Ontogeny refers to the development of an organism. With humans, ontogeny begins with the fertilized egg and continues through embryonic and fetal development, birth, maturation, and ultimately senescence. Clearly, many developmental phenomena in humans are found in other animals and seem to be related to events in the evolutionary history of the group. Phylogeny relates to the development of a group. Phylogeny is a history of a group of organisms from the beginning of life to the present time.

The terms ontogeny and phylogeny are grouped together frequently because of the fundamental biogenetic law espoused by the German scientist Ernst Heinrich Philip August Haeckel (1834-1919). Haeckel's law, also known as the theory of recapitulation, states that ontogeny is the short and rapid recapitulation of phylogeny. Haeckel, as had others before him, noted the similarity of embryonic forms within a group. Certainly a mouse, elephant...