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Show 272 IMPERFECTION OF GEOLOGICAL RECORD. [CHAP. IX, rupted successi.o n °f ch ap t ers, mafy l ':Dr epreensteonmt btehde ianp poaur-r entl abruptly changed forms o I e, . 0 h' consye cuti.v e b u t WI' de I Y separated' formations1. d' n· t· hlS V.I ew, the difficulties above discussed are great y Iminis - ed, or even disappear. CHAP. X.] GEOLOGICAL SUCCESSION. 273 CHAPTER X. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. On the slow and successive appearance of new species-On their different rates of change-Species once lost do not reappear-Groups of species follow the same general rules in their appearance and disappearance as do single species-On Extinction- On simultaneous changes in the forms of life throughout the world-On the affinities of extinct species to each other and to living species-On the state of development of ancient forms-On the succession of the same types within the same areas-Summary of preceding and present chapters. LET us now see whether the several facts and rules relating to the geological succession of organic beings, better accord with the common view of the immutability of species, or with that of their slow and gradual modification, through descent and natural selection. New species have appeared very slowly, one after another, both on the land and in the waters. Lyell has shown that it is hardly possible to resist the evidence on this head in the case of the several tertiary stages ; and every year tends to fill up the blanks between them, and to make the percentage system of lost and 11ew forms more gradual. In some of the most recent beds, though undoubtedly of high antiquity if1neasured by years, only one or two species are lost forms, and only one or two are new forms, having here appeared for the first time, either lo~ ally, or, as far as we know, on the face of the earth. If we may trust the observations of Philippi in Sicily, the successive changes in the marine inhabitants of that island hav~ been many and most gradual. The secondary form~ twns are more broken ; but, as Bronn has remarked, ne1ther the appearance nor disappearance of their many now extinct species has been simultaneous in each separate formation. 12* |