During the period in which we have proof that Cetacea have existed, the evidence in the shape of bones and teeth, which latter enduring characteristics in most of the species are peculiar for their great number in the same individual, must have been abundantly deposited at the bottom of the sea; and as cachalots, grampuses, dolphins, and porpoises are seen gambolling in shoals in deep oceans, far from land, their remains will form the most characteristic evidences of verte brate life in the strata now in course of formation at the bottom of such oceans. Accordingly, it consists with the known characteristics of the cetacean class to find the marine deposits which fell from seas tenanted, as now, with vertebrates of that high grade, containing the fossil evidences of the order in vast abundance.
The red crag of Suffolk and Essex contains petrified frag ments of the skeletons and teeth of various Cetacea, in such quantities as to constitute a great part of that source of phos phate of lime for which the red crag is worked for the manu facture of artificial manure. The scanty and dubious evidence of Cetacea in secondary beds seems to indicate a similar period for their beginning as for the soft-scaled cycloid and ctenoid fishes which have superseded the ganoid orders of mesozoic times.
We cannot doubt but that had the genera Ichthyosaurus, Pli,osaurus, or Plesiosaurus, been represented by species in the same ocean that was tempested by the and Dio plodons of the miocene age, the bones and teeth of those marine reptiles would have testified to their existence as abun dantly as they do at a previous epoch in the earth's history. But no fossil relic of an enaliosaur has been found in tertiary strata, and no living enaliosaur has been detected in the present seas: and they are consequently held by competent naturalists to be extinct.
In like manner does such negative evidence testify to the non-existence of marine mammals in the liassic and oolitic times. In the marine deposits of those secondary or mesozoic epochs, the evidence of vertebrates governing the ocean, and preying on inferior marine vertebrates, is as abundant as that of air-breathing vertebrates in the tertiary strata ; but in the one the fossils are exclusively of the cold-blooded reptilian class, in the other, of the warm-blooded mammalian class. The Enaliosauria, Cetiosauria, and Crocodilia, played the same part and fulfilled similar offices in the seas from which the lies and oolites were precipitated, as the Delphinidec and Balawidte did in the tertiary seas, and still do in the present ocean.
The unbiassed conclusion from both negative and positive evidence in this matter is, that the Cetacea succeeded and superseded the Enaliosauria. To the mind that will not accept such conclusion, the stratified oolitic rocks must cease to be trustworthy records of the condition of life on the earth at that period.
So far, however, as any general conclusion can be deduced from the large sum of evidence above referred to, and con trasted, it is against the doctrine of the Uniformitarian. Organic remains, traced from their earliest known graves, are succeeded, one series by another, to the present period, and never re-appear when once lost sight of in the ascending search. As well might we expect a living Ichthyosaur in the Pacific, as a fossil whale in the Lias: the rule governs as strongly in the retrospect as the prospect. And not only as respects the Vertebrata, but the sum of the animal species at each successive geological period has been distinct and pecu liar to such period.
Not that the extinction of such forms or species was sudden or simultaneous: the evidences so interpreted have been but locaL Over the wider field of life at any given epoch, the change has been gradual; and, as it would seem, obedient to some general, continuously operative, but as yet, ill-compre hended, law. In regard to animal life, and its assigned work on this planet, there has, however, plainly been "an ascent and progress in the main." Although the maminalia, in regard to the plenary develop ment of the characteristic orders, belong to the Tertiary division of geological time, just as "Echini are most common in the superior strata; Ammonites in those beneath, and Producti with numerous Encrini in the lowest". of the secondary strata, yet the beginnings of the class manifest themselves in the formations of the earlier preceding division of geological time.
We are not entitled to infer from the Lueina of the per mian, and the Opis of the trial, that the Lamellibranchiate Mollusks existed in the same rich variety of development at those periods as during the tertiary and present times; and no prepossession can close the eyes to the fact that the Lamelli branchiate have superseded the Palliobranchiate bivalves.