when Jussieu first enunciated his scheme of classification; nor did it receive its later improvements until so late as 1846, when, after the publication, in succession, of the schemes of De Candolle and Endlicher, Lindley communicated his finished system to the world. And there certainly existed no even tolerably perfect system of zoology until 1816, when the "Animal Kingdom" of Cuvier appeared. Later naturalists—such as Agassiz, in his own special department, the history of fishes, and Professor Owen in the invertebrate divisions—have improved on the classification of even the great Frenchman; but for purposes of comparison between the scheme developed in geologic history and that at length elaborated by the human mind, the system of Cuvier will be found, for at least our present purpose, sufficiently complete. And in tracing through time the course of the vegetable kingdom, let us adopt, as our standard to measure it by, the system of Lindley.
Commencing at the bottom of the scale, we find the Thallogens, or flowerless plants which lack proper stems and leaves—a class which includes all the algæ. Next succeed the Acrogens, or flowerless plants that possess both stems and leaves—such as the ferns and their allies. Next, omitting an inconspicuous class, represented by but a few parasitical plants incapable of preservation as fossils, come the Endogens—monocotyledonous flowering plants, that include the palms, the liliaceæ, and several other families, all characterized by the parallel venation of their leaves. Next, omitting another inconspicuous tribe, there follows a very important class—the Gymnogens—polycotyledonous trees, represented by the conifers; and cycadaceæ. And, last of all, come the Dicotyledonous Exogens—a class to which all our fruit, and what are known as our "forest trees," belong, with a vastly preponderating majority of the herbs and flowers that impart fertility and beauty to our gardens and meadows. This last class, though but one, now occupies much greater space in the vegetable kingdom than all the others united.
Such is the arrangement of Lindley, or rather an arrangement the slow growth of ages, to which this distinguished botanist has given the last finishing touches. And let us now mark how closely it resembles the geologic arrangement as developed in the successive stages of the earth's history.
Fig. 1.[4]
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.
Fig. 2.  |
Fig. 3.  |
And here let me remark, that the facts of Palæontological 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 already quoted, Solomon is said to have discoursed of plants, "from the cedar tree that is in Lebanon, to the hyssop that springeth out of the wall,"—from the great tree to the minute herb; and Cowley rose, in his metrical treatise, as has been shown, from descriptions of herbs and flowers to descriptions of fruit and forest trees. And as in every age in which there existed a terrestrial vegetation there seem to have been "trees" as certainly as "herbs," the palæontological botantist finds that he has, in consequence, to range his classes, not in one series, but in two—the Gymnogens, or cone-bearing trees, in a line nearly parallel with the Acrogens, or flowerless, spore-bearing herbs. But the arrangement is in no degree the less striking from the circumstance that it is ranged, not in one, but in two lines. It is, however, an untoward arrangement for the purposes of the Lamarckian, whose peculiar hypothesis would imperatively demand, not a double, but a single column, in which the ferns and club mosses would stand far in advance, in point of time, of the Coniferæ. In the Coal Measures, so remarkable for the great luxuriance of their flora, both the Gymnogens and Acrogens are largely developed, with a very puzzling intermediate class, that, while they attained to the size of trees, like the former, retained in a remarkable degree, as in the Lepidodendra and the Calamites, the peculiar features of the latter. And with these there appear, though more sparingly, the Endogens—monocotyledonous plants, represented by a few palm-like trees (Palmacites), a few date-like fruits (Trigonocarpum), and a few grass-like herbs (Poacites). In the great Secondary division, the true dicotyledonous plants first appear; but, so far as is yet known, no dicotyledonous wood. In the earlier formations of the division a degree of doubt attaches to even the few leaves of this class hitherto detected; but in the Lower Cretaceous strata they become at once unequivocal in their character, and comparatively abundant, both as individuals and species; and in the Tertiary deposits they greatly outnumber all the humbler classes, and appear not only as herbs, but also as great trees. Not, however, until shortly before the introduction of man do some of their highest orders, such as the Rosaceæ, come upon the scene, as plants of that great garden—including the fields of the agriculturist—which it has been part of man's set task upon earth to keep and to dress. And such seems to be the order of classification in the vegetable kingdom, as developed in creation, and determined by the geologic periods.
Fig. 4.[5]
The parallelism which exists between the course of creation, as exhibited in the animal kingdom, and the classification of the greatest zoologist of modern times, is perhaps still more remarkable. Cuvier divides all animals into vertebrate and invertebrate; the invertebrates consisting, according to his arrangement, of three great divisions—mollusca, articulata, and radiata; and the vertebrates, of four great classes—the mammals, the birds, the reptiles, and the fishes. From the lowest zone at which organic remains occur, up till the higher beds of the Lower Silurian System, all the animal remains yet found belong to the invertebrate divisions. The numerous tables of stone which compose the leaves of this first and earliest of
