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(the doctrine of irreversibility of evolution), a doctrine first clearly formulated in 1893 by the Belgian paleontologist L. Dollo. According to Dollo, an organism cannot revert, even in part, to its former state through which several of its ancestors have already passed. For example, land vertebrates in the course of secondary adaptation to life in water do not become fish again nor do they acquire the characteristics peculiar to fish (for example, gills). That is why ichthyosaurs, which became adapted to an aqueous environment, remained in all their structural features typical reptiles; similarly, whales remained mammals with their own special structural features.