The most ancient period of whose organisms any trace remains in the rocks seems to have been, prevailingly at least, a period of Thallogens. We must, of course, take into account the fact, that it has yielded no land plants, and that the sea is everywhere now, as of old, the great habitat of the algæ,--one of the four great orders into which the Thallogens are divided. There appear no traces of a terrestrial vegetation until we reach the uppermost beds of the Upper Silurian System. But, account for the fact as we may, it is at least worthy of notice, that, alike in the systems of our botanists and in the chronological arrangements of our geologists, the first or introductory class which occurs in the ascending order is this humble Thallogenic class. There is some trace in the Lower Silurians of Scotland of a vegetable structure which may have belonged to one of the humbler Endogens, of which, at least, a single genus, the _Zosteraceæ,_ still exists in salt water; but the trace is faint and doubtful, and, even were it established, it would form merely a solitary exception to the general evidence that the first known period of vegetable existence was a period of Thallogens. The terrestrial remains of the Upper Silurians of England, the oldest yet known, consist chiefly of spore-like bodies, which belonged, says Dr. Hooker, to Lycopodiaceæ,--an order of the second or acrogenic class. And, in the second great geologic period,--that of the Old Red Sandstone,--we find this second class not inadequately represented. In its lowest fossiliferous beds we detect a Lycopodite which not a little resembles one of the commonest of our club mosses,--_Lycopodium clavatum_,--with a minute fern and a large striated plant resembling a calamite, and evidently allied to an existing genus of Acrogens, the equisetaceæ. In the Middle Old Red Sandstone there also occurs a small fern, with some trace of a larger; and one of its best preserved vegetable organisms is a lepidodendron,--an extinct ally of the Lycopodiums; while in the upper beds of the system, especially as developed in the south of Ireland, the noble fern known as _Cyclopteris Hibernicus_ is very abundant. This fern has been detected also in the Upper Old Red of our own country, mingled with fragments of contemporary calamites. With, however, these earliest plants of the land yet known, there occurs a true wood, which belonged, as shown by its structure, to a gymnospermous or polycotyledonous tree, and which we find associated with remains of Coccosteus and Diplacanthus. [Illustration: Fig. 2. CYCLOPTERIS HIBERNICUS. (Nat. size.)] [Illustration: Fig. 3. CONIFER OF THE LOWER OLD RED SANDSTONE. Cromarty. (Mag. forty diameters.)]