60 THE PALEONTOLOGIC RECORD 



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 due 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 con- 

 densation without omission, through the earlier inheritance of char- 

 acters acquired in the adult or adolescent stages of life. By the un- 

 equal acceleration of characters an overlapping, or telescoping, as 

 Grabau calls it, may be introduced. It follows, therefore, that accel- 

 eration 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 paleo- 

 biologist, 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, Wiirtenburger, Buckman, 

 Smith and others among the Cephalopoda. It is shown that Ammon- 

 ites pass through a goniatite stage, and that, as phrased by Zitttel, 

 " The inner whorls of an ammonite constantly resemble in form, orna- 

 ment and suture line the adult condition of some previously existing 

 genus or other." The nautilus grows at first straight or orthocera- 

 form, 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 



