If inheritance were perfect, the individual would take exactly the same course in development as its ancestors. That it does not do this in all cases is a more remarkable fact than that in so many cases it follows the ancestral mode of development so closely. This loss of inheritance is clue to a progressive condensation of ontogeny, or as it is commonly called, acceleration. Most embryologists misconceive the law of acceleration, limiting it to the omission of characters or stages. With the classic formulation of the law by Hyatt we are all familiar. According to Hyatt, acceleration involves not only omission, but condensation without omission, through the earlier inheritance of characters acquired in the adult or adolescent stages of life. By the unequal acceleration of characters an overlapping, or telescoping, as Grabau calls it, may be introduced. It follows, therefore, that acceleration may be by elimination, by condensation without change in the order of appearance of characters, and by condensation with change in the order of appearance, or telescoping. As conceived by the paleobiologist, the law of acceleration is an explanation of recapitulation, as well as an explanation of the failure to recapitulate.
Another factor in inheritance is retardation, so named by Cope. By the operation of this law, characters that appear late in the ontogeny may disappear in the descendents, because development terminates before the given characters are reached. In this way the ontogeny may be shortened and simplified, and many ancestral characters may be lost entirely. The result of the continued operation of retardation is retrogression, since the loss of the characters of nearer ancestors, with the continued repetition in early ontogeny of the characters of remote ancestors, must eventually cause the species to resemble the remote, rather than the nearer, ancestors.
II
Of the numerous cases adduced by paleontologists, in which there is clear evidence of recapitulation, I shall mention a few only.
Probably the best known examples of recapitulation are those made known by the researches of Hyatt, Branco, Würtenburger, Buckman, Smith and others among the Cephalopoda. It is shown that Ammonites pass through a goniatite stage, and that, as phrased by Zitttel, "The inner whorls of an ammonite constantly resemble in form, ornament and suture line the adult condition of some previously existing genus or other." The nautilus grows at first straight or orthoceraform, then arched or cyrtoceraform, and finally at the close of the first volution of the shell, becomes close coiled. The impressed zone appears in ancient nautiloidea in the neanic stage, where the whorls first come into contact, and is indeed a result of contact. In modern nautilus, and in Mesozoic and Tertiary nautilus the impressed zone appears in
