Phylogeny and ontogeny are, therefore, the two coordinated branches of morphology. Phylogeny is the developmental history [Entwickelungsgeschichte] of the abstract, genealogical individual; ontogeny, on the other hand, is the developmental history of the concrete, morphological individual.

Allgemeine Entwickelungsgeschichte der Organismen (1866), Vol. 1, 60. Trans. Stephen Jay Gould, Ontogeny and Phylogeny (1977)