the germ of life on some sunny and serene spot ;-it may charm the materialist to claim for Life the eternity he does for Matter, by referring to a metamorphism which is continuously obliterating the fossils in the deepest seated rocks; but the paleontologist is debarred from such reveries, and is bound down by a rigid chain of facts as they occur in nature. He has traced life so early as the Cambrian slates; should it be detected still lower, he is ready to accept it. To him, in the mean time, the Metamorphic schists are a tabula rasa; the Cambrian slates form his furthest verge and boundary; and the spirit of induction restrains him within its limits. And, after all, fossil evidence itself is greatly in favour of the view, that we have here attained, or all but attained, the furthest limit of life. We see it increasing and spreading into higher and higher forms as we ascend in the geological scale, and decreasing and narrowing into lowlier forms as we descend: numerically the forms are fewer, physiologically they become less important; and it is but fair induction to believe that in the few scattered forms of Cambria we have all but reached the zero of organic existence.*

From the Cambrian the palaeontologist passes into the Silurian age-a period characterised by its lowly sea-weeds and doubtful traces of land plants-by genera and species of protozoan, radiate, molluscoid, molluscan, and articulate types, but by few, if any, even of the lowest vertebrate order. Its strata consist of shales, sandstones, conglomerates, and limestones-the solidified muds, sands,

*It is right to mention, however, that the tendency of recent discovery is to carry the traces of life further and further back among these slaty and semi-crystalline strata. The detection of new graptolites and trilobites in the schists of Bray Head, Skiddaw, Bohemia, and North America, is a fact too significant to be overlooked in geological speculation.

F

pebbles, and coral-growths of seas and estuaries. It is customary for a certain class of geologists to talk of "the deep, turbid, and shoreless seas" of the Silurian epoch, as if the globe was then enveloped by one dreary monotony of ocean. Do such generalisers ever for a moment think that such a vast thickness of sediments could never have been produced without the existence of broad lands from which they were transported by rivers, or of sea-shores from which they were abraded by waves and tidal currents? Could conglomerates be formed without wave-exposed beaches, sands without open sea-shores, or could shells that are truly littoral, and corals that flourish only from twenty to sixty fathoms, have existed without water of limited depth for their development? The eye of the trilobite would have been useless in a turbid ocean; a turbid ocean would have been death to the growth of corals; worm-burrowed, ripple-marked, and rain-pitted sandstones could have been formed only on shores exposed to the alternate ebb and flow of the tide; and conglomerates are merely the broken-down and water-worn fragments of an older rocky shore. In fine, there is not a single feature in the rocks of the Silurian period which might not take place in the ocean of our own day. The existence of deeper and shallower seas—of waves, currents, tides—of lands, shores, and rivers-of sunlight, and rains, and winds—are as clearly impressed on its strata as they are upon those of every other geological epoch. It differs alone in the geographical distribution of its sea and land-the greater insularity, perhaps, of the land-masses-their consequent climatologyand the specific characters of its plants and animals; though, knowing the wide extent of its deposits (and they occur alike in the continents of the Old and New World, in the northern and in the southern hemisphere), geology is not yet in a position to map with accuracy the geography of the

period, nor to define with certainty the external conditions to which its flora and fauna would be necessarily subjected.

When we turn to its biological aspects, the outline, though far from complete, is at least, as far as it goes, homogeneous and intelligible. Fucoids or fucus-like seaweeds, some carbonaceous fragments of unknown stems, spore-like organisms, apparently from land plants, and a few lepidodendroid twigs that may have belonged to some ancient form of club-moss, are nearly all we know of the silurian Flora; though, judging from the extent of anthracite deposits in various regions, vegetation (aquatic and terrestrial) must in certain centres have existed in some

1

FRAGMENTS OF SILURIAN FLORA.

1, 2, Fucoids-Cruziana and Chondrites (?); 3, 4, Lycopodites-Lepidodendroid twigs from

the Upper Silurians of Lanarkshire.

exuberance. On the whole, the silurian Flora is of a very lowly character, and its scanty fragments find their nearest affinities in the sea-weeds, liver-worts, and club-mosses of existing nature. Of course, the imperfection of the geological record is fully and frankly admitted, for it cannot be

supposed that in strata so eminently marine, we are likely to discover more than the merest indication of a terrestrial vegetation. Still we can only reason from what we know, and shape our inferences by the results of our observation.

When we turn to the Fauna of the system, we find the record much more complete and legible. We are presented with infusorial organisms from its shales; graptolites or sertularian-like zoophytes in inconceivable numbers; corals of

1. Oldhamia; 2, Protcvirguiaria; 3, Graptolites; 4, 5, Diplograpsus 6, Didymograpsus ; 7, Rastrites.

many genera and species; encrinites of various forms; star-fishes, independent and free-floating; and sea-urchinlike cystidea, attached to the sea-bottom by their jointed foot-stalks. In molluscan life we have representatives of every order-brachiopods, acephalans, gasteropods, pteropods, and cephalopods - vegetable-feeders thronging the shores, carnivorous orders in the open sea, and infusorialfeeders in the deeper waters. The great preponderance of brachiopods over acephalans and gasteropods is one of the most noticeable features in the molluscan life of the period -a feature now reversed, seeing that acephalans and gas

teropods are the predominating forms in existing waters.* In the articulate division we have numerous annelid markings-the trails and burrows of sea-worms; the calcareous crusts and shell-like cases of serpule and spirorbes; and a vast and characteristic display of trilobites (three-lobed), a form of crustacean almost restricted to the period; together with the larger and higher forms of eurypterites (broadfins-in allusion to their paddle-like swimming limbs). These

1. Heliolites; 2, Catenipora; 3, Cyathophyllum; 4, Taxocrinus; 5, Cystidea; 6, Palæaster

trilobites, along with some smaller bivalved forms of crustacea, have been long and familiarly known; but the euryp

*We abstain, in this as in other instances of comparison, from numerical tabulations, as every year of further discovery and nicer discrimination of species disturbs, if not destroys, the value of such statistics. Not many years ago the Brachiopoda were supposed to be on the very verge of extinction, and yet the application of the dredge to deeper waters has revealed the existence of nearly a dozen genera in modern seas. Every year, too, discovery adds some new form to our lists of fossils, while former lists of so-called species-Continental, British, and American-are being examined with more rigorous care, and reduced to their proper value.