Haeckel's Biogenetic Law, often summarized as 'ontogeny recapitulates phylogeny,' posits that the development of an individual organism (ontogeny) follows the same progression as the evolutionary history of its species (phylogeny). Although influential in the late 19th and early 20th centuries, this theory has been largely discredited by modern biology due to its oversimplification and inaccuracies in developmental processes.