Phylogeny is the science that deals with the history of the development of an organism from its original ancestors. Thus to trace birds back to a reptilian stock, this through a yet earlier amphibian type to an animal without a vertebral column but only a dorsal chord, and thus back to the invertebrata, is to trace the phylogeny of birds. The two principal methods by which this study is carried on are by paleontology or the study of the actual shells or skeletons of extinct animals, and by embryology. The latter is of value, as it is thought that the actual history of the development of an individual (or ontogeny) is a summary of the life-history of its race; or, as it is technically expressed, ontogeny is a summary of phylogeny. This is no doubt correct in the main, but the ontogeny has been so much altered by the development of structures used only for the attachment or nourishment of the embryo, that it must not be too literally accepted. Thus in the larva of the Sea-urchin a series of calcareous rods has been developed forming an easel-like skeleton, whence it is known as a Pluteus. These have been merely secondarily acquired for the support of the larval appendages, and it would be quite erroneous to conclude that a former ancestor of the Sea-urchin resembled the Pluteus stage.