A phylogeny (or phylogenesis) is the origin and evolution of a set of organisms, usually species. A major task of systematics is to determine the ancestral relationships among known species (both living and extinct), and the most commonly used methods to infer phylogenies include cladistics and phenetics.
During the late 19th century, the theory of recapitulation, or Haeckel's biogenetic law, was widely accepted. This theory was often expressed as "ontogeny recapitulates phylogeny", i.e. that the development of an organism exactly mirrors the evolutionary development of the species. The early version of this hypothesis has since been rejected as being oversimplified and misleading. However, modern biology recognizes numerous connections between ontogeny and phylogeny, explains them using evolutionary theory, and views them as supporting evidence for that theory. See the article on ontogeny and phylogeny.