Fossil remains begin to abound in the lowest limestone which follows this sandstone, as is well known. Hitherto, it must be remarked, that almost every substance of this nature is of marine origin. Thus, it has been supposed to be proved, that few or no terrestrial plants, or animals, existed in these earlier states of the globe ; although it is evident, that proofs of such a negative must always be im perfect and unsatisfactory, from various geological consi derations, on which it is unnecessary here to enter.
But, beyond this point, vegetables begin decidedly to appear, together with shells ; which, from various appear ances and from concomitant circumstances, have probably been the Inhabitants of those fresh waters in which these deposits, constituting the coal strata, must have been formed. In this series also, throughout, there is an irre gular, and sometimes an apparently capricious, distribu tion of these organized bodies ; obviously for the same reasons that they ale similarly uncertain in other situa tions, namely, the quantity of sandy materials which seem, in all cases, to have been unfitted for their habitations. The mixture of different kinds of organic remains in these deposits, is a question which is examined elsewhere.
That extensive sandstone, the red marl of England, which follows the coal series when that is present, or some of the inferior rocks when it is absent, contains many or ganic bodies, as does that important limestone, the lias, by which it is succeeded. In this latter, plants are also found, and amphibious animals first appear. From gene ral considerations, we have every reason to expect that these latter should also be found in the coal series, but we are not aware that any such example has occurred. This also is a case in which an absolute negative can never be proved; and in which we must still believe it probable that such remains may thus exist, though an instance should never be discovered.
It is not necessary to prolong these remarks on the secondary strata upwards beyond this poin , as the organic remains begin here to be abundant and frequent, and as it will be necessary to notice these strata again for other purposes. It has been shown, that, in a general sense, there is an increase in the numbers, or quantity, of or ganized bodies as we proceed upwards through the series; and it will hereafter appear, that in a sense equally gene ral, the number of species also increases. Some reasons have already been assigned, why, in every part both of the ancient and recent strata, the succession is imperfect, interrupted, and capricious. But others ought here to be
enumerated.
As the living tribes are known to dwell in colonies, and not to be universally diffused in every part of the bottom of the sea, it must be expected, not only that their dead prototypes should be absent from some strata, but even from parts of one stratum which in other places may con tain them. Where their proportions to the earthy parts of a stratum vary, different causes may be assigned; from considering, in the same way, the mode in which the ma terials of the land are deposited tinder the ocean, and the differences which attend the propagation and destruction of living species. 'Many colonies may have been exter minated by causes unknown to us, or they may have been mixed with undue proportions of earthy matters, or entire ly overwhelmed by them. The propagation of some may have been tedious, and thus the substances in which they have lived may have preponderated. The decomposition of others may have been considerable, or even complete, so as to have left nothing but inorganic limestone as scanty testimonials of their former presence ; and, as al ready remarked, moved by their natural instincts, they may have abandoned, or refused to propagate in, sub stances unfitted for their habitations, such as quartz sand. Hence, as already observed, the fossils abound chiefly, as the living tribes did, in limestones and shales; and when they are wanting, even in beds of this nature, we have no more reason to be surprised, than that the bottom of the present ocean, wherever we examine it, is not always covered with colonies of shell-ash.
General Sketch of the Nature of Organic Remains.
In giving a general sketch of the nature of organic re mains, they may be considered in two ways; according to their geological relations, or to their situations when liv ing, or else according to their living relations, as they be long to animals and vegetables, and to distinct divisions among these. We shall in this sketch make use of the former arrangment, but shall adopt the latter when we come to give an account of the subjects themselves. They will now therefore be distinguished into marine and terres t•ial ; and these latter may again be subdivided into tut-me and aquatic. The marine remains, we need scarcely re mark, are almost exclusively of an animal nature; almost all, if not really the whole of the plants that have yet been discovered, belonging to the terrestrial deposits.