tem. 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 _Zosteraceae,_ 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 Lycopodiaceae,--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 equisetaceae. 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.)]

And here let me remark, that the facts of Palaeontological science compel
us to blend, in some degree, with the classification of our modern
botanists, that of the botanists of an earlier time. In a passage
alr