PHYLOG'ENY (from Gk. Of/Xop, phylon, tribe -76Eta, -yeneia, production, from -yl-yeecrOat, yiynest hat, to become). While ontogeny (q.v.) is a term expressing the development (embry ology) of the individual, phylogeny means the development or genetic history of the members of the group or class as a whole. The conception of such a process did not arise until a multitude of facts had been accumulated 'hiring the study of the embryology of different animal types. the re sults being further checked and complemented by the facts of paleontology. The doctrine or theory of phylogeny, then, rests (1) upon a series of life-histories of animals, including their em bryonic and post-embryonic stages of develop ment; (2) upon what we know of the geological succession of animal forms: and (3) upon the theory of descent. Phylogeny is the equivalent of genealogy, and involves the idea of blood-rela tionship. A genealogical tree of a single human family, when earried out through many genera tions, is almost indefinitely branched and sub divided, the members or descendants from the earliest human pair affording an instance of divergent evolution. The ancestral tree or pedi
gree of any class of animals is a matter of theory, hut the facts of affinity and homology are so numerous and natural that the theory is fairly well grounded. The phylogeny of any group is an expression of one's view of classification, and since in respect to many groups there is a diver gence of opinion as to the systematic position of this or that member, so the views as to the pedi gree of certain classes or general groups differ with different systematists. The success of the endeavor to work out a phylogeny of any group depends very largely on knowledge of the earlier fossil forms; yet each attempt blots to the more careful study of this or that characteristic, to tracing its growth, development, and final disap pearance, or replacement by a later acquired fea ture, while the mental discipline resulting tends to render the work of the systematist more phil osophical. See PALEONTOLOGY; EVOLUTION.