that have been recently advanced to account for the great chronological elimination of vitality. It is (if anything man shall ever comprehend) the gradual unfolding of a predestined scheme-a divine conception, to the realisation of which the various forces of nature, co-related and coadapted, are in ever-active co-operation.

THE MIDDLE PAST.

MESOZOIC SYSTEMS-THE TRIAS, OOLITE, AND CHALK.

We now take leave of the palæozoic aspects of the world, and pass on to those of the Mesozoic or "middle life" period-characterised by forms and species which hold an intermediate place between those of the more ancient and those of the more modern epochs. The grand primeval types and patterns are still the same-radiate, molluscan, articulate, and vertebrate-but the modifications of the types are new, and the consequent organisation higher and more complex. The "differentiation" of the vital functions (as zoologists express it) now becomes more marked and apparent that is, instead of organisations in which several functions are performed by the same organ, each function has an organ specially devoted to its purpose. The expression of Creative thought has become more specialised, and the plants and animals of the newer epochs bear the impress of that specialisation, and find in new external conditions a fitting habitat for their growth and elimination.

We now take farewell of the graptolites, cystideans, trilobites, and eurypterites of Silurian seas-of the gigantic crustaceans and bone-cased fishes of the old red sandstone -of the sigillariæ, stigmariæ, lepidodendra, and other endogenous forms of the coal period-of the cup-in-cup, honey

comb, chain-pore, spider-web, and other corals of the Devonian and mountain limestones—of the huge reptile-like fishes that swarmed in carboniferous waters; and are introduced to other species and newer forms of vitality. The vegetation that adorns the lands of the mesozoic period bears a closer resemblance and affinity to the tree-ferns, cycads, zamias, palms, and subtropical pines of the present day; and the botanist feels he can now institute comparisons with some prospect of success, and attempt restorations with greater confidence and certainty. So also in the animal world the approximations are becoming closer and closer; the divergence from existing families is less perceptible even to the unscientific observer; and the zoologist now meets with all the great divisions of vertebrate lifefishes, reptiles, birds, and mammals. A vast progress has been made in the great onward evolution of vitality— whole families of lower life have died out, and higher ones have taken their places-and orders only beginning to come into existence in the primeval world are now approaching their culmination, or point of greatest numbers, variety, and development.

Besides these gradational advances from lower to higher forms, which are common to every geological epoch, there are also some curious external characteristics which must arrest the notice even of the least scientific and the least geological of observers. So noticeable are these features, that if the fossils of the paleozoic cycle were arranged on one side of a museum, and those of the neozoic on another, the difference would strike the casual observer as strongly as would the difference between the brute-man sculptures of the Ninevites and Egyptians on the one hand, and the man-god sculptures of the Greeks and Romans on the other. It is like passing from the Assyrian and Egyptian chambers of the British Museum to those devoted to the Greeks and

Romans. The expression of human thought is not more clearly indicated by the remains of these ancient civilisations, than the expression of creative thought is indicated by the fossil forms of the paleozoic and mesozoic Earthperiods. Thus, in the palæozoic endogens the ultimate development of the leaf is, for the most part, stamped in permanent beauty on the tall sculptured stems, whereas in the neozoic exogens it ascends to the more exquisite but evanescent beauties of the flower and fruit. Again, the palæozoic leaf, being endogenous, has a venation wholly parallel, whereas the neozoic leaf adds the reticulated venation of the exogen to that of the endogen. Further, as the floral arrangement of the endogen is formed by three, and that of the exogen by five, all the paleozoic flowers and fruits are stamped by the normal number three, whereas fives and threes are equally normal in the neozoic flora. So also in the animal kingdom: the corals of the paleozoic cycle had their septa or ray-like partitions arranged in fours, while those of the neozoic are arranged in sixes. In the palæozoic cephalopods the arms are for the most part void of sucking discs, while those of the neozoic seas are, on the other hand, generally furnished with them; and in the chambered shells of the same order, the palæozoic species have their sutural junctions plain and simple, while those of the neozoic are often foliated and of most intricate pattern. The paleozoic crustacea, even in the highest forms yet discovered, are more larval-like or abdominal in their segmentation than the neozoic, in which head, thorax, and abdomen become distinct and definite. Again, the palæozoic fishes had all the heterocercal or unequally-lobed tail (which marks the embryonic condition of fish-life in general), while in the neozoic order, the heterocerque is subordinated, and the homocerque, or equally-lobed, and the undivided tails become the general and normal forms.

These and other distinctions, upon which our limits will not permit us to dwell, stamp the paleozoic as a life-period widely different from that of the mesozoic, and yet there was no break, no discontinuity in the great evolution of vitality. As the life of one system runs imperceptibly into that of another, and the two have always some forms in common; so the paleozoic runs into the mesozoic, and it is only when viewed as a whole, and at a sufficient distance, that its distinctive characters stand out in bold and peculiar relief. So in like manner we shall find it with the mesozoic life-period, when we have reviewed the forms of its triassic, oolitic, and cretaceous systems. It has a facies peculiar to itself, and though approaching in some of its features, yet as a whole unmistakably different from the facies of the cainozoic period, which is now running its course, and bearing us along with it.

And first, we turn to the Trias or upper new red sandstone, with its "triple" series of various coloured sandstones, shelly limestones, and saliferous and gypseous shales. These party-coloured deposits, in which ferruginous tints predominate, are clearly the sediments of circumscribed oceanic areas-areas which, in the northern hemisphere at least, were of no great depth, and subjected to repeated elevatory and depressing movements. This new arrangement of sea and land, accompanied by no gigantic rivers or estuaries, and apparently by a somewhat arid climate, is characterised by a numerical as well as specific paucity of life-a paucity which is greatly aggravated by the unsuitable nature of the sandstones and marls for the preservation of organic remains. Physiologically, however, the forms are still on the advance; cycads and conifers are more decided in their characters; brachiopods diminish, and true bivalves increase; cold-blooded air-breathers become more numerous,