being quite enough to produce, at the present day, a marked difference in the appearance of the group of inhabitants of adjacent districts, which difference is perfectly appreciable at the first glance by one accustomed to observe with any degree of accuracy.

The general nature of the animal remains is not the only means possessed by the Naturalist, or made use of by the Geologist, to determine the circumstances under which submarine deposits may have taken place. There is now a chain of observations extending over the whole series of known strata; and, regard being had to the present advanced state of knowledge of the existing species of animals, it has been distinctly proved, that the more carefully and strictly these observations are compared and brought to bear upon one another, the more manifest is it, that, ever since the first introduction of animals upon earth, there have been successive creations of species similar to one another, but not identical,-—performing often the same office, but perfectly distinguishable,—and involving a constant introduction of new species, although never, not even in a single instance, involving the repetition of a species that has once died out.

The statement of this fact, namely, that species, like individuals, have a certain limited term of existence, has already been made indirectly in the preceding chapter; and I have there described a number of animals very much unlike any that now exist, although not without such resemblances as indicate the possession of analogous habits, and exhibit proof of unity of design running through creation, and connecting even the most ancient species with those yet surviving; but I did not there speak concerning that peculiar suc

cession of animals which the pursuit of Geology has proved to exist. It should, however, be clearly understood, that, in making use of this term "succession," I have no intention of assuming a gradual modification of species in the way of the development of a higher organization, as if animals originally created imperfect were subsequently, and by manifest gradation, at length enabled to perform functions of a higher kind; for this is by no means the case, so far as the observations of Geologists have hitherto been able to determine. The order of nature seems rather to be a succession of this kind; namely, that first of all, as we have seen in the last chapter, representatives were introduced of each of the principal natural subdivisions of the invertebrated animals, combining many typical characteristics subsequently kept separate, and that the species thus originally introduced were gradually displaced by others in which distinctness of typical character was more marked. Some animals, as the coral polyps, remained stationary in point of development; others, as the encrinites, lasted for a long time, but at length were partly superseded by higher types of the group, performing offices which required greater powers of locomotion; others, again, as the brachiopods, exhibited almost immediately the greatest abundance, variety, and extent of their development, and were only superseded, after a long interval, by the higher conchifers, which at first were sparingly introduced; while, again, others, (and those the most important in every respect,) such as the Cephalopoda, at once assumed an importance which hardly increased, although it varied, for a long period, and at length actually became less; these animals

being ultimately succeeded by a group (the gasteropods) of much lower organization, although admirably fitted for the work they had to perform. We shall see hereafter that the course of development of the fishes and reptiles was very similar to this; so that there is no evidence of these animals having gradually passed into one another, or of any such order of succession having been a part of the plan adopted by the great Director of the universe.*

Since it is the fact, that, according to some general law, species of animals are introduced, last only for a limited period, and are then succeeded by others performing the same office, it will readily be seen that in any group of strata the absence of a certain number

* I have dwelt the more earnestly on this subject, because there appears to be a strong tendency in the minds of many persons to conclude, that since the Invertebrata appear to have been first introduced, and to have been in course of time succeeded by the vertebrated animals in something of the order of their organization, there was a succession and a gradual development of higher types of existence in a certain order of creation. So far as Geology in its present state affords evidence on this subject, the facts seem decidedly opposed to any such view; and I make this statement the more unhesitatingly, because I find that it perfectly accords with the conclusions arrived at by one of the most philosophical of living naturalists, who brings to a close his investigation concerning the extinct Reptiles in the following manner :

66 Thus, though a general progress may be discerned, the interruptions and faults, to use a geological phrase, negative the notion that the progression has been the result of self-developing energies adequate to a transmutation of specific characters; but, on the contrary, support the conclusion, that the modifications of osteological structure which characterise the extinct reptiles, were originally impressed upon them at their creation, and have been neither derived from improvement of a lower, nor lost by progressive development into a higher type."-Professor Owen's Report on British Fossil Reptiles; Report of Eleventh Meeting of the British Association at Plymouth, 1842, p. 202.

of species common in beds of an older formation, and the presence in that group of other species which are analogous, would lead the naturalist to conclude either that a great change had taken place suddenly in the depth or relative position of the sea bottom receiving deposits, or else that a period had elapsed between the deposit of the lower or older beds and those which overlie them, and that this period was longer or shorter according to the amount of difference in the species examined. And this brings us to the subject referred to at the commencement of the present chapter, viz. the existence of a break in the continuity of strata observed in the case of the rocks of the second period in Scotland and Devonshire.

With regard to these localities, however, the evidence requires a yet more detailed statement. In North Britain, the beds resting on and wrapping round the gneiss, the mica schist, and other old rocks, consist, for the most part, of coarse conglomerate or pudding-stone, evidently made up of the broken fragments of the old granitic rocks, rolled and tossed about for ages in a troubled sea, the hardest stones being rounded into bullet-shaped pebbles by their long and incessant attrition against one another. These coarse, gravelly masses are not, however, universal; and on the north-eastern coast and in the Orkneys they are often replaced by more regular strata of hard, darkcoloured, bituminous schists, abounding with the fossil remains of fishes.

On the frontier of Wales, a deposit, in many respects very similar to the conglomerate of Scotland, and expanded to an equal and enormous thickness, is found to cover up, by regular gradation, the newest

strata of the silurian period; and from this deposit are obtained occasionally the fossil remains of fishes of the same species as those found in Scotland.

Lastly, in Devonshire and Cornwall, between the granite of Dartmoor and a series of black strata of the same geological age as the carboniferous beds which elsewhere overlie the conglomerate of Herefordshire and Scotland, there is a large series of sandy and slaty rocks, containing numerous fossil shells and other organic remains; and these appear, on examination, to possess a character intermediate between that of the silurian and that of the newer or carboniferous series.

Now it will be readily admitted, that a sea in which the coarse, gravelly conglomerates of Herefordshire and Scotland were being deposited, would be hardly likely to contain the remains of delicate shells and the skeletons of polyps and encrinites, because, even if the animals could have lived in such a sea, their hard parts would be ground and pounded into ten thousand atoms as soon as they were exposed to the rough beating of the shingles; while, on the other hand, the clayey and sandy bottom of the more southern sea might readily preserve such remains as were left by animals of this kind. It would not, therefore, be singular that we should find a number of fossils in the devonian beds very different from those in Scotland, even if they were being formed at the same time; and the evidence of contemporaneity offered, by comparing the fossils with those of beds whose position in the series was known, would be sufficient to establish the position of the group in question. In this way the devonian strata were discovered to be of the same