The Edinburgh New Philosophical Journal

The Theory of Successive Development in the Scale of being both Animal and Vegetable, from the earliest periods to our own time, as deduced from Paleontological evidence.

THE theory of successive geological development, so generally adopted by geologists, is clearly stated by Professor Sedgwick in his admirable Discourse on the Studies of Cambridge. "There are traces," he says, "among the old deposits of the earth of an organic progression among the successive forms of life. They are to be seen in the absence of mammalia in the older, and their very rare appearance in the newer secondary groups; in the diffusion of warm-blooded quadrupeds (frequently of unknown genera) in the older tertiary system, and in their great abundance (and frequently of known genera) in the upper portions of the same series; and lastly, in the recent appearance of man on the surface of the earth." (p. xliv.) "This historical development," continues the same author, "of the forms and functions of organic life during successive epochs seems to mark a gradual evolution of creative power, manifested by a gradual ascent towards a higher type of being."—Ibid. p. cliv. "But the elevation of the fauna of successive periods was not made by transmutation, but by creative additions; and it is by watching these additions that we get some insight into Nature's true historical progress, and learn that there was a time when Cephalopoda were the highest types of animal life, the primates of this world; that Fishes next took the lead, then Reptiles; and that during the secondary period they were anatomically raised far above any forms of the VOL. LI. NO. CI.-JULY 1851.

reptile class now living in the world. Mammals were added next, until Nature became what she now is, by the addition of Man."-Ibid. p. ccxvi.

Sir Charles Lyell, in his important Anniversary Address to the Geological Society, February 1851, rejects this theory: before, however, going into detail whether of fact or argument on the question, he makes the following preliminary statement of the principal points which he expects to establish in opposition to the development theory.

First, in regard to fossil plants, it is natural that those less developed tribes which inhabit salt water, should be the oldest yet known in a fossil state, because the lowest strata which we have hitherto found happen to be marine, although the contemporaneous Silurian land may very probably have been inhabited by plants more highly organized.

Secondly, the most ancient terrestrial flora with which we can be said to have any real acquaintance (the carboniferous) contains Coniferæ, which are by no means of the lowest grade in the phænogamous class, and, according to many botanists of high authority, Palms, which are as highly organized as any members of the vegetable creation.

Thirdly, in the secondary formations, from the triassic to the Purbeck inclusive, gymnosperms allied to Zamia and Cycas predominate; but with these are associated some monocotyledons or endogens, of species inferior to no phænogamous plants in the perfection or complexity of their organs,

Fourthly, in the strata from the cretaceous to the uppermost tertiary inclusive, all the principal classes of living plants occur, including the dicotyledonous angiosperms of Brongniart. During this vast lapse of time four or five complete changes of species took place, yet no step whatever was made in advance at any one of these periods by the addition of more highly organized plants.

Fifthly, in regard to the animal kingdom, the lowest Silurian strata contain highly developed representatives of the three great divisions of radiata, articulata, and mollusca, shewing that the marine invertebrate animals were as perfect then as in the existing seas. They also comprise some indications of fish, the scarcity of which in a fossil state, as well

as the absence of cetacea, does not appear inexplicable in the present imperfect state of our investigations, when we consider the corresponding rarity and sometimes the absence of the like remains observed in dredging the beds of existing

seas.

Sixthly, the upper Silurian group contains amongst its fossil fish cestraciont sharks, than which no ichthyic type is more elevated.

Seventhly, in the carboniferous fauna there have been recently discovered several skeletons of reptiles of by no means a low or simple organization, and in the Permian there are saurians of as high a grade as any now existing, while the absence of terrestrial mammalia in the paleozoic rocks generally may admit of the same explanation as our ig norance of most of the insects and all the pulmoniferous mollusca, as well as of Helices and other land shells of the

same era.

Eighthly, the fish and reptiles of the secondary rocks are as fully developed in their organization as those now living. The birds are represented by numerous foot-prints and coprolites in the Trias of New England, and by a few bones not yét generically determined, from Stonesfield and the English Wealden.

Ninthly, the land quadrupeds of the secondary period are limited to two genera, occurring in the inferior oolite of Stonesfield; the cetacea by one specimen from the Kimmeridge clay, the true position of which requires further inquiry, while an indication of another is afforded by a cetacean parasite in the chalk. But we have yet to learn whether in the secondary periods there was really a scarcity of mammalia (such as may have arisen from an extraordinary predominance of reptiles, aquatic and terrestrial, discharging the same functions), or whether it be simply apparent and referable to the small progress made as yet in collecting the remains of the inhabitants of the land and rivers, since we have hitherto discovered but few freshwater, and no land mollusca in rocks of the same age.

Tenthly, in regard to the paleontology of the tertiary periods, there seems every reason to believe that the orders

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The Edinburgh New Philosophical Journal, Том 51