forms, corresponding to that already noticed in the ancient animal types.  For instance, in the Carboniferous period we have only Cryptogams, Ferns, Lycopodiacae, and Equisetaceae.  In the middle geological ages, Conifers are introduced, the first flowering plant known on earth, but in which the flower is very imperfect as compared with those of the higher groups.  The Coniferae were chiefly represented in the middle periods by the Cycadae, that peculiar group of Coniferae, resembling Pines in their structure, but recalling the Ferns by their external appearance.  The stem is round and short, its surface being covered with scars similar to those of the Ferns; while on the summit are ten or more leaves, fan-like and spreading when their growth is complete, but rolled up at first, like Fern-leaves before they expand.  Their fruit resembles somewhat the Pine-Apple.

The mode of growth of the Coniferae recalls a feature of the Equisetaceae also, in the tufts of little leaves which appear in whorls at regular intervals along the length of the stem in proportion as it elongates, reminding one of the articulations on the stem of the Equisetaceae.  The first cone also appears on the summit of the stem, like the terminal cone in the Equisetaceae and the Club-Mosses.  Thus in certain types of the vegetable, as well as the animal creation of earlier times, there was a continuation of features, afterwards divided and presented in separate groups.  In the present times, no one of these families of plants overlaps the others, but each has a distinct individual character of its own.

At the close of the middle geological ages and the opening of the Tertiary periods, the Monocotyledons become abundant, the first plants with flower and inclosed seed, though with no true floral envelope:  but not until the two last epochs of the Tertiary age do we find in any number the Dicotyledonous plants, in which flower and fruit rise to their highest perfection.  Thus there has been a procession of plants from their earliest introduction to the present day, corresponding to their botanical rank as they now exist, so that the series of gradation in the Vegetable Kingdom, as well as the Animal Kingdom, is the same, whether founded upon succession in time or upon comparative structural rank.