thus avoid the painful impression that there are any suppressed facts of recent date which clash with the theories of the succeeding Lectures, destroying their value and impairing their unity. And it may be well to remind him that there are two schools of Geology, quite at one in their willingness to bring all theories to the test of actual discovery, but widely differing in their leanings as to the mode in which, a priori, they would wish the facts brought to light to be viewed. The one, as expounded in the following Lectures, delights in the unfolding of a great plan, having its original in the Divine Mind, which has gradually fitted the earth to be the habitation of intelligent beings, and has introduced upon the stage of time organism after organism, rising in dignity, until all have found their completion in the human nature, which, in its turn, is a prophecy of the spiritual and Divine. This may be said to be the true development hypothesis, in opposition to the false and puerile one, which has been discarded by all geologists worthy of the name, of whatsoever side. The other school holds the opinion—though, perhaps, not very decidedly — that all things have been from the beginning as they are now; and that if evidence at the present moment leans to the side of a gradual progress and a serial development, it is because so much remains undiscovered; the hiatus, wherever it occurs, being always in our own knowledge, and not in the actual state of things. The next score of years will probably bring the matter to a pretty fair decision; for it seems impossible that,

if so many able workers continue to be employed as indu ously as now in the same field, the remains of man and higher mammals will not be found to be of all periods, if a periods they existed. In the meantime, it is well to know actual point to which discovery has conducted us; and th have taken every pains most carefully to ascertain.

The Upper Ludlow rocks - the uppermost of the Siluri -continue to be the lowest point at which fish are fou Up to that period, during the vast ages of the Cambri where only the faintest traces of animal life have been tected in the shape of annelides or sand-boring worms, throughout the whole range of the Silurians, where shell-f and crustaceans, with inferior forms of life abounded, traces of fish, the lowest vertebrate existences until latest formed beds of the Upper Silurian, have yet appear There are now six genera of fish ranked as Upper Silurian, Auchenaspis, Cephalaspis, Pteraspis, Plectrodus, Onchus M chisoni, and Sphagodus. The two latter-Onchus Murchiso and Sphagodus are represented by bony defences, such are possessed by placoid fishes of the present day. Sir Ro erick Murchison at one time entertained the idea of placin the Ludlow bone-bed at the base of the Old Red Sandston but its fish having been found decidedly associated with Sil rian organisms, this idea has been abandoned.

The next point to which public attention has been specially directed, is the discovery of mammals lower than they had formerly appeared. Considerable misconception has arisen on this head. The Middle Purbeck beds, recently explored by Mr. Beckles, in which various small mammals were found, occur considerably farther up than the Stonesfield slates, in which the first quadruped was detected so far back as 1818. But this discovery involves no theoretical change, inasmuch as all the mammalian remains of the Middle Purbecks consist of small marsupials and insectivora, varying in size from a rat to a hedgehog, with one or two doubtful species, not yet proved to be otherwise. The living analogue of one very interesting genus is the kangaroo rat, which inhabits the prairies and scrub-jungles of Australia, feeding on plants and scratched-up roots. Between the English Stonesfield or Great Oolite, in which, many years ago, four species of these small mammals were known to exist, and the Middle Purbeck, quarried by Mr. Beckles, in which fourteen species are now found, there intervene the Oxford Clay, Coral Rag, Kimmeridge Clay, Portland Oolite, and Lower Purbeck Oolite; and then, after the Middle Purbeck, there occurs a great hiatus throughout the Weald, Green Sand, Gault, and Chalk, wherein no quadrupedal remains have been found; until at length we are introduced, in the Tertiary, to the dawn of the grand mammalian period; so that nothing has occurred in this department to occasion any revolution in the ideas of those who, with my

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husband, consider a succession and development of type to be the one great fixed law of geological science. The reader will see that in the end of Lecture Third such remains as have been found lower than the Tertiary are expressly recognized and excepted. "Save," says the author, "in the dwarf and inferior forms of the marsupials and insectivora, not any of the honest mammals have yet appeared."

But while attaching no importance to the discoveries in the Middle Purbeck, except in regard of more ample numerical development, it is necessary to admit the evidence of marsupials having been found lower than the Stonesfield or Great Oolite; even so far back as the Upper Trias, the Keuper Sandstone of Germany, which lies at the base of the Lias. I must be permitted, on this point, to quote the authority of Sir Roderick Murchison, as one of the safest and most cautious exponents of geological fact. "In that deposit," says he, referring to the Keuper Sandstone of Wurtemberg, "the relics of a solitary small marsupial mammal have been exhumed, which its discoverer, Plieninger, has named Microlestes Antiquus. Again, Dr. Ebenezer Emmons, the well-known geologist, of Albany, in the United States, has described, from the lower beds of the Chatham Secondary Coal-field, North Carolina (of the same age as those of Virginia, and probably of the Wurtemberg Keuper), the jaws of another minute mammal, which he calls Dromotherium Sylvestre. Lastly, while I

write, Mr. C. Moore has detected in an agglomerate which fills the fissures of the carboniferous limestone near Frome, Somersetshire, the teeth of marsupial mammals, one of which he considers to be closely related to the Microlestes Antiquus of Germany, and Professor Owen confirms the fact. From that coincidence, and also from the association with other animal remains, the Placodus (a reptile of the Muschelkalk), and certain mollusca, Mr Moore believes that these patches represent the Keuper of Germany. If this view should be sustained, this author, who has already made remarkable additions to our acquaintance with the organic remains of the Oolitic rocks and the Lias, will have had the merit of having discovered the first traces of mammalia in any British stratum below the Stonesfield slates." "Let me entreat,"

says Sir Roderick, in a passage occurring shortly after that we have quoted, "Let me entreat the reader not to be led by the reasoning of the ablest physiologist, or by an appeal to minute structural affinities, to impugn the clear and exact facts of a succession from lower to higher grades of life in each formation. Let no one imagine that because the bony characters in the jaw and teeth of the Plagiaulax of the Purbeck strata are such as the comparative anatomist might have expected to find among existing marsupials, and that the animal is, therefore, far removed from the embryonic archetype, such an argument disturbs the order of succession of classes, as seen in the crust of the earth." So far from disturbing the order of succession,