LECTURE SECOND. THE PALEONTOLOGICAL HISTORY OF ANIMALS. AMID the unceasing change and endless variety of nature there occur certain great radical ideas, that, while they form, if I may so express myself, the groundwork of the change, the basis of the variety, -admit in themselves of no change or variety whatever. They constitute the aye-enduring tissue on which the ever-changing patterns of creation are inscribed: the patterns are ever varying; the tissue which exhibits them for ever remains the same. In the animal kingdom, for instance, the prominent ideas have always been uniform. However much the faunas of the various geologic periods may have differed from each other, or from the fauna which now exists, in their general aspect and character, they were all, if I may so speak, equally underlaid by the great leading ideas which still constitute the master types of animal life. And these leading ideas are four in number. First, there is the star-like type of life, life embodied in a form that, as in the corals, the sea-anemones, the sea-urchins, and the star-fishes, radiates outwards from a centre; second, there is the articulated type of life, life embodied in a form composed, as in the worms, crustaceans, and insects, of a series of rings united by their edges, but more or less moveable on each other; third, there is the bilateral or molluscan type of life, life embodied in a form in which there is a duality of corresponding parts, ranged, as in the cuttle-fishes, the clams, and or plane; and life embodied the snails, on the sides of a central axis fourth, there is the vertebrate type of life, in a form in which an internal skeleton is built up into two cavities placed the one over the other; the upper for the reception of the nervous centres, cerebral and spinal, — the lower for the lodgment of the respiratory, circulatory, and digestive organs. Such have been the four central ideas of the faunas of every succeeding creation, except perhaps the earliest of all, that of the Lower Silurian System, in which, so far as is yet known, only three of the number existed, — the radiated, articulated, and molluscan ideas or types. That Omnipotent Creator, infinite in his resources, - who, in at least the details of his workings, seems never yet to have repeated himself, but, as Lyell well expresses it,* breaks, when the parents of a species have been moulded, the dye in which they were cast,-manifests himself, in these four great ideas, as the unchanging and unchangeable One. They serve to bind together the present with all the past; and determine the unity of the authorship of a wonderfully complicated design, executed on a groundwork broad as time, and whose scope and bearing are deep as eternity. The fauna of the Silurian System bears in all its three great types the stamp of a fashion peculiarly antique, and which, save in a few of the mollusca, has long since become. obsolete. Its radiate animals are chiefly corals, simple or compound, whose inhabitants may have somewhat resembled the sea-anemones; with zoophites, akin mayhap to the sea-pens, though the relationship must have been a remote one; and numerous crinoids, or stone lilies, some of which consisted of but a sculptured calyx without petals, while others threw off a series of long, flexible arms, that divided and subdivided like the branches of a tree, and were thickly fringed by hair-like fibres. There is great variety and beauty among these Silurian crinoids; and, Fig. 46. CYATHAXONIA DALMANI. from the ornate sculpture of their groined and ribbed capitals and slender columns, the Gothic architect might borrow not a few striking ideas. The difference between the older and newer fashions, as exemplified in the cup-shapedcorals, may be indicated in a single sentence. The ancient corals were stars of four rays, or of multiples of four; the Fig. 47. GLYPTOCRINUS DECADACTYLUS. (Hudson River Group, modern corals are stars of six rays, or of multiples of six. But though, at a certain definite period, — that during which the great Palæozoic division ended and the Secondary division began-nature, in forming this class of creatures, discarded the number four, and adopted instead the number six, the great leading idea of the star itself was equally retained in corals of the modern as in those of the more ancient type. The articulata of the Silurian period bore a still more peculiar character. They consisted mainly of the Trilobites, -a family in whose nicely-jointed shells the armorer of the middle ages might have found almost all the contrivances of his craft anticipated, with not a few besides which he had failed to discover; and which, after receiving so immense a development during the middle and later times of the Silurian period, that whole rocks were formed almost exclusively of their remains, gradually died out in the times of the Old Red Sandstone, and disappeared for ever from creation after the Carboniferous Limestone had been de Fig. 48. CALYMENE BLUMENBACHII. posited. The Palæontologist knows no more unique family than that of the Trilobites, or a family more unlike any which now exists, or a family which marks with more Fig. 49. ༢ན་ Fig. 50. Fig. 51. ORTHISINA VERNEUILI. LITUITES CORNU-ARIETIS. LINGULA LOWISII. certainty the early rocks in which they occur. And yet, though formed in a fashion that perished myriads of ages ago, how admirably does it not exhibit the articulated type of being, and illustrate that unity of design which, amid endless diversity, pervades all nature. The mollusca of the Silurians ranged from the high cephalopoda, represented in our existing seas by the nautili and the cuttle-fishes, to the low brachipods, some of whose congeners may still be detected in the terebratula of our Highland lochs and bays, and some in the lingula of the southern hemisphere. The cephalopods of the system are all of an obsolete type, that disappeared myriads of ages ago, a remark which, with the exceptions just intimated, and perhaps one or two others, applies equally to its brachipods; but of at least two of its intermediate families, the gasteropoda and lamellibranchiata, several of the forms resemble those of recent shells of the temperate latitudes. In its general aspect, however, the Silurian fauna, antiquely fashioned, as I have said, as became its place in the primeval ages of existence, was unlike any other which the world ever saw; and the absence of the vertebrata, or at least the inconspicuous place which they occupied if they were at all present, must have imparted to the whole, as a group, a humble and mediocre character. It seems to have been for many ages together a creation of molluscs, corals, and crustacea. At length, in an upper bed of the system, immediately under the base of the Old Red Sandstone, the remains of the earliest known fishes appear, blent with what also appears for the first time, -the fragmentary remains of a terrestrial vegetation. The rocks beneath this ancient bone-bed have yielded, as I have already said, no trace of any plant higher than the Thallogens, or at least not higher than the Zosteracea, plants whose proper habitat is the sea; but, through an apparently simultaneous advance of the two kingdoms, animal and vegetable, though of course the simultaneousness may be but merely apparent, the first |