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Definition: A unidirectional trend in the evolutionary history of a lineage as revealed by the fossil record; also, a now discredited, anti-Darwinian evolutionary theory, popular around 1900, postulating that genetic momentum forced lineages to evolve in a predestined linear direction that was independent of external factors and often led to decline and extinction.
Source: Hickman, C.P., et al. 2011. Integrated Principles of Zoology. Fifteenth Edition. McGraw-Hill Higher Education.