D'Orbigny's stage basically appears as an epoch similar to the present one: a past state of nature showing lands and seas crowded with their respective animals and plants. In the course of his development of this concept, d'Orbigny also uses the term stage for (1) a natural chronological division of the earth history, (2) an accumulation of rock strata, and (3) a biostratigraphical unit. If, to some extent, stages are defined and delimited after physical criteria (discordances, etc.), the main argument relies on the vertical distribution of fossils. These paleontological criteria include (1) the first appearance of certain organic forms, (2) the dominance or abundance of other ones, and (3) the paleontological zones to which stages correspond. The biological characteristics of each epoch or stage result essentially from the creation of new sets of organisms proper to the stage, whereas the end of each stage is marked by catastrophic extinctions and geological revolutions. D'Orbigny's zones are chronologic indices characterized by given species selected out of all those occurring in the stage on the basis of their wide geographical and environmental distribution and of their confinement to a given stage or part of a stage. D'Orbigny's stratigraphic zone is comparable to a geographic zone; it delimits and locates 'stratigraphic latitudes' of distinctive character. [Quotations from d'Orbigny's original French texts are appended.]

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