the earth within the records of history. No doubt, many animals have thus been removed from their localities by the various changes which have taken place on the surface of the globe, and may have been either partially or totally extirpated in the revolutions which have changed the ocean into land, and levelled continents into the ocean.

Of the whole number of fossil animals yet discovered, amounting to about six thousand six hundred, the greater proportion are marine-consisting of coral zoophytes, molluscous animals, and fishes.! From the sedimentary strata hitherto examined being almost exclusively marine and fluviatile, this was a circumstance which might readily have been anticipated. In the primary or lowermost series of sedimentary rocks, no organic traces can be found. It is only in the greywacke and older sandstones that they begin to make their appearance; and they do so at a distinct and definite point, evidently shewing a period of commencement of organized existence on the earth.*

* "In the grey wacke of the Cambrian mountains, there occur organic remains in the upper and middle portions, but none in the lower, though similar in all other respects. The same is the case in North and South Wales. I may state generally, as the result of my own observations on the older strata of this island, that there is a line in the descending series where organic remains seem entirely to disappear, and that this is by no means co-ordinate with mineral changes, nor produced by igneous action.”PROFESSOR SEDGEWICK, in Geological Transactions.

In the first fossiliferous strata, the animal remains are of rare occurrence, and confined to a few species, consisting of crustaceous animals and fish, supposed to be the inhabitants of deep seas. In the succeeding strata, or mountain limestone, these animals disappear and are succeeded by encrinital polypi, coral zoophytes, and shell mollusca. The coal measures deposited in the hollows of the elevated limestone contain few animals, and those chiefly fluviatile, but abound in plants and large trees. Then we have the new red sandstone partially lying over the coal measures, containing very few traces of organized beings, but come again to the lias and oolitic beds, which are full of shells and animals, that have had their localities in shallow seas near the shores of continents. Over these is superimposed the chalk, a marine deposit, also containing marine shells and crustacea, and a series of tertiary strata, both fresh water and marine, terminates the whole.

Now, although each of these formations, generally speaking, contains a certain amount of distinctive species, yet there are some tribes of animals which range throughout the whole. Thus, various species of coral zoophytes are found in all the strata; terebratulæ, also, are common through the whole; ammonites extend throughout all the strata, except the tertiary; spirifers and productæ extend through all the series to the oolite; while belemnites only

appear in the lias, oolite, and chalk; and the echinæ in the chalk alone. In short, these fossil animals appear to have strictly conformed in their habits to recent species. They had certain localities which they frequented as being suited to their organization; some inhabited deep seas; some littoral situations, and others the shallow estuaries of rivers. And, when certain changes of the sea, affecting its depth, temperature, and other circumstances, occurred, the races either changed their localities, or became extirpated. Thus, when the deep and ancient bed of the greywacke system was elevated and formed a shallower sea, the inhabitants of the locality found it no longer a fit abode for them; but this revolution prepared a suitable bed for the encrinites and mollusks of the succeeding limestone deposit. This bed, too, after a certain period, suffered an elevation, and became a shallow estuary for the reception of drifted vegetables. In an adjoining bed of the ocean, certain currents were accumulating the debris of a neighbouring region, charged with numerous shell animals, and rivers flowed into these beds, where saurian reptiles formed appropriate habitats. But the question will be asked, were the species of the respective beds co-existent ? The proposition has more of analogy to support it than the alternative of separate and distinct creations. If we find a low and marshy plain, where only reeds and mosses have hitherto grown, by some means

gradually filled up with dry rich soil, and this soil, in the course of a few years, covered with a variety of luxuriant vegetation, we do not hastily conclude that a new creation of vegetables suited to the particular soil has been effected, but reflect that seeds from other similar situations may have been transferred, and finding a suitable soil have here taken root and flourished.

Those portions of strata hitherto examined may have thus been successively stocked, from some common centre, with animals suited to the nature of the localities. Those animals may have been, as it were, the pioneers of Nature, and after having performed their allotted duties, may have suffered extinction in the successive changes which the respective strata that they occupied have evidently undergone; while other races, forming the great majority, in more favoured localities, may have remained to extend over the seas and oceans of the existing period.

It does not by any means follow, that because we find successions of different animals occupying beds superimposed upon each other, that the extinction and new creation of species have been the cause. In the successive beds of some strata we find the same species repeatedly alternating with other distinct species, and in some instances recurring in such numbers as to form the almost exclusive relics of the higher beds. Now, we cannot surely

here have recourse to repeated creations of the same species.*

That species belonging to the four great divisions of the animal kingdom existed at various periods of the secondary formations, is now put beyond all doubt. Dr Hitchcock has discovered what he conjectures the remains of mammiferous animals, in the greywacke of America.† The footmarks of reptiles and birds have been evidently traced in slabs of new red sandstone; the bones of a didelphis in the oolite or Stonesfield slate, near Oxford; and bones of birds in the Wealden beds of Sussex, below the chalk.

Considering that all these formations are of aqueous deposition, the few remains of land animals which they exhibit are sufficient proof that terrestrial beings existed as contemporaries of the marine on some part of the surrounding soil.

That the marine animals of the earlier formations differ so much from those of the oolite and chalk, which are both strictly of marine origin, may arise not so much from a difference of age in those strata, as

* Mr Mammalt, in his elaborate work on the coal field of Ashby de la Zouche, full of the idea that "strata are characterized by their fossils," maintains, that every shale bed of this coal field has vegetable impressions peculiar to itself. An examination of his plates, however, which are truer to nature than his theory, shews the same species of fossils repeated many times in different beds.-GREENOUGH'S Address to Geological Society, 1835.

+ Silliman's American Journal, 1837.